Dan @AI6XG did some experiments with EFHWs. Results on his blog: https://www.ai6xg.com/post/efhw-xfrmr-capacitor and https://www.ai6xg.com/post/end-fed-half-wave-antennas-more-about-the-primary-capacitor
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Yes I observed the same on mine. Without the cap, my antenna is not usable on 10m. It helps to flatten the curve.
Today I went for sota activation and here is my observation :
- no tuner on 20m, easy 1.1 SWR
- Tuner on 40m, no problem to get 1.1 SWR.
- Did not try other bands
Conclusion for 40m :
- maybe I will build a coil and add it in the end with ~ 2m wire extension
- maybe I will just use the tuner
Silly idea I got :
Create a bypassable coil that I put at the base of my 20m EFHW antenna to transform it into a 40m EFHW without adding any lenght. I asked google, it seems like others did it before (Adding a loading coil to a 40m Vertical for 80m operation | M0MCX - Amateur Radio - Ham Radio), it’s not looking that good but I might give it a try.
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From my experience with a 40/20/15/10 m version (20,x m wire) that is extended by a 110µH coil (plus some 1,6m wire) to allow also 80m is that the usable part of the 80m band is very minimal without a tuner. Maybe 50 kHz maximum. Then the SWR is too bad.
So if your tuner adds already enough inductance to also get 40m why mess around with the coil and 2 m extra wire. This will only make things more complicated to deploy and handle. Keep it simple … for SOTA. The chasers have the good antennas to pick you up 
73 Joe
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Yep. Built an 80/60/40/20 vertical on that principal: what coil do I need to add 2m up a 20m vertical to make it resonant on …
Presuming centre loaded is more effective than base loaded, which is what an ATU is effectively doing.
So I have a 20m 1/4 wave vertical on a sota pole with a coil 2m up: 1 coil, 3 taps and bypassed for 20m.
However, a better solution seems to be winding the coil on spaced former that lets you clip on to any turn in the coil so you can tune for the paricular operating conditions you find yourself in. Details on reflector(below)… yet to try it here.
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Oh I see. So yes you are right, I will not bother with it and just keep it like this. I will do some more tests to check if I can tune on the other bands between 20 and 10m.
Because if I have to bring the tuner , maybe a random wire with a non resonant length would be a smarter choice. From the tables, it looks like 16m is an ideal length. It a bit too long for me but if can easily get 1.2 swr on all bands from 40 to 10m … I could consider the trade off.
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Why aren’t u using the 20m? For a huge amount of summit’s 20m wire fits perfectly. And you are resonant on 40-20-15-12-10m without an tuner. For peaks with less space you could use a vertical antenna with coils.
I’am using 20m of course. But if can have 40, why not ? I’m not resonant on 40m without a tuner at all though … Neither on 15 nor 12.
If you are building your balun as I mentioned here (64:1):
I’m pretty sure it will work. And with a little bit of fine adjustment its resonant also on 15 and 12m.
I build like 5 antennas now with different wires and cores and it was working every time.
Deployed as Sloper or as Invereted V. For adjusting I used every time the Sloper setup.
The lentgh is like 19,8m for the wire. Use like 21m of wire and bend it. Not cutting so you have more room to adjust.
Deployed as Sloper. For perfect resonance the Balun needs to be like 1m above the gorund.
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Oh you mean the 20m wire long ? I though you were talking about the 20m band … Well, I dont use it because it is too long. For many summits I guess it could be fine but I think for the summits I’m aiming for, it’s impossible.
But I could give it a try and build a second one since I have like 30m of wire left …
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I know what you mean, the higher mountains like 3000m+ are restricted with space…
You could build a vertical with different loading coils and switch then between the bands…
Or build something like the ATX1080
Becoming 40m resonant without loadingcoils on a EndFed antenna and a short wire idk if it is even possbile. Maybe if u have a tuner that can tune a really high swr.
Of course monoband is not the problem. A lot of windings and the coil is ready 
Check out this dipole:
https://www.qsl.net/g3pto/shortant.html
Saw sometimes some random wire projects with an 4:1 or an 9:1 Balun. It seems that they are working with less wire length. But remember more wire radiates better
Also this OCFD Antenna is 20m overall length…
I’m building something like this for peaks with little space:
Heinz @HB9BCB modified his AX2-Antenna:
73
Julian
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I have a little zm2 tuner and I successfully tune my 9.5m wire on 40m. I didn’t try with shorter wire yet.
The extensible antennas are also an option indeed. I have my eyes on the Komunica HF-PRO-2 Plus T …
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9.5m wire sounds good!
Komunica HF-PRO-2 Plus looks really nice! Adding a counterpoise and you’re good.
Yes, it’s about 8m footprint, which is suitable for most of the summits here.
The komunica is nice but kind of heavy for the genre (425g), especially compared to AX antennas, but it is multiband and very versatile from what I can see. There is 80m band and even 2m band !!! That’s incredible.
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Yes very wide range.
I’m curious how its working on 80m. I can’t imagine that it works well on 80m…
But you can save some money and maybe build something like I do.
I’m also curious about how it will perform
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Yes I will wait for you report on this cheap solution. Anyway, I’m not planning to buy / build this type of antenna before a year or so, since it’s winter and the tricky summits are not very accessible at the moment.
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okay, hope to build it over the x-mas holidays