Weird homebrew antenna issue

[K2BRZ]

Hi All,
I have a kinda weird situation with a homebrew 20m QRP antenna I'm making. I call it the (j)A(nk)X2 because it's a janky DIY copy of the AX2. Instead of using a toroid in the windings, I'm using an air coil. The first prototype I built works fine, however it's a little bit "bulky" as it's made from an old medicine bottle. For version 2, I found a smaller bottle and used the same website calculator (M0UKD) I used for v1 and for my 40m loading coil for my 5.6m whip. As I said, v1 works fine and I've made a contact or 2 with it. For v2, however, it seems to be resonant in the 10-11 MHz range, nowhere near 14MHz.

When I rechecked my work, I noticed I had an extra winding on it, so I removed that but nothing changed. The only way to get it resonant on 20m is to collapse the whip (39"/.99m) almost completely down. I've tested both versions with the same tripod mount, same counterpoise, and same calibration on my nano-vna but I'm at a loss of where to check next.

Any ideas?
Brian, K2BRZ

[KL7MJ]

I have a $20 telescoping portable antenna (based on a thread on QRZ). It consists of a 17 foot vertical and four 18 foot radials.
A 17 foot (5.8m) vertical is resonant around 13MHz without any coil. Try bypassing (shorting out) the coil and see how that works.

73,
Mike, KL7MJ

[K2BRZ]

Thanks for the response, but I can't access the ends of the coil as they are inside the base.

20251022_133306v2.jpg (71.87 KiB) Viewed 32798 times

The first version was made the same way, but with a bigger bottle and that one works fine. I'm not quite sure of where the issue is.

[K2BRZ]

Quick update, I found another calculator that said I had too many windings, so I removed a bunch and now am quasi-resonant for 20m, resonant with a tuner but that is to be expected with these antennas.

[KL7MJ]

Take a look at some of the designs on QRPGuys.com and QRPBuilder.com. They have designs that use easy to obtain parts and great documentation. You can either buy one of their antenna kits (DS1 at QRPGuys and QX1 at QRPBuilder) or look at their instructions and build your own.