Here’s a simple problem. How can I fit a 20 1/4λ GP antenna onto a 5m pole?
A 1/4λ needs a radiating element 5m long and 3 or 4 elevated radials each 5m long. Ideally for a 50ohm match you have the radials slope down at 45deg. Under those conditions, the feed point is 5m x sin(45) = 3.5m AGL. 3.5+5 = 8.5m You need an 8.5m long pole. Now you can fit a 20m on a 7m pole by changing the radial angle which changes the impedance and then adjust the lengths to get the match back to 50ohm.
Well I have always used 5m poles since I started SOTA because that’s what I had so I have made antennas that are designed for a 5m pole. From about 2018 onwards I stopped using full size 5m poles (1.2m collapsed) and started using 5m travel poles (57cm collapsed). I do have a 7m pole but I really don’t want to go back to using unwieldy poles.
We can move the feed point to about 1.6m AGL which is convenient for someone my height and I can trim lengths to get a match. But how do I physically support the radiator as it will be 1.6m beyond the pole. If I use wire it wont stand up. I can put a loading coil into the radiator to shorten it but I don’t want to do that as it will be less efficient that a full 1/4λ radiator.
Suggestions on how to solve this mechanical issue please?
A variation on @VK1AD 's idea, a short length of garden cane to slip on the top of the pole then leave the last bit loose and flapping around? Not perfect but might work.
For my 20m 1/4λ GP antenna I have the feed point about 600 mm off the ground and it works - I can make contacts with it although it might not be as efficient as it could be. 600 mm is not much use if you want to put the guys at the feed point, or use the radials as the guys.
I favour simple and dirty: Using the 1.6m high feed point I would terminate the radials with a loop, either loop the wire and solder it or solder on a tag, then use fishing line or tent guy cord from the loop or tag to a suitably placed tent peg - or just tie it to suitable rocks, vegetation etc. (Postscript: you can vary the position of the tent pegs to change the angle of the radials as a tuning aid but why complicate matters?)
What I’m wondering is why you no longer favour your wire version of a Buddipole, my copy works very well indeed, and I based it on a dipole centre T from Moonraker with three banana sockets so I can put two radials as an inverted T.
As you know, I have a lot of successful experience of building and using quarterwave GP antennas for all bands 30m to 6m.
Some things are more/less crucial than others. The length of the radiating element is the most crucial - though there is more leeway if you use thicker core wire. Best to cut it to the exact theoretical length though because then the advantage of thicker core wire will be the wider usable frequency range.
The height of the feedpoint AGL is important. The antenna can be made to work (or be made appear to work) with the feedpoint on the ground, but elevated is better by far. One section of pole above the ground seems fine for the feedpoint for 30m/20m. Obviously if the driven element is hanging from the top of the pole, the feedpoint will be higher on 15m/12m/10m (etc) antennas.
The length of the radials and the angle of them to the vertical seem not so crucial. I cut my radials also to quarterwave and add some guy cord so that they also are entirely above ground. But this does put them more like 70-75 degrees to the vertical as opposed to 45. In my experience, this does not matter.
To your specific conundrum, you need to extend your 5m pole by approximately the length of a trekking pole. I wonder if a small section of plastic drainpipe could sleeve over your walking pole handle and base of pole to achieve this? Then you could fit a proper 20m quarterwave GP antenna on with the radials elevated.
make it an inverted-L antenna and take the excess wire at the top of the mast down one of the guy cords.
As Armin suggests - make the driven element a helical by wrapping it around the mast.
I have taken both of these approaches on my J-Pole 20m vertical and they work. How much difference there is to having a 7 metre mast and keeping everything straight I have not tested, so you may lose out with these approaches but you’ll get on the air and make contacts.
With the wind we always have on GM summits just adopt Ed’s suggestion leaving 3m to blow out horizontally from the top of the pole - you could always add a drogue chute to the end of the wire to assist
The DX Commander commercial vertical folds the end of the vertical element back on itself on 40m. I e-mailed Callum as I was trying to work out if this “linear loading” approach could be used to make a 160m dipole a bit more manageable on a summit. Anyway probably a bit trial and error but I think that if you fold the end back on itself you will need between 2 and 3 times the missing length folded back - so if it was 70cm short you may need 140-210 cm folded back. I’ve not tried this and I have no idea of the impact on efficiency but it would probably be easier to manage than winding helically and more aesthetically pleasing than the inverted L approach, and less ropes and tent pegs to get knotted. 73 Paul
It works and worked well* but on 20m it has a loading coil so it won’t be as good as a full size element and also the feed point is low down. Exactly how much better a 1/4GP with elevated feed point and full size elements I don’t know but the improvement may be worthwhile when you are QRP. If the mechanics of the situation can be solved and made workable on a summit, which we all know is very different to a back yard test, then it should be possible to improve higher band performance. Also it should not be beyond my skill to add links/traps and get a 20/17/15 vertical.
That foldback idea is bringing back memories. I looked it up and it is briefly mentioned in my Radio Communication Handbook. There is also the idea of a folded ground plane antenna. IIRC a folded dipole for 20m would be close to resonance on 15m, so I suspect that the same would apply to a folded ground plane. I might try that sometime using 300 ohm window line for the radiator.
I can see that you are looking for a simple KISS principle design, but you will have to be careful it doesn’t become more complicated in order to get it to work. Won’t adding a glass fibre rod to the top overload the end section of the travel pole? I’d favour adding a section underneath the pole, but that’s more weight and it would need to be in two sections to match the telescoped length of the travel pole. Indeed, that will apply to adding a rod at the top.
For several years I used a 20m / 17m link vertical with links in both the vertical element and the counterpoises / radials. I mounted it on a 7m pole with a feed point about 1.7m and used it mainly in the Trans-Atlantic S2S events, but it was not easy to change bands. I needed something simpler and easier to use. What I’ve ended up with is a pole mounted vertical using a Slidewinder coil and quite frankly I haven’t noticed any difference in performance.
Anyway, what’s this 20m thing?.. I thought you were in love with 30m. You get that too with a Slidewinder.
I have such a thing somewhere. It was made by and given to me by Andre GM3VLB (of IOSA, Scotia and Lothians Radio Society SOTA GM Island award). When I was cleaing out the shack, for the first time in 21 years when I had new windows fitted, I found it and put it somewhere safe. It’s very safe because I cannot find it now
I would add some string to the end of the radiator and extend it sideways, i.e. turn the radiator in an upside-down L-shape. Worked for me with a 20m radiator on a 4 m pole with feed-point abt. 1m AGL.
I would not fly the loose end in the wind. Fluctuations of height above ground would probably affect SWR.
I don’t seem to have such a thing any more. I’ve possibly sold it and forgotten (but not on ebay) or lent it to someone or given it to someone. Damn!
I found some suitable wire for a 20m 1/4wave GP but before making one thought I would do some tests on various antennas just to check they still worked when they haven’t been used for a while. Mainly to test I remembered how to deploy them as I was using a different pole and they all mount slightly differently.
10m and 2m 1/4wave GP. This was the original 2013 12m challenge antenna. Worked fine 1.2 : 1 match. The same radials are used with a different top section, 1.6 : 1 match. Sorted.
The Buddistick clone. 30m 1.4 : 1 sorted. 20m 3.5 : 1 match. No amount of radial length tweaking worked. I checked the loading coil connections many time. I could get 1.3 : 1 match when the coil was shorted out but that’s not right. The trees/bushes where I tested this have grown a lot in the last 12 years and I think it was being detuned. 17m 1.2 : 1 no problem. 15m 1.6 : 1 and touchy which is how I remember it. I changed bands many times to ensure the setups were repeatable. They were. I change the top section and could get 12m 1.5: 1 and 10m 1.2 : 1.
I fetched the KX2 (I’m still learning how it works, I have to think which buttons to press still) and checked I knew how to check it was happy without using the ATU. Yes, worked. I listened an Deutsche Wetterdienst DDK9 on 10.1008MHz and it was not it’s usual crushing signal. All bands sounded a bit flat.
Finally I setup the 40/30/20 EFHW with my AA5TB match, arranged as as inv L with about 4.5m vertical and the rest sloping to about 1m AGL. I was able to get a match trivially easily on 40m, 30m, 20m, 15m, 24m. It would never match 17m but I now have a link at the right place and 17m match was simple with the link open. 28m was never better than 2.1 : 1 so the KX2 ATU fixes that. Listening on it showed DDK9 was 3 S points louder. I always knew the Buddistick was sub-optimal on 30m, it worked and I made QOS but the EFHW has a lot more capture area.
I also played with EFHW counterpoise, it’s a piece of wire 4m long thrown out on the ground. When laying under the sloping wire there was no issue adjusting the AA5TB to get a match. When the counterpoise was 90 to the sloper matches were very touchy. That should tell me something. I should enter this into something like MMANA or better and see what is shows.
A very nice man sent me a couple of the cores Stephan HB9EAJ uses in his 7 band antenna. I need to pull my finger out and make one and the 1/4wave 20m GP (with bendy top). I did intend to make the GP today but I spent 3 hrs playing with these extisting antennas just to re-familiarise myself with them. It’s not an issue with the assorted Inv V dipoles, put them up, set links, antenna is already matched.