G4NSJ - Flowerpot antenna how it works and construction
Vložit
- čas přidán 3. 07. 2023
- G4NSJ - Flowerpot antennas how they work and construction. In this video, I explain how a flowerpot antenna works. I also explain how to construct one for the air band.
Link to the article...
gm3sek.files.wordpress.com/20...
Dimension calculator...
nomonsuhendar.blogspot.com/20...
www.g4nsj.co.uk - Věda a technologie
Servisol 10 in the background and the video title reminds me of many decades ago when I was on the bench a TV came in for repair, complete with a fully flowering pot plant on top with a wire coming down from it poked into the socket. Customer says that's the TV's proper aerial. I can't remember at all what the repair was but the customer assured me his TV worked fine like that in his home, maybe because he lived just south of Bedford near transmitter.
Ray, like you I have a lot of respect for the late G3TXQ and his analysis and don't disagree with any of it. The flowerpot antenna as described IS a centre fed halfwave dipole precisely as you said. However you could argue too that it is a quarterwave vertical with a single leg groundplane. It is definitely NOT an endfed. I guess pedantically you could argue that it is closer to a 75ohm antenna than a 50 ohm one but it is close enough. For newcomers the concept of standard coaxial cable being effectively 3 separate conductors at RF is a difficult one to grasp however it is correct.
For those that are in doubt about the "secret world" inside the cable I would suggest a VNA test of through loss with maybe a metre of RG58. Take a sweep, save the plot then wind maybe 10 turns of the cable around a ferrite and try again, nothing changes. The changes are only in common mode impedance which does not impact the "secret world" inside the cable.
Hope that made sense... might make a useful video 73
That makes very good sense, thanks for all your thoughts. It's not easy for newcomers to grasp but, hopefully, the video will help. Cheers for now, Ray.
Hi Ray. Thanks for making your video easy to understand and enjoyable to watch, it’s probably one of the best I’ve watched for a long time as you covered the making of the air band antenna in easy to understand stages. I always found the technical side of radios and antennas difficult to digest but you explain things more simply. All the best. 👍🏻
Hi Dave, I'm pleased that the video has been useful to you. Thanks for your kind comments. Cheers, Ray.
Beautiful explanation of how the RF choke works Ray, i never completely understood it untill this video. Great work mate and i really enjoy the way you present your videos.
Thanks. That’s very nice of you. All the best, Ray.
I thought this video was about receiving episodes of Bill and Ben! 😮😂
😂🤣
Hi Ray, firstly a big thank you for ALL your presentations in particular this type of vertical dipole. I have a large amount of low loss satellite 75 ohm coaxial cable so decided to build a VHF 2 M version using the online calculator. To cut a very long story short and having used an awful lot of electrical tape in testing I can confirm that the online calculator is perfectly fine for the top and bottom measurements of the coax but the length for the RF choke has to be considerably longer at 132 cms which gives around a 1.2:1 SWR across the entire 2 M band. I'm awaiting a VNA analyser in the next week or two so hopefully will be able to give more information on its bandwidth. Cheers and 73's
Hi, thanks for all the information, that’s brilliant! Looking forward to the VNA results. Do keep me posted. All the best, Ray.
Thank you for another great video. This is helps to put all sorts of "bits and pieces" together in one place where they sum up to common sense.
Thanks, glad you found the video useful.
Great vid, especially enjoyed the root in the flowerpot pun. Very clearly explained, makes perfect sense, thank you!
Thanks!
Thanks for the video Ray! That flower pot antenna reminds me of a "no ground plane needed" marine antenna. Hope to try one someday. Thanks very much.
Yes, I've heard of the marine antenna. They work well, apparently.
@@g4nsj take care, have a good one!
Great informative video Ray.Keep them coming :)
Ray, you have a such a wonderful attitude about learning - from someone who struggles with a lot of these concepts, thank you! I agree with some of the prior comments, you are a great teacher. I'm planning to build one of these for 2m outdoor base station use. Sadly, the inside of my roof sheathing is lined with aluminum (UGH) making attic antennas impossible. This should be easy to make, and pretty easy to hide. Thanks again, I've subscribed!
Thanks! I'm pleased that you find the videos useful. OK about your loft... that's unfortunate! However, the aerial will work even better outside. Cheers, Ray.
Great video Ray. I explained skin effect to my girlfriend yesterday regarding the very unwieldy Hyperflex 13mm coax that we have just bought for UHF use. Anyway, in the first couple of minutes of this video my girlfriend said that you were talking about skin effect regarding that antenna. Then you said that very thing! I must be doing something right then. Also i have bought that same frequency counter after watching your last video about it. Works really well for the money and the tone decoding is a fantastic feature. Best wishes, David.
You're definitely doing something right, David! The frequency counter is great. Cheers, Ray.
👍Good video, thanks. I have made a few of these for 2m ham band and they certainly work. You can make them without the conduit so they can be coiled up for portable use - hang it from a pole, tree or window frame, etc. I would describe it as Sleeve Dipole. The ones I have made have the upper element slightly longer than lower element. So, I would say they are (slightly) Off Center Fed (OCF) dipoles. Very useful when you want a vertical antenna without ground plane or radials.
I think the velocity factor would be slightly different between just the inner and the outer, though slightly off-centre-ing it might be the best idea for a 50 ohm match. I haven't experimented though, I'm just guessing and going on what others have said. I've seen two different designs, one where the top half is longer, one where the bottom half is longer. Probably doesn't matter which!
Hi Ray, thanks for an interesting and informative video. I remember back in my previous Sydney home, some of my FM radios received harmonics of Aircraft transmissions on the FM band, as we were not that far from the airport. All that was possible even just on an inbuilt whip antenna. Anyway, all the very best. Rob in Melbourne Australia.
Interesting, thanks. 👍
I built one fer the chicken band using 75ohm coax. Its flat across the band with swr less than 1.5/1 works great.
Super video, as usual!
Thanks!
Thanks Ray. I understand more after your explanation. 73
Thanks John.
Ray, Thanks so much for making these videos. I've watched your original flowerpot antenna build video several times. I finally made one a couple of weeks ago and have been having lots of fun chatting on repeaters. I made mine for 2m, but have also used it on 70cm and it works pretty well. It's in my attic beneath concrete roof tiles, and I've been amazed at how well it works. I just used the calculator you linked and hoped for the best.
A couple of questions have been troubling me. I wasn't sure what to enter into the calculator for "working frequency", so just shot down the middle with 145MHz. I'm confused as to whether that was the correct approach or not. Also, a more experienced ham has kindly lent me a Nano VNA, and I've since found that the SWR is showing as about 1.6 or so on 2m and 1.8ish on 70cm. I'm a bit clueless with the "trimming" aspect of the build, in that I can fairly easily snip a bit off the top, but altering the length of the lower element seems that it would be a bit vague. I also wonder how much the terrain (junk) in my attic affects the SWR. I guess I don't want to make it worse as the antenna currently works pretty well. Any pointers would be of great help, but either way thanks for the videos. They are perfect! All the best :)
Yes, the working frequency needs to be the middle of the band. As for SWR, you can add or remove one turn of the coil if the SWR is too far out. That might help. Adjusting the top length should bring the SWR to within reasonable limits. Junk in the attic will affect the SWR. I moved my aerial around in the attic, keeping it clear of stuff, for the best results. Glad you like the videos. Cheers, Ray.
Two things to look at with the VNA, the actual resonant frequency (the frequency of lowest SWR) and then the actual VSWR itself. If the "dip" is at 145MHz then your length overall is right - but the impedance at the feed point may be a bit out. It may be possible to try creating a new antenna with the feed point moved a little bit up or down to see if the SWR rises or falls. One design uses 457mm on the top, 447mm on the bottom, but this may be optimised for 146MHz, but you can see the thing is slightly a-symmetric. Pete Waters, of Waters and Stanton has another design in one of his videos where the dimensions are different again. Can't remember what Tim, G5TM has done but you could look for his videos - and he's commented on this video as well.
Hi mate, i'm in the outback of australiand I have been a radio ham for weeks i have watched all your videos Please keep them up and you can waffle as long as you want thanks mate
Thanks! It's great to hear from you. More videos on the way. Cheers, Ray.
@@g4nsj I am going to make my first contact tomorrow. I am really looking forward to it. I feel like the first time I lost my virginity.
@@mattmiegel Excellent!
@@mattmiegel How many times did you lose it? ;-)
Great presentation Ray; not sure what career path you chose in life, but you would've made an excellent teacher. My knowledge of coax is limited to fitting PL259 connectors to it during my CB radio days (early 1980s!). I had no idea the braid functioned as two different conductors; very interesting 👍
Coax is interesting stuff. I don't know about being a teacher. I think the kids would have driven me mad! Cheers, Ray.
Such a simple antenna. Great for portable too when made for 2m IMHO it’s electrically a half wave dipole but physically end fed. 73 Ray I’ll work you some day from Yapton.
Yes, I look forward to a contact as you're only up the road from me. Cheers, Ray.
"Flowerpot" Reminds me of the old 'Portable Ground' joke.
I remember that!
very interesting!
Thanks!
Thanks
Thank you, thank you, thank you!
Today you explained a chole brilliantly.
@@jamess1787 thanks!
Thank you for this, Ray. Explained very clearly and well. I will need to make a few of these as I'm a big fan of dipoles. And some lovely shirts too. Have a fantastic day, Sir. And I look forward to future videos.
Thanks!
Brilliant as always one thing to note the black plastic and fishing poles contain graphine to colour the plastic which in turn is conductive so if you made the same antenna in white pvc you'd see a difference on am analyser . I made that mistake. Be great to demo a duel band J-pole build. 73
de M0JFE
Good point. What I do it place a sample of the material in the microwave. If it gets hot, it's no good!
@@g4nsj you got it 👍 remember to put a bit of water in a mug as well with it .
@@johnm0jfe Yes, I did that. 👍
Excellent description Ray and another great video.
73 from Hilversum.
G7HFS/PA3IKH.
Thanks, Ian. Hope all is well with you. Cheers, Ray.
@@g4nsj All good here thanks Ray.Back to the UK on 11th but then back here on 24th.Its all going well thanks and I hope all is good with you.When I'm back hopefully we can catch up on 2mtrs/70cms from Beachy Head when I'm up there.👍
@@ianharling9569 Look forward to chatting, Ian.
It's a halfwave vertical dipole, except it relies on the so-called "choke" as opposed to a traditional coaxial dipole which uses anther sleeve over the bottom half.
The conceptual issue with this 'design' (sic) is that it's trying to use a 'choke' at a current null (voltage peak), which is just daft.
Chokes, being magnetic, choke-off current. They're inherently ineffective at a current null / voltage peak.
Of course it'll still "work", but there will be currents on the outside of feedline, even past the so-called choke.
Every Ham should know all of this...
You're right, hams should know this.
Mmm…ham.
Yes baluns really belong at the high current end of the radiating element where they are most effective. However I suspect some clever impedance inversion may be going on at the section of quarterwave (labelled lower radiator) in between the terminated braid and coiled up coax line.
@@jagmarc "...clever impedance inversion ..." Nothing mysterious about it; impedance inversion (
@@JxH Those wire ended neon bulbs light up nicely on the hi z end of radiating element. A graphical demonstration of impedance transmogulation or whatever is called.
They work great made loads for 11 meter
Excellent!
Skin affect is a real pain. I built an EMC chamber to work up to 20GHz, keeping the RF where you wanted it was painful. if the RF had a chance to swap from an inner skin to an out skin at a joint it would. lots of copper tape and tin foil saved the day more than once.
Excellent! It's nice to hear from you. Cheers, Ray.
Well explained Ray, that makes sense.
I remember the original video and making the aerial the next day, since then made a few for friends that like listening to Airband 😊
The T2LT coax antenna must work on a similar principle as it’s almost identical? They seem to be popular on 11m for mobile static dx, really simple but effective and rolls down to nothing.
73 Franco
Hi Franco, I'm not sure about the T2LT coax antenna. I'll look into it. Glad you liked the video. Cheers, Ray.
T2LT is just another name for this antenna.
100% facts!
Thanks!
Is this based around 25mm dia. conduit ? Obviously the dia. will affect the length of coax for the choke, I've looked at the calcualtor you linked but cannot see any ref. to the tube dia. Cheers
I used 20mm conduit.
Hi Ray, i have been told you shouldn't use Black plastic tubing for aerials, you should use white, because the black has got something in it that effexts RF, ideally though fibred glass tube is best...that's just what i have been told Ray...take it or leave it hi...from Fred in Essex g4vvq.
Hi Fred, nice to hear from you. You are quite right, some pipe has carbon or something in it. I stick a piece of pipe in the microwave to see if it gets hot. If it does, then it has something in it that RF doesn't like. Cheers for now, Ray.
@@g4nsj Ok Ray sounds like a good idea,,,73 Fred.
Might be a useful effect, to widen bandwidth.
@@jagmarc indeed.
@@g4nsj Simple quick way to test effect black plastic, run a white plastic one then look for VSWR change with another black tube taped to it.
Just a quick question, whats the best Coax cable to make this `Flower Pot` antenna?
RG58 will be fine
Ray, is the coil just functioning as a common mode choke?
Yes, it chokes off the RF at the base of the aerial.
Hi, when you say the inner is the top half, and the outer is the bottom half - how can you have a top and bottom half when you’ve cut the braiding off ? You just have a centre core going up the conduitI. I’ve seen other videos where the centre core was 23 inches long, and the braiding was also 23 inches long.
The braiding was separated from around the centre core ( not cut off ) and twisted and the antenna ended up like a T shape, with the upright of the ‘T’ being the coax going back to the receiver.
So a hole would have been made half way up the conduit, and the centre core fed up one way, and the twisted braiding fed down the other way.
Like your relaxed style of presenting, but cannot get how you describe top and bottom when you only use one whole centre core.
The braid still acts as the bottom half, down to the choke, even though it’s cut off. The antenna is a centre fed dipole, the top half is the inner and the bottom half, down to the choke, is the outer. There’s a very interesting article here…. gm3sek.com/wp-content/uploads/2019/01/G3TXQ-RC.pdf
A wire running downwards for a quarterwave alongside the outer braid is tightly coupled to the braid anyway so it might as well be the same braid. The choke gives it something to push against and create a hot end node. Ferrite works well as an end stop for HF bands. Above that even with speciality ferrites they are too quirky and unferrited coiled coax is more predictable.
Edit though having said that, as another youtuber pointed out in another comment thread this video, offering an impedance at a current null seems daft because the higher source impedance at current minimum (highest voltage) would go straight past it as though it's not there. I suspect the key to it is the magic quarterwave thingy.
@@jagmarc thanks! 👍
Great video, the only bit i dont get is the two x 54cm pieces of coax, how are these joined together
Glad you like the video. The two pioeces aren't joined together. The inner is the top half and the outer is the bottom half.
Im still not getting it lol
@@AndrewCollins23770 HI Andrew, when you say, joined together, what do you mean? The inner comes out and is the top section. It’s not joined to anything. Can you draw a diagram for me? Cheers, Ray.
Hello Ray, my, name is Rob, I wonder if you can help me out. I have watched a couple of your videos and you seem an amicable chap, I live at the foot of the south downs, and I recently bought a tecsun pl 990x s.w. radio.I know relatively nothing about radio, although I am eager to learn. In the meantime can you recommend a indoor/balcony aerial to get me going, would be most appreciative, all the best Rob steer.
Hi Rob,
The best aerial for short wave reception will be a length of wire strung out in the open. If you only have a balcony, run the wire around the balcony or, if possible, drop a wire over the balcony so it hangs down vertically. That will give you some short wave reception. I hope this helps. All the best,
Ray.
@@raysrants Tp
Throw a bit of wire over the balcony, hmm, OK, thanks. RS.
@@christophersteer505 Ideally you want 50 feet or more of wire but, as you're restricted, there's not a great deal you can do.
Not happy regarding the length of coax for the choke. The inductance of the choke is proportional to the square of the number of turns and the number of turns depends upon the diameter of the conduit.
Hmm, good point. I'll read Steve's article again. Thanks,. Ray.
Is this antenna safe from lightning? Compare to other antennas?
If lightning is nearby, nothing is safe!
I`ve heard people say avoid the cheap basic RG58
It’s fine for this type of antenna.
@@g4nsj Thanks! great video & great information
@@celtus2004 thanks!
i have one for the 40 metre band! the choke is wound on a toroidal ferrite core.
Excellent!
Do you have it positioned horizontally or vertically? 20m sticking straight up would be something!
@@paulsengupta971 horizontal at about 4 metres up.
Thanks for the information 👍🎛
108-138Mhz but what about a Slim-Jim for the Aviation Band.
Yes, that would work well.
They are both half wave antennas, the difference being that the Slim Jim/J-pole is end fed with a matching section, and the flowerpot is centre fed. They should give approximately the same performance.
A useful little video Ray. I must build one and see how it performs. 73 de GI8WFA.
Let me know how you get on. 👍
"Promosm" 🎊
Hi, what is Promosm ? Sounds interesting.
RF electrickery
😆 de G7JDZ