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Magnetic Loop Mysteries Explained
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- čas přidán 15. 05. 2012
- A number of years ago I designed and patented a way of feeding a small wire transmitting loop using a ferrite transformer and since then have read almost everything that I came across referring to Small Loop Antennas. Surprisingly I found that there were a lot of disagreements about how they work the current distribution, Voltage distribution and the radiation resistance or feed point impedance.
In order to try to get these disputes settled once and for all I have conducted a number of practical experiments and made a video of them. I hope you enjoy watching it, may be a little boring at points, but seemed necessary however you can Fast Forward when necessary. Thanks for your interest and please email me if you have any questions.
Cheers Ben G0CWT
www.magnetic-loops.info
Music: Kevin MacLeod
What happens when you minimise the losses and put a high quality low-loss capacitor in the gap? From standard electrical theory, voltage causes current to flow between two points; if you put a high voltage across a capacitor, there will be a high current related to the frequency (rise-time/period). If you put a slab of ferrite around the conductor, it provides a high impedance to the current, and damps the resonance, thus reducing the current.
As you and I well know, any outside influence on a tuned circuit will cause a change in the resonant frequency: With a narrow band-width high Q circuit like a magnetic loop, clipping an ferrite slug onto one end of the inductor will shift the resonant frequency away from the applied power/signal, and the current will drop immediately as a consequence.
Ben, you are right on the money and your webpage is very clear.
All the old hams should know a properly loaded AC circuit has the Voltage 90 degrees behind phase to the Current (Cosine, Sine), except where measuring reflectance. The antenna ends must be maximum voltage and minimum current. The centre feed point for a (coax sheath) balanced aerial will be maximum Current and minimum voltage, the part that best does the radiating..
Thanks Ben for teaching us all something useful.
that was more info in one shot then iv seen in a long time leave it to the ol timmers to do things right
Thanks. I repeated your measurements and my conclusions are little bit different. First the current measurement: in my meter circuit I couldn't get rid of unwanted E-field sensitivity without a screen around the pick-up windings, so I measured a combination of induced current AND parasitic E-field. Probably the showed demonstration is also affected by this unwanted crosstalk. Secondly: the fluorescent tube does not indicate current, but only the common-mode voltage with respect to ground (your hands, your body) and the gradient of the E-field (difference between two high-voltage ends). The loop is floating, but the capacitance to ground is sufficient to have bright fluorescent response near the high-voltage ends of the resonating loop. Thanks for inspiration, by the way. Brightnoise, Netherlands (PA0FSB).
An excellent demonstration, my late father did something broadly similar to illustrate standing waves in a transmission line. (a slotted coaxial approximation and two probes, one capacitative and a current probe.) Very graphically demonstrated how the line behaved while matched/shorted or open.
Thank you Ben, congratulations. I made similar experiment on EH star model home made, modified by myself. I used home made sensors for E and H field and i used an oscilloscope, two tracks, with exploration of RF field close and not close to the EH antenna. The results were obviously different compared to your magloop antenna. I have also a home made magloop antenna (also home made - my project - variable capacitor 3D printed, for high voltage, similar to butterfly type but not the same), i modified controloop also and i obtained better sensitivity and better matching (SWR 1:1, return loss around 10 - 12 dB on 14 MHz). Thank you for your very interesting video. It is a stimulus to experiment new measures and to test the setup of my magloop project. 73/72 de IS0FQK - Pat.
+acart0 you are completely correct about the voltage current distribution. max current is always at the current feeder. the magnetic loop is basically acting like a folded dipole that is capacitively shortened. The voltage end does little to radiate so why not.
People who say its the same current around the loop are ignoring the basic laws of AC and resonant tuned circuits where voltage and current are 90 degrees out of phase.
Yes, on a ferrite, 8~9 turns makes it work well for 50 ohms.
Thank you for this informative and educational video. It clears up many things in my mind. I am experimenting with a ferrite feed loop now for personal use.
I never considered that the current around the loop is constant for the following reason. Assume a straight dipole half a wavelength long. fed at the centre. NOw we can assume that the dipole is made up of a lot of capacitors where incremental plates of the same area run all along the dipole starting at the centre. The distance between the plates making up each incremental capacitor increases as one approaches the ends. So the largest capacitors are around the centre and these incremental capacitors bleed current away from the main line, the dipole itself, Now with the capacitors around the centre being the largest as the distance is lowest, these will form a loop of displacement current and so the highest currents are around the centre and the minimum or almost zero current at the end this having the highest voltages. Now if we had to bend the straight dipole slowly to shape it into a loop, what is in fact happening is that we are making the end capacitor slightly bigger as the ends are coming closer. This procedure will effect the current and voltage distribution in the "bent dipole" with the heaviest currents still placed around the centre and the highest voltages placed around the ends. Now if we had to place an artificial additional capacitor in addition to the " natural one at the ends" well the distribution of the current and voltage will depend on the value of the additional capacitor placed at the end to increase the end capacitance. I would say that if this capacitor is too big, then the current could reach a high level. So a loop is really a bent dipole with an additional capacitor placed at the ends.
If one uses a mult turn magnetic loop as I use when demonstrating the magnetic loop to my students, I lower the frequency , well with a multi turn loop then the final capacitor will have copper running beside it or through it ideally and so in a multi-turn loop, there is a possibility that the current around the loop is more equally distributed rather than that in a single loop with an end small capacitor.
The loop is tuned to frequency with the meter and ferrite in place at 90 deg and any change in current due to the presence of the relatively small amount of ferrite involved is negligible.
As I am only trying to detect the change in current around the loop and not to actually measure the amount of current present it is not sufficient to cause a problem at the frequencies in use. Thanks for your interest.
Thank you, Ive found those exact numbers in some work I'm doing with mag-loops and Fractal Geometry, this was very helpful since you labeled it for me..impedance. More science like that please, at present I'm in SDR radio, but hope to get certified soon.
Thank you Ben, this is a great video. I also have made videos on Mag Loops so I know how much work you have put into this.
Yes, thanks. Built one 16 feet on a side (64' total length) and worked on both 80 and 160 m using recommended ferrite transformer and proper taps ... the antenna seemed to belt out one helleva ground wave vertically oriented in direction in the 'plane' of the loop. Free-standing using fiberglass poles was quite the trick too, since I don't have a mast or tower to hoist a 'spreader' as your website shows ...
did you put the capacitor at the top or did you have it at the bottom for ease of adjustment?
If the current is NOT the same all round the small loop, where does the "missing" current go ?
Concerning current drop off at the base of the loop: What would happen if you ran a thin wire from the top (10 and 2 o clock positions) of the loop, straight across the air gap to the base of the loop, in order to make that current flow? Is that practical and what would be the effect on the signal, if any?
You're using a small loop to feed the larger loop. The smaller loop will influence the E and H fields. Also, I don't see any shielding on your homemade current sensor, so perhaps it is influenced by the E field. So I didn't find you demo to be 100% convincing, but it was a good effort.
thank you for the helpful explanatory video. A friend constructed an 80 mtr loop and it's great for receiving on 80 and 40m. Not helpful for transmitting as yet..
How do you know the lamp is measuring current and not RF power density?
If the peak current flow is at 180 degrees on the main loop, then couldn't the displayed system be improved if the inner loop was rotated 180 degrees from where it is? This is assuming that the inner loop has the same characteristics. I am under the impression that maximum power transfer will occur if the peak current portion of the fed loop is tightly coupled to that of the resonating loop. I'm watching these videos to figure out how to build such an antenna, so I have no hands-on experience here, so my question is: "Does the rule 'amp-turns-per-inch' apply to this antenna?" That's where my comment is coming from.
Us aliens do this With Our Saucer.
This is a really good demonstration that speaks volumes about current and voltage distribution in the magnetic loop and it’s made me reconsider the tuning capacitor. We know the voltage and current are out of phase but I had it in mind that the tuning capacitor had to be of high voltage and high current capability whereas from this very practical demonstration clearly it needs to be a good high voltage device and its current handling capability is far less important. Thank you for demonstrating the point so perfectly clearly, well done!
Kind Regards . . . Andy
GWØJXM
It is important to have a good current handling capability of the capacitor as well. There is a lot of reactive energy at its connection point. I recently tried to tune an aluminum strip mag loop of 1/3 wave circumference by having a second piece of strip on top and coming into close proximity to the loop gap. I witnessed heating at only 20W carrier power, which caused the resonant frequency to change by 500khz at 28mhz.
I tried a capacitor made from 2 strips of copper hanging down from the top gap and this eliminated the heating problem.
Even with 40W carrier for 10 minutes, nothing gets warm at all anymore, no more de-tuning whatsoever either.
Losses are a very important consideration.
Nylon bolts and nuts are bad to use at this point. They heat up. White plastic chopping board (presumably very similar to polyethylene) has proved to be excellent and even when used as a dielectric material for the capacitor, it does not warm up at all. I don't know how many volts a capacitor made from a 3mm thick piece of this stuff can withstand, but I am going to test it at 100W in a loop to see how it goes.
Ben, how does the ferrite sleeve affect the loop's tuning (i.e. resonant frequency) as it is moved around the loop sampling the loop's current?
About the difference in current measured around the loop: this must be ostensibly due to the 'displacement currents' (as theorized by Maxwell) created by the differential HV (High Voltage) existing either side of the capacitor (and with the differential HV tapering off to zero as one gets to the point directly opposite the tuning cap).
-uploadJ
What an excellent video! Are these measurements only valid when the loop is made resonant by the capacitor rightly adjusted?
Yes The loop must be resonant.
very interesting bit long winded but still where you did the impedance testing,
maybe you would have a higher ohm reading if the loop was tuned.or if thats as good you can get it you need to look at your design you should easy get your swr down and under 1.5 and your impedance will go up but its still good see the bulb test gives others an idea how they work.
Can the ragtime piano. Do you play that while you transmit also?
Very impressive how much time was spent waving that light bulb around.
does the smaller feeding loop have to be located in the primary loop or can it be outside of the loop. I can imagine it needing to be on the same plane or axis as the radiating loop.
one last thing. can you connect the inner conductor, if your using coax, completing a full loop and mounting the cap to the sheild as if its a copper tube loop antenna?
Thank you! In your view, is it better to have the cap on top of bottom for DX vs NVIS? Thanks
My personal experience shows the opposite using a field strengh meter away from the antenna. The arrow should be pointing twards the variable capacitor.
Hi and thanks for the video..
what is the set up for the fluorescent tube. ?
Is it just on a stick to hold it, or is there more going on there?
I wanted to make something like that for a very different project..
Hi the tube is on a stick
Is it mathematically possible to make a 2 meter loop and a 440 antenna loop where the 2 meter will act as reflector for the 440 and the 440 will act as a director for the 2 meter?
Why do you have the cap on right and the feed on the left? horizontal ? that can't be right.
Is that how you run that antenna ????
I am looking to build an antenna like this so do you put the Cap on top or bottom,
I'll be waiting for the 'talkies'...
Great video. I am currently experimenting with loops. I have to ask, what is that catchy music track??
73, VE3NJK, Canada
in interest of public safty. would it not be best to place the small fed loop at the bottom. upside down, from the way most run, with cap at bottom.
Nice video.
could be 60 sec. long
great vblog
you know how metal detectors work with a loop soon as metal gets near the loop it changes the frequency going thru the loop so when you put anything near that loop it changes the loops properties. you should use multiple fixed lamps and multiple fixed meters at various parts of the loop then everything stays the same.
I use a neon to check a whip antenna for the right length the neon lamp will be the brightest at the end of the whip at resonance.
73 OM
de n8zu
Please give me piano chord from this video ?
Thanks...
Could have done without the rag time shakey's pizza soundtrack lol
Great show. The loop antennas are my favorite. From his experiment the impedance of the feeding point to the capacitor was among many ohms? See my channel. I loved the dog!
with vidéo I learn nothong at all !
Please doit 100 times more
Three thoughts: First, thanks for making the video. Second, your demos were effective but way too long. Viewers really do not need to see you wave the fluorescent tube over the loop 50 times to get your point. Third, why no voice-over? It certainly would have been more helpful than the on-screen text and that really ghastly music.
cat got your tongue?
Ben Edginton the author of this video is now in his 90's and he has not updated his web sites in quite a while.
Can't you folks speak up about what you're trying to say in this video. Cat got your tongue?
Boring
OK, with all respect, what is wrong with your hands? see a doctor, dude. And, we get it. Showing us the same thing over and over is boring.