Why is Coax 50 Ohms? (
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- čas přidán 24. 05. 2024
- Prompted by a question to Ham Radio Answers, let's look at why coax cable used in ham radio is generally is 50 ohms characteristic impedance. There really is a reason.
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I am 76 y.o. retired. I have been wondering "Why 50 AND 75?" for about 65 years. Now I know. Thank you.
When I did my FAA tech ops interview this was an interview question. I was interviewing for environmental (air conditioning and generators) and since I was Navy ET for 6 years they threw the communications packet at me also. I never got a call back since I bombed interview even though it was a 2 hour long interview and the Navy guy said he knew I could do the school but the air force guy in charge didnt like me since I didnt do 20 years and got out early (he said he only hired retirees typically so I knew I was fighting uphill battle). He asked me ohms law and to do a wire gauge sizing problem (both I got right and both he had to look up). Ultimately I didnt get job so it didnt matter. But this was a question anyway.
Why wonder about something for 65 Years that you can easily look up and read about?
@@lambda7652 because the internet was just only starting to be used around 1990. Wikipedia started in 2001 and youtube in 2005. Being able to easily look up and read about stuff is a really recent thing
@@lambda7652 because you usually think of things like this in the middle of doing something more important and never remember to go looking for the answer when you're done.
LoL, I'm 69 and not looking forward to 70. I do the best I can. Keep safe my friend.
40 years ago I was asked this as an interview question for a job at the Motorola company as an RF design engineer. I spent over two hours filling the board with math to give a deterministic answer for both the 30 Ohm,(max power) and the 75 ohm(least loss) cable impedence answers... but could not for the life of me conjure up the math to justify why 50 ohm cable impedence was chosen. After a very long time of me sweating over the math during which all the grey beards were well amused, they finaly spilled the beans and told me it was a comprimise...and there was not a determinustic answer! I was not amused by them taking so much joy out of my apparent stupidity, so politely gave them the bird after receiving my travel expenses and ended up in the Marconi Company.
Sounds like they were assessing how you would tackle trying to find a solution to an unfamiliar problem under pressure, while also checking for sticks up your backside.
Don't get me wrong, plenty of people prefer strict professionalism at work, and that's fair. All the better to discover such differences at the interviewing stage, though. It sounds like you were both better off for it.
So much of engineering involves the art of finding solutions to conflicting requirements with imperfect mathematical models that involve a combination of empirical and theoretical approaches. This is what separates the great engineers from the great mathematicians. The mathematician enjoys the quantitative perfection of math. The engineer enjoys the qualitative best-fit compromise to conflicting requirements using imperfect mathematical models and empirical tables. Like medicine, engineers "practice" their craft.
I've met many engineers who are brilliant at mathematical theory but fall short when these models don't perfectly predict reality. It's a lesser version of Dustin Hoffman's idiot savant character in the movie Rain Man. Your Motorola interviewers were brilliant. They asked a question to qualitatively evaluate your problem solving skills when math cannot provide a single quantitative answer. They were also able to evaluate your ability to handle the stress of not being able to reverse engineer this kind of real-world engineering solution. Your responses revealed a lot about your approach to engineering.
I encourage you to look at this from the interviewer's point of view and consider what they were trying to evaluate. Their objective was not to humiliate you or to provide them with entertainment at your expense. (That was just your bonus gift to them.) Your response told them volumes about your approach to real-world engineering and how you respond to these kinds of real-world problems. I think these kinds of questions are some of the best interview questions. They reveal so much about the prospective candidate.
I too have failed at similar interview questions. I never got angry at the interviewer, only at myself. It would never have entered my mind to give the interviewer the bird for asking this kind of question. If you did this forty years ago and still harbor anger toward the interviewers... well, I don't know what I can tell you. The problem is yours, not theirs.
Fantastic strory!!! Thanks for sharing!
anyone who ever had to deal with Motorola engineer back in the day would understand LOL
in the 90s I had run ins with the team that was over the 680?? chips and early 2000s with the repeater guys, oh it was a shit show.... Im out here testing and re engineering (trying to make it better, bigger better faster) what you didnt
I love your reaction to the interview abuse. I am thinking you chose well. Is not Marconi the guy who figured most of this out anyway? I could not do it with the math all thou I could see it on a “SA” or a sweep cart when we used to sweep our ESM systems before deployment; it’s good to know when locked on to fire control radar. Can still hear it ringing in my ears over 4 decades later. I was a nuts and bolts tron.
I love it when things are explained in their historical context - it always clears up all the why’s.
More years ago than I care to remember I worked for Radio Shack during the CB (Citizen's Band) radio boom. The normal (single) antenna used RG58 (50 ohm) cable. Some buyers wanted dual antennas to be mounted on either side of a truck for a better sending and receiving pattern. The dual antennas used (if I remember correctly) RG59 (75 ohm) cable.... Of course half of the radio's output power went to each. The way the two cables were connected their presented impedance was close to 50 ohms at the coax connection to the radio.
Before the general adoption of twisted pair Ethernet, some computer networks used 50 ohm coax. That's what I built my first home network with in 1995.
In the late 70's and early 80's I was a cable TV tech for a system in Reston Va. The system was first built in the early 70's using 1 inch direct bury 75 ohm coax for the main trunk lines. Aluminum coax as is used now was not available back then. Our cable consisted of a solid copper center conductor, polyethylene dielectric, a corrugated copper shield which was coated with a waterproofing gel and finally the polyethylene waterproof jacket. The entire system was underground. We had an issue with a particular pedestal amplifier located in a low lying swampy area where the outer jacket had apparently been nicked allowing the water to get between the outer jacket and the shield. The water would percolate along the inside of the jacket and come out inside the next amplifier which we had to go out, open up, drain, then replace the amp. After the other tech had replaced the amp 4 times, I was assigned to solve the problem. We didn't have the budget at that point to replace or dig up the cable, so I removed about an inch of the outer jacket just before it went into the amp allowing the water to leak out before going into the amp.
Around 1979, the Reston cable system got its first satellite dish. About a year later the tech on call started getting hundreds of calls in the middle of the night that all the satellite channels were out. He went to the head-end but couldn't find the issue. After a couple of hours of frustration, he called me in. Between the dish and the building, we were using a 2 inch coax, very similar to the one you showed a picture of with air dielectric. The cable was run underground through a conduit to the dish about 50 feet from the building. To keep water out of the cable, there was a small air compressor which kept about 1/4 pound of pressure in the cable. After a minute of studying the situation, I asked, why is the compressor running continuously? The dish was the type where the accumulated signal was reflected from the large dish back to a small cone shaped reflector which reflected the signal into a square horn where the actual antenna and low noise amp (LNA) was located. The opening of the square horn was covered with a flexible plastic shield which kept water out. It had been raining most of the day. I went out to the dish and discovered the plastic shield had ruptured allowing the rain to fill the horn and cable going back to the building. We jerry rigged a plastic bag over the horn, disconnected the cable from the receiver in the building and drained about 2 gallons of water out of it. Hooked back up and the service was restored until we could get a replacement (UV protected this time) horn shield.
Thank you for sharing this! Awesome stories!
Those Coax ethernet networks were awful, no happy memories there for me. Good stories!
@@hexadecimal7300What even fewer people know is the coax Ethernet that came before! The coax Ethernet most people remember is the one using BNC connectors and RG58 or similar cable, using T-adapters and BNC terminators. This kind was called ThinNet. So why “thin”? Because the very original Ethernet - retroactively named “ThickNet” used coax cable as thick as a garden hose! Connectors weren’t used at all for the terminals, instead a conical reaming tool was used to carve out an opening for a “vampire tap” that literally jabbed into the center conductor. The vampire tap was part of the Ethernet transceiver which was physically attached to the cable, typically in the ceiling or raised floor. The transceiver connected to the terminal’s Ethernet interface using a 15-pin D-sub cable. This is known as the “AUI” interface. (The earliest ThinNet interfaces were simply a regular-for-the-time Ethernet card, but with a ThinNet transceiver instead of the ThickNet one. Later the transceiver was built into the card, where it has remained to this day.)
A guy a mile from me was apparently an avid short wave enthusiast, who excelled at DXing. When he passed, I visited his property in the woods and saw his very tall antenna tower. I Googled his name and discovered his fame. Someone apparently bought the tower, because the next time I visited it was gone and I saw cut stubs of 1"+ cable I did not recognize. It was "Heliax" I believe you called it, I am positive. Regarding your citation of Bell Labs research, my dad started at Bell Labs in 1928. In his things after he passed, we found volumes of technical articles authored at BL from that time forward that were of the nature you cite - seeking to reduce noise, maximize power transfer, increase reliability, etc. What an amazing period of innovation he worked through, from '28 to '69. Excellent job on this video.
It's called "Heliax" because it uses an air dielectric, with a flat helical spacer (polyethylene, I think) to keep the inner conductor centered. Imagine a long, thin strip that's twisted, with a hole through the center. It's normally pressurized with dry air or nitrogen. And isn't it amazing that Bell Labs seems to come up with so many inventions/discoveries? They invented the transistor, of course. They also discovered the background radiation predicted by the "Big Bang" theory.
That's Heliax, all right. Before I retired from a Fortune 100 I was a comm tech (later renamed IT Field Services tech, then Communications Networks tech) we had a few decades of 2GHz microwave. Ours used 1.75 and 2.something inch Heliax. It has been a while since I fought with that stuff - it was stiff and smooshable at the same time - but once in place it was plenty reliable. Well, except the center connection on our older stuff used a "bullet" that went inside the center conductor. That had a bad history of becoming intermittent at the antenna end, where winds liked to work it back and forth a bit.
Sometime around Y2K all the 2 GHz licenses were bought out for PCS, the first step toward modern cell phones. I don't miss them a bit.
Years ago while researching into the origins of VHF (AM) offset carrier transmission (simulcasting) I ended up in the under ground archives of the British library, Chancery Lane in London. Most of the original sources had yet to be digitised. After several days searching through dusty volumes of abstracts I located the original paper - but on the previous page was a paper on the optimum rate of twist for minimising cross-talk on telephone distribution frames. These papers were dated 1946. The bottom line is that someone, somewhere, spent a lot of time figuring such things out, often long ago, and the standards we take for granted were often based on the physical characteristics of the materials used. Good video by the way - and a good question too...
Oliver Heaviside was a genius, that many in Electrical Engineering community no less about. He introduced the concept of (impedance) "matching", whether it is a twisted pair, flat cable (like for old TV set to its antenna) or co-axial cable. Conceptually, this is the backbone of Electrical Engineering.
The also formulated what we now know as Maxwell equations.
Twisted pairs. In the days of open wire telephone lines, if you followed along one pair, about every 5th to 15th pole (depending on the system) you could see a bracket with 4 insulators. The 2 wires would switch positions at this point. This was the same as twisting the wires. The distance between twists, depended on numerous factors such as how close the pair was to the next pair, what the length of the line was, to what frequencies were being transmitted along the line. If you look at the difference between cat 3 and cat 6 cable you can see the cat 6 cable has much tighter twists allowing a higher carrier frequencies, thus more data along the pairs without getting so much crosswalk that the signals on adjacent pairs would interfere with each other.
Thanks Dave! Might I add more information about why 50 Ohm coax. Years ago an engineer for the Hewlet Packard instrument labs explained it this way.
At about the start of WWII there was no standard. With a world war approaching, a standard was needed. The military brought in experts. The experts said the same thing you had about different ohms. He had a third possible number of 45. He said that the military person, with no prefect choice took the three values (30, 45, and 75) and added them up and divided by three. The results was 50 ohms. By the end of the war, there was such a supply of 50 ohm equipment that it was the new standard
50 ohms takes me back… That was the large one with a screw on connector on my vidicon camera. Broadcast TV was 75 ohms with a type F connector. The early 1960s were fun. Didn’t know they stopped calling it the Heaviside Layer!
As an electrical engineer I took an electromagnetics 2 class. We went over all of this and did the math to "match impedances" as it is necessary at high frequencies
Thanks Dave. Good explanation. Characteristic impedance of a transmission line is a hard enough concept to grasp. Understanding 377 ohm Free Space is the next step. Been licensed since 1960 when you had to demonstrate 20 wpm Morse at the FCC office, in front of a grumpy FCC examiner. Pretty intimidating to a teen. No Wikipedia or Internet back then. Saved up for months just to buy my ARRL Radio Amateur's Handbook. Keep up the good work. 73 DE AA4KW. Bob
Good explanation Dave. I'm a EE with a BS degree. I specialized in transmission line theory and RF communications. This is exactly what we were taught at Iowa State many moons ago. The characteristic impedance for CATV coax is 75 ohms. For telephone cable it is 900 ohms with loaded cable and 600 ohms with non-loaded cable.
Thanks Dave. I am a retired Electrical Engineer, this was an excellent explanation . I never really thought about all the specific details you explained. Good job. Joe
I'm retired and also disabled at 66. I just recently got my technician license and now studying for the general-class-License. I love your videos and how you cut down on the bs thank you.
Thanks for the idea and inspiration. I will take early retirement in 4 years. Maybe I will do something similar. So, Jerry: one year later, bring us up to speed on your progress.
Hi Folks, Interesting discussion! I seem to recall reading decades ago that the 50 Ohm standard came about from the US Navy. The long wire dipole elements that were suspended from the ship's conning tower to the bow and stern at a declining angle lowered the nominal feed impedance from 72/73 ohms to about 50 Ohms. Also, that the first cable TV coaxial cable impedance was actually 72 Ohms and that knurled (obsolete) S-connectors (similar in design to a very small PL-259 connector) preceded the use of 75 Ohm F-connectors. I'm not sure if it was RCA or Jerrold who originally set the 72 Ohm standard. In my earlier cable TV years I had the honor to meet and work with some of the Jerrold Electronics engineers who developed many of today's current CATV standards. Ken Simmons, Hank Arbitter and Don Rogers to mention a few. That's what I remember!
As a Canadian I appreciated you randomly switching between Zee and Zed.
I love the rock band, Zed Zed Top.
Then a fisher zee might also float your boat
Use of Zed (to distinguish from Cee on A3 transmissions) among US hams has been common for ages.
@@amazing7633 Z will always be Zulu in my Non PC world, C for Charley. Zee is ocian in Dutch and we all know salt water and electronics are never a good mix ;)
zed seventy one sounds better
Best condensed yet great I ever heared of this subject after 37 years working RF. Kudos!!
Allow a correction - Heaviside invented “operational calculus” not differential calculus. Newton and Leibnitz independently invented differential calculus in the late 1600s. Leibnitz’s version was closer to the modern notation using dy/dx. Operational calculus is a method of solving differential equations (which might be a source of confusion here, with the term “differential”) . Differential equations are equations containing derivatives. Operational calculus converts a differential equation into a problem of algebra, which is a simplification.
I think the term everyone is looking for Integral Calculus developed independently by Newton and Leibnitz. Granted newtons methods are a bit harder to follow as he delt more with infinite series. Derivative Calculus has been around since the time of the Greeks. While these are the main breachers, And there are many branches of Calculus. Such as differential calculus, multi-variable calculus, vector calculus, etc...
I don't know who discovered which branch or who did what with which, Derivative Calculus and Integral Calculus are the 2 main branches which everything else expands from.
Heaviside was also (largely) responsible for vector calculus. It is his reformulation of James Clerk Maxwell's equations into the four we know today as Maxwell's equations (Maxwell had 20 different equations in x,y an z co-ordinate form). He was also produced half the terminology used in electromagnetism. Conductance, impedance, permeability, reluctance a whole lot more.
Heaviside was a major league eccentric genius. Probably Britain's equivalent to Nikola Tesla but without the annoying cultists and virtually unknown to all but a tiny proportion of the population.
Thanks for the clarification, I was starting to think my brain had already melted and forgot about my tons of math classes from engineering.
We had a semester almost entirely learning Heaviside operational calculus methods (tho there was more to it too).
@Hypercube Jones I wouldn't. You have to keep a sandbag in the back to offset the imbalance of the heavy side. :P
Third order equipment has distance, velocity & acceleration. The physical world is third order with sunlight reflection refraction and sun spots as random noise
Very interesting, first time I have seen such good info on coax. Ref to the open copper cadmium wires on telegraph poles, in the UK they were run in parallel pairs on short local routes, but on long routes they were run twisted by rotating the wires working from the outside insulator working towards the other outside insulator, then dropping down to the arm below and then working to the outside of the arm and then finishing back at the original start position further down the route. I can’t remember with certainty but I think it was every 4th pole the process was repeated. In 1969 there were still a few local routes left on British Telecom. I was taught that when looking for a short circuit (where wires got twisted together in high winds) if you looked at the apparent short whilst walking and the short didn’t move it was shorted at that point. If the twist moved then that point was clear. Short circuits were cleared by throwing a stick at the offending wires. If a route needed re-regulating then during the 1960s it would be replaced by overhead poly cable.
With all the cables that I saw, the twists were not that tight, the twists were all identical and cross talk did not occur. If a jointer made a mistake during repairs and two lines had their legs muddled then crosstalk was as good as a solid connection. The telephones were 600 ohm impedance, lines were usually max 1000 ohms loop resistance dc test and both legs had to be electrically identical in resistance otherwise mains hum would occur. I spent 30 years on telecoms and any engineers that I knew qualified in open copper wires were well gone prior to 1995.
We used Heliax on our high power transmitter to antenna connections and we filled the space inside with a dehumidified pressured air supply slightly greater than atmospheric pressure to keep out moisture
This is like "Real Housewives of Wherever" for Electrical Engineers. Thank you so much for your videos!
From Amphenol: Experimentation in the early 20th century determined that the best POWER HANDLING capability could be achieved by using 30 Ohm Coaxial Cable, whereas the lowest signal ATTENUATION (LOSS) could be achieved by using 77 Ohm Coaxial Cable. However, there are few dielectric materials suitable for use in a coaxial cable to support 30 Ohm impedance. Thus, 50 Ohm Coaxial Cable was selected as the ideal compromise; offering high power handling AND low attenuation characteristics.
For people using 75 ohm cable, you can get RG11 which is very close to RG8's size, and you can get flooded cable which has a water absorbing gel to protect the shield from water damage.
6:30 "An extremely low loss cable available at an extremely high price" absolute classic! Can't wait to adapt that to future conversations..... Cheers mate...
I wonder how you would terminate such a cable.
The cable is Andrew 1 5/8" air cable which requires pressurization (Nitrogen or dried compressed air). There is a foam version (LDF7-50) of this cable as well. Dimensionally they are the same. The cable's impedance is mainly the ratio of inner to outer conductors. The air cables have spiral jackets while foam cables have straight ribs (corrugated). Air has the least capacitance (parasitic loss) per foot. At frequencies above 800MHz the loss gets real bad.
This same cable in 75 ohm has a smaller center conductor. (LDF7-75) 'N' connectors for 75 ohm have smaller center pins but otherwise are identical to the 50 ohm connector.
The military still uses some 62 ohm stuff. I'm not sure of the application though.
TV originally used 300 ohm flat cable (balanced) and to use 75 ohm cable with TV antennas requires a balun which couples the 300 ohm antenna (balanced) to the 75 ohm coaxial cable (unbalanced). These are usually designed into modern antennas.
The twisted telephone is a balanced (600 ohm) transmission line. what improved cross talk was adaptation of better electronics in the system which allowed lower signal levels to be used on the lines. (Telephones adopted built-in amplifiers)
@@ronreyes9910 1 5/8" and 7/8" was widely used in mobile telephony stations (BTS) up to recent times. 7/8 was used for feeders lengths between 20 and 50m, and 1/5 8 was used for feeders greater than 50m between radio equipment and antennas.
Now the trend is using distributed system, I mean the controller and BaseBand Units are installed on ground level (inside some kind of rack/shelter) with Remote Radio Units (RRUs) installed near the antennas...
Thanks, brought back my early electronics , radio and TV days in College in Cork Ireland back in the eary 80's
Nice treatment of the topic! I appreciate the time taken to trace the historical roots. Cheers!
Thank you for this profound and comprehensive explanation (like many before). Your videos are always a pleasure to watch.
All the best for the new year to you, the ones you love and all viewers of this channel! 73 from Germany
Interesting. I was getting cross talk on my landline about 25 years ago. I could hear one particular residence talking to other people, but only when I was on the line to someone else. We discovered that they could hear us one day when I told my other party to shut up and listen to the second line. The other crossed line heard me and became freaked out and hung up. LoL. I don't remember hearing it again so they probably made a complaint to the phone company. However, I've heard cross talk on landlines for many years before. You couldn't always hear clearly what the other conversations were, but you could hear talking on the line. Sometimes two or more conversations at once. It never really occurred to us to try and listen, mostly because you had to be on the line with a call. The other reason was, you really couldn't hear them that well and thirdly, their conversations sounded boring. Just that one time that I first mentioned that I could hear them well. That went on for a few months for sure. I mostly ignored it. Just that one time I actually tried to listen. More for curiousity of whose line we were crossed with. Anyway, that's it.
There you are! Meow😺😹
In the sixties home phones were being rolled out at a high rate and the infrastructure could not cope. The solution was to roll out temporarily with line sharing called 'party lines'. This meant that you could pick up a phone and hear any ongoing conversations . You managed with this by politeness and hanging up.
RG 68 & RG 59 are both 75 ohm. Can't believe I remembered that, but I double checked my memory before posting. Both were used on the radar sets back in the 70's and 80's that our unit maintained. Definitely preferable to RG 6. ALSO, I would like to say thank you for answering many questions I have had for decades, but didn't find the time to research. Comprehensive and concise. Very much appreciated.
Your CZcams channel a recent discovery, I much enjoyed your very clear presentation on 'Why 50 Ohms?' Very useful to know the 'why' of it.
That was very interesting, I'd always wondered why transceivers use 50 ohms while TV used 75 ohm coax, now I know.
You mentioned early on the idea of using copper pipe for coaxial transmission line. In high power broadcast, particle accelerators, etc., this is exactly what is used. The outer conductor is a standard copper water pipe diameter-- three common ones being 3 1/8 inch (3 inch ID), 6 1/8 inch (6 inch ID) and 8 3/16 inch (8 inch ID). The inner conductor was often also a common water pipe size, but today is usually a custom size that gives a Zo of 50 ohms. This line typically comes in 20 foot lengths, and is connected together with a sleeve or bolt flange on the outer conductor, and a silver plated 'bullet' to connect the inner conductors together without an impedance 'bump'. Teflon spacers at the ends, and teflon pins in the middle of the section keep the inner conductor centered inside the outer pipe. Elbows, etc. are built so this like can go around corners, up towers, etc. Although air is the usual dielectric, sometimes dry nitrogen under a slight pressure (to keep moisture out) is used. Rarely, an exotic dielectric gas like sulfur hexafluoride is used. Depending on the line size and the frequency, these lines can handle tens to hundreds of kilowatts of RF power. 50 ohm line is the most common, but 75 ohm rigid line is also available and is popular for television service, where the low loss in long runs for tall towers is really needed.
I agree, if you try to build a coaxial guide using commercial (copper) water pipe, like it would have been done in 1930, then you would most likely end up with a rather low impedance like 30 or 50 Ohm. History shows that some bold decision are often pure accidents.
My father was the original electrical engineer that set up a TV station just after the war. I remember once while in college, a friend and I were at my house for some college break and my father had to go check on some work being done at the transmitter and he asked us along. When we got there, the work was out back, but we wanted to see the inside of the building. He said "You're both MIT EEs, I trust you to not kill your selves or break anything, go ahead".
Out in the back room, behind all the electronics, we found the coax, which was up at ceiling level and about 6 inch copper pipe (as you say a common size). It was very interesting to follow around. There were pipes coming out of each of two redundant transmitter banks and several places where different shaped bits could be swapped in to connect to different things. One was swapping between the two transmitters for which was connected to the actual tower. But there was some additional switches that could connect either to a large cylinder, about the size of a water heater, where the main coax pipe went into one side, near the top another connected straight up, and near the bottom one went straight out to the side. We wondered a bit at this three connection component. Some examination showed the lead out the top went through some very non-coax style bends, and the lower lead went straight to the back wall of the building and ended. We asked my father about it, and it was the "dummy load" for testing the transmitters when you didn't want to actually transmit. It _was_ actually a water heater although not a normal one, the 6 inch water main into the building could be set up into the top, and apparently with the TV transmitter at full power, the water came out the back of the building boiling.
When Santa said "I'll go way beyond the answer" I decided to stay and watch!
Congratulations Dave! You should be very proud of the thousands and thousands and thousands of views for this video. Well-deserved. Keep up the great work - you represent our hobby at its finest! KJ6ER, Silicon Valley
An excellent video Dave. Funnily enough I am currently putting together a talk for our local club, explaining why when people say "I get out better with an SWR of 1.4-1.5:1 than I do with 1:1 and the height and feed point impedance is one of the main points of that talk. Many people actually "detune" their antenna from the resonant frequency they want, just to get a low SWR!
A 50 ohm resistor gives a 1:1 SWR. It's not much of an antenna, though. Obsessing over SWR is often counter-productive. If it's over 3:1 with a long transmission cable, then you start to have noticeable losses, and the voltage starts to approach flashover with higher power levels.
Dave, if my math teachers explained things as well as you do, my school years would have been so much better. Thank you.
Most detailed explanation of this technical question i could find so far! Thanks a lot, Dave!
Once again dave, I find job as always. This really answers a lot of questions that I have encountered with fellow hams over the years. This has a great job to explain things. One of my ham friends actually uses 75 ohm cable and some of his setups. Thank you again for another great video. 73 from KY4ROB.
- I've always built-tuned my center-fed dipoles by erecting them with a 120° angle between the two sides, trimming to lowest SWR, then re-erecting as a flat top dipole.
The SWR increases to ~1.35:1 but seems to perform better than those with a lower SWR by other means of tuning/trimming.
When I was studying physics in the 1970's, wave propagation in coaxial waveguides was a natural subject to examine, because we were thinking about waves and their confinement between conductors. But a century earlier, when Heaviside began thinking about it, signal frequencies were probably in the low kHz range, which is essentially DC as for as wave propagation is concerned. So it makes sense that a coaxial waveguide was a novel idea. My recollection is that this concept only became a useful technology when the right dielectric was invented at Bell Labs - I believe in the 1920's.
When I was an apprentice/ trainee back in 1959..
Base stations normally employed a centre feed folded dipole mounted on a boom on the side of the mast feed with 75ohm feeder ...
Vehicles employed a vertical whip with the horizontal vehicle roof becoming the counterpoise, employing 39ohm feeder ( incidentally the thick copper centre conductor also made an excellent element for an AC solder gun) As the ground plane was at right angle to the whip except of course when it thrashed about when mobile !...
To cut the whip to resonate we used a scrap telescopic car radio antenna with a piece of whip rod soldered to the end...
Monitoring the TX power with a vswr or a radiation Meter the antennas length would be adjusted for min vswr and max radiation...The whip was then cut to this length ...Later on 50 ohms became the standard for both base and mobiles...When I asked why 50 ohms? I was told because it was about half way between 75 and 39 ohms !😊
Excellent video! This has reminded me of my days at the Army SEE ( UK ) in the early '70's. My trade was telecommunications and we dealt with all kinds of HF/VHF transceivers and the associated antenna. I still have technical info on matching O/P and I/P Z's. Oh such long distant memories! Thank you so much.
Heaviside did not invent differential calculus. But he did invent the Heaviside Operator which provided a simplified means for solving certain types of differential equations.
Not differential but operational calculus.
Very interesting video. I am no ham radio user, but i used to do computer networks. The old coax cabling back in the day needed 50 ohm terminators. Which always went missing. I made my own from 50 ohm resistors. Those were, and probably still are hard to come by. The man in the electronics shop made weird faces when i asked for those, assuming i was mistaken. I had my whole home testing network connected up with this coax cabling at the time, only giving it up when my network interface cards were no longer supported in newer operating systems in the late nineties. Which i hated, as they were all working fine otherwise. So there’s my 50 ohm story. Greetings and a happy 2021 from the Netherlands!
My 50 Ohm story...:
I was working for a telecommunications company and they tried to install coax ethernet at one of their sites and it didn't work. I told them that they needed a terminator at each end. That was after the movie Terminator 2 came out. They naturally believed I was bullshitting them, even though I tried to explain the physics behind that.
They then asked another expert. The other expert told them the same bullshit. They got really pissed off by that.
Then they thought the cables were broken and went to a computer store to get new ones. The guy at the store also tried to be "funny" and asked whether they wanted terminators as well - and showed them what they looked like and that they were obviously designed to fit the T connectors. They finally got the message...after a hands-on demonstration.
@@klausstock8020 that's a great story. thanks, you made me laugh.
Ne2000 coax network card?
The way you explain makes me wanna watch all the videos you have! It's all backed up with something, clear speech, and a thorough storyline...Really good job! 👌👍
Thank you, Mr. Casler. A very well thought out presentation.
The low losses are not important only for reception, but becomes very important mainly for continuous transmission: 1dB cable attenuation is not that much of a deal for a receiver, but with a 10kW transmitter it means 2kW of power would get turned into heat within the cable, which means you need to take that away by the cable cooling. In real installations all the connections have to allow the purge and cooling gas to flow freely (the circulating gas does double duty: Maintains the temperature, as well as it keeps the moisture and oxygen out to prevent corrosion). But too high losses would mean too much heat to deal with.
Wow. This was an interesting and very well explained answer to a very good question! Thanks, Dave!! I coulda read those Wikipedia articles myself....but I never woulda come away with the understanding that you extracted and conveyed. Nicely done.
Hands down, probably the best video I have ever watched regarding anything amateur radio. Ever. Thank you Dave.
Awesome presentation! My dad schooled me on some of that when I was a kid, and of course I've forgotten most of it some three decades later. This was very cool.
I'm radio ignorant, but TV savvy and appreciated and understood your explanation. Thank you !
I worked for Harris, RCA, Raytheon over the past 40 years. There are many other Z over a wide range of Coaxial cables.
Yes 50 & 75 are the most common.
Very interesting video. I've been a ham since '89 and didn't know all of this.
You are a natural teacher! Highest compliments! First video of yours I've seen!
Thanks David. It all makes sense when you have all the facts.
"Of course I'll go waaay beyond the question as I always do" -and that's exactly why we tune in!
Press Like if Dave is your Elmer!
IDK who is Elmer, but Dave is very interesting to listen to.
Thank you for your lessons for those of us who have never taken time to search out these problems before. God bless.
Wasn't searching for this, but very glad I ran across this video. Very informative.
Olivier Heaviside is a Hero of early Electrical Engineering.
Here here!!!
Yes he certainly did a number on Maxwells equations, made them easier to understand, I just hope he did not leave anything out.
I've been a fan of Heaviside since I was a student. The university library had his original volumes.
@@hexadecimal7300 4 equations of electromagnetism we use today should be called Heaviside's eqs, not Maxwell's eqs.!
@@imeprezime1285 Ok , thankyou, only ever studied them 40 years ago at college, never ever needed to use them since.
When I worked in TV I remember we used coax a lot there. We also used coax for older ethernet. I always remember we had to use terminators on the cables and one of them was a 50ohm terminator while the other was 75.
We only used 50 ohm term. To connect a terminal we had to drill a hole into the coax to get at the centre conductor, all very messy, all for 1Mb! UTP and RJ45 so much easier.
@@hexadecimal7300 Yes I remember - bee sting tap.
@@earthstick Thanks, what a pain they were, had a lot of faults, late finishes and right runarounds with them. The coax was also pretty inflexible and would not go where people said to put it. So happy when twisted pair and RJ45 connectors took over.
@@hexadecimal7300 I remember the first place I worked the network was always going down because it was early coax ethernet. I just couldn't remember if it was the ethernet with the 50ohm terminator or the black and burst signal we feed to all the TV equipment. One was 50 the other 75.
@@hexadecimal7300Well, I remember that t.p./RJ45 were pretty unreliable , too. Talking about time 40 years ago…
David, thanks for filling us all in on a subject that many of us simply took for granted. You earned my sub. de N6ABM living in 7 land. 73's.
Always wondered about that.
But learned even more, about telephone wiring, shielding, coax, and twisted pair.
I have done some telephone, RF, but now mostly audio, and some digital cabling.
Great information, thanks for sharing.
it's a treat to hear a well thought out, reasonably technical explanation in a sea of surface level information. keep up the good work!
THIS is the sort of nerdy information that I love. Thanks for the history lesson!
Thank you SO, so much for this video. I've been pondering this question for quite a while. Thank you, sir.
Mr. Casler. Thank you for your presentation. Spot on explaination.
Also it should be remembered that the Z0 of free space is 377 ohms. Hence there is a mismatch between an antenna and free space (for maximum power transfer) also a mismatch between the feeder and antenna and a mismatch feeder to receiver. A dipole is just a transmission line pulled apart into two arms. That is why some recommend ignoring the E field and instead concentrate on the H field, with a loop antenna.
Fun video, thanks Dave! Transmit only cable is 25 ohms like broadcast tv and fm radio coaxial cable
Absolutely the best explanation I have seen. Thank you sir!!
Thank you for making this explaination video.
I enjoy learning about this kind of general principles / electrical engineering stuff.
Awesome video/information Dave. December 2020 I passed my EXTRA test! I say this because my brain cells, after all that studying, seem to be better at understanding all you've presented here! I'm big into 40m wire beams and mostly use cheap RG-58. Never a problem. RG-58 is 52 ohms I believe. I did have to replace some as I guess water got into some and there's a black corrosion on the braid. I subscribed! 73 de, WA3RSL.
Wow, such a great explanation; thanks a ton. :)
I slid by your live stream on Thursday 2/18 and noted you being surprised about the popularity of this video. I decided to leave a note addressing this. Background, I'm a 50 something electrical engineer with a masters degree in electromagnetics as well as a technician level ham KD5BAP. I happened across this video totally by accident and it may have been the first of yours I viewed. Looking at the title line I asked myself, well, why 50? Hmmm, clearly I know that nearly everything I deal with is Zo 50 ohm but I did not know why and don't remember _why_ being a topic over _it is_. They'll never teach something like that in a class so I watched to see your take on it. You have a very good voice and approach to instructional videos and I was following all the way to the end.
The interest continued and I have watched a group of your other videos subsequently. I particularly like videos that take complicated ideas and transform them into something that is more understandable to a larger audience rather than only your peers. I use this concept to evaluate a person's understanding of a topic because nearly everything can be refined and explained to someone without an equivalent level of experience as the presenter and still deliver very useful information to the audience.
If you want another idea along this path how about a video that discusses how we ended up with the connector types that we have, where they are good, and where/when they are not. A modern example is the type of connectors that are being used in the upcoming wireless systems in vehicles.
Thank you for the details, research and time. Most of all, thanks for sharing.
Ah, but there is a ware-out mechanism, curves. Tighter bends in a cable run will put pressure on the inner conductor and can overcome the physical strength of the dielectric. This throws off the relationship between the inner conductor and outer shield at that curve, and changes the impedance there. This can cause reflections and seriously effect the SWR. Keep the bends in your cable runs as gentle as you can.
Heaviside also reduced "Maxwell's" equations into their modern differential form.
Yes, a magnificent achievement that on its own was enough, but he did so much more.
@Sean M True--I have heard 21. Heaviside rewrote them into the current 4.
The modern formulation due to Heaviside, uses the vector calculus not the differential calculus. The differential calculus IS due to Newton, but the modern notation (calculus) is due to Leibniz.
@@nosnibor800 Heaviside was autodidactic and derived his own calculus.
@@ChrisWilson999 Correct. From the age of 21 until he was 28, he had returned to his parents house and attic bedroom, where he worked all night, then slept until noon, then went to the local library to teach himself maths. He wanted to understand Maxwell's treatise, which he mastered, and then reformulated the 22 equations into the 4 using vector calculus, taught today. I believe someone else also reformulated Maxwell's equations, using Quaternions. Later, he did indeed develop his 'p' operator calculus, in order to solve transient response problems - his method was taught widely until the late 1930's, to Electrical Engineering students. The Laplace transform was then substituted and used universally. Later someone (I forget who) demonstrated that the 'p' operator and Laplace transform were equivalent. The 19th century certainly was a fascinating period. Heaviside was indeed brilliant but later started dressing up in women's clothes and painting his fingernails pink. He thus became an embarrassment and was hidden by the establishment - a bit like Alan Turing.
Thanks... I learn a big ton of important things having lifelong value to me from your video, precious information.
I remember using Reels of 50 Ohm coax for computer networks in the -90's. So many games of Blood3D, DukeNukem, Doom, Diablo, Master of Orion played using those cables, T-connectors and the 2 mandatory end-terminations, so many yells and screams, from some idiot-mom disconnecting her sons computer wrong and messing up the net, and so many electroshocks from shorting the pins with fingers when 3 dozen dodgy homebuilt PC connected with un-grounded extension-cords were connected simultaneously! Thank you for explaining why 50 Ohm, and thank you to my teachers in electronics, and mathematics, since it is their hard work that made me actually understand the explanation!
Dave,. You're looking much better. Your color is back. Good job OM.
I was taught coaxial means along the same axis. My last job we used both 50 and 75 ohms in the 70 MHz range.
Coax is both around the same axis. I have used a lot of different coax cable. They all had the center wire and then the other wire around that.
Thank you for this video! You explain everything beautifully.
I found this video extremely informative. Thanks for diving into the history of it all.
The reason that the exact 35, 50, and 75 Ohms are chosen is also that they make rather good quarter wave combiners and splitter transformers. Note that a perfect vertical is 35 ohms.
A perfect vertical?
@@bendermi I think he forgot "dipole" (antenna)
@@karhukivi
Okay I'm not that much into this material but I'll Google dipole antenna right away.
@@bendermi Correction - he meant a "vertical antenna" usually referred to as a "vertical". A dipole antenna has a different impedance s it is essentially a folded antenna. Plenty more on the net!
@@karhukivi
I'm fast but not that fast I'm still watching videos are those antennas for me I'm only listening to music on FM?
I just go ahead and do the thumbs up for Dave's videos in case I forget later.
I worked in TV broadcasting. We used rigid 50 Ohm 6 inch coax which was just copper tubes with plastic disc spacers. Early rigid coax was 51.5 Ohms because the manufacturers of transmission line used the closest standard copper pipe size available. Eventually, custom pipe sizes were manufactured, and 50 Ohm transmission line became more common.
Cables are great for antennas with matching circuits tuned. For wire antennas I gave up on coax in favour of 600 ohm open wire ladder line. Admittedly I was skeptical at first but now I don’t ever intend to use coax on a wire antenna again. Great explanation on coax OM 73
Did the relatively high output impedance of tube technology of the time affect bell lab's choice of 30 ohms for max power transfer? I wonder if 50 ohms today is still the best balance of power/loss given modern low impedance output stages?
The long and short of it is that 50Ω couples well with a half-wave vertical dipole (or quarter wave & ground plane) without needing a balun. In telephony and pro audio there's no reason to transfer power, and so the nominal impedance for twisted pair voice cable is 600Ω. Telegraphy and telephony had very little in common, aside from the rights-of-way that they shared.
The telegraph was a switched DC technology, while the telephone was AC. Early telegraph networks got away with using the earth as a common return path because before then there was no market for electrical wire; instead iron fence wire was used because it was cheap. Telephony needed copper wire and a balanced line topology to reach any distance. Despite Heaviside's genius, coax was not the savior of telephony, and bundles of twisted pair continue to be used to this day for local loop circuits. One advantage that coax has is the potential to carry frequencies far beyond audio frequencies, which is why we see it used for radio. So it never was about crosstalk!
The 75Ω nominal impedance became the norm for standard coax types that did _not_ convey power to a transmitting antenna. This includes baseband video in the TV studio, broadband receive-only TV in master antenna, community antenna and cable TV plants, and inside telephone plants, carrying broadband voice and/or data. One notable exception was 10Base-2 Ethernet that used 50Ω coax. Woe be the technician who had a T3/DS3 Internet connection to a 10Base-2 Ethernet LAN, and mixed up the cables that both had BNC terminators!
It's worth noting that a balanced line is more efficient, but coax is more idiot-proof. A good 300Ω twin lead from a TV antenna will preserve moreof the signal, but must be installed with standoffs to maintain dielectric separation from any other object that might affect signal propagation. So while 300Ω twin lead had the potential to perform better than 75Ω coax, it only did if installed and maintained by skilled people. Coax was far less sensitive, and so any idiot could install it successfully.
As a TV engineer I have to laugh at the reverence for Andrew Heliax. In broadcasting, Heliax is the smallest coax that we use, when we need flexibility. Hard line coax (essentially two copper plumbing pipes of very specific size, silver plated) is a lot less lossy, and for UHF we use waveguide that does away with the center conductor altogether, instead creating a standing wave in a rectangular tube. If you think that stuff is costly, you should see the transmitter electric bill--that explains why saving a dB here and there really adds up!
Yes, but Heliax is used very extensively in FM broadcasting - all the way up the tower to the antennas. But hard line is only used in very short runs.
@@chansetwo not sure what you mean by that. As I noted, the whole point of using less lossy coax is to reduce losses. You're not going to save enough over "very short runs" to make any sense at all; _of course_ rigid coax and waveguide only makes sense over long enough distances for it to make a difference! That's the entire point of using it.
Thanks Dave. Great Explanation. understood every Bit thoroughly.
I was asked to rebuild some of the feedlines at a local 50KW AM station here in Louisiana. I note as I was taking down some of the 20' 4" hardlines, some of the sections were 51.5 ohms. This transmitter site had two phasors. a 4-tower day (50kw) and a 6-tower 5kw night. On the driver tower, there was an old TV antenna at the top that hadn't been used in years. I don't know for sure but I think the radio station begged, borrowed, or stole some of the TV hardline (51.5 ohms)
This can of worms was a challenge, to say the least.
Thanks for sharing this.
Cheers from Louisiana.
Thank you Oliver Heaviside for inventing the coax we know today
Cables are like dogs treat them right and they won’t bite
Thanks for the explanation. Much appreciated. It never quite twigged that the construction of the cable led to the impedance of it but now it makes perfect sense.
Excellent lesson ! Math & myself are the worst lifelong enemies... so over my decades of electronics work, got bogged down in the numbers part.. gave up & just accepted, not needing to be an engineer in designing. Thanks for all the extensive work you did in putting this together !👍.
Answer starts at 9:15 if you know how coax cable works.
30? 60? 77? I thought the answer to everything was 42.
As a telecomms guy I was taught that the defination of the charistic impedance of a transmission line is the impedance of an infinite length of the line OR the impedance of a finite length of line which is terminated in it's charistic impedance.
This is the difference between engineering and physics.
I hope no one snaps at my ankles but I've been laboring under the impression that 50 Ohm Coax became the industry standard because 50 ohms is the characteristic impedance of a 1/4 wave vertical ground plane antenna. If I've been wrong all these years I'm going to grab hold of a good ground and stick my tongue in a wall outlet! I'm long retired now but besides being a GC ham ( WA2ERQ ) since 1966, I had a Land Mobile Radio / Marine Electronics shop for over 15 years. During those years we installed countless 1/4 wave VHF / UHF mobile antennas on police, fire, emergency and commercial vehicles. During those years we ran RG58 and later RG8X coax to 1/4 wave antennas. I don't ever recall any RF transformer or impedance matching network between the transceiver and antenna. We tuned ( CUT ) the antennas using an in-line RF Watt Meter capable of reading true forward and reflected power.
So to summarize, it only seems logical to me why 50 ohm coax was and still is standard. On the other hand I am prepared to be humbled with differing or apposing "facts".
Wakodahatchee Chris
Correction: That last sentence should have read " Apposing *Theories* " not "Apposing facts".