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Hello Bill,

You have explained your problem quite well.
Presumably the leading edge of the vertical stabiliser on your aircraft will be faired with fabric, fibreglass or timber.
If it will be faired in aluminium, this antenna will not work and you will need to go to an external 1/4 wave whip as used on the majority of light aircraft.
This would also require a metallic "ground plane" to be fitted to the fuselage, but let's not get ahead of ourselves for a moment.

A couple of questions:
1) does the coaxial cable shield connect directly to one half of the dipole and the centre connect directly to the other half?
Note: I am guessing the design has the coaxial cable shield (braid) connected to the copper strip on the top of the fuselage.
2) are there any ferrite cylinders fitted to the coaxial cable near the antenna?


Either way, here are a couple of things to try.
1) do as Ken suggested and adjust both halves of the antenna equally and adjust for a minimum at around the centre frequency of 126MHz.
Does this satisfy your requirements?

If not:
2) does the copper tape have conductive adhesive?
If "YES" run another length of tape beside both of the existing pieces, overlapping the edges slightly.

If "NO" still run another length of tape as above, but you will need to solder the joint along its length.
You can just spot it every inch or so.

Reason: this makes the antenna electrical bandwidth wider which should drop the VSWR at the band edges.
You may have to fiddle the lengths again slightly to keep the minimum centred around 126MHz. (or there abouts, don't stress too much)
BTW, if the tape does not have conductive adhesive, you will need to spot solder any pieces you add to the ends.
NOTE: if you need to buy more copper tape, just buy a roll of wider tape and replace the existing pieces.

Hopefully your problem is solved now.

But if not, try fitting 3 or 4 type 43 ferrite cylinders to the coaxial cable near the antenna connections.
These are commonly sold through electronics stores for EMI (electrical interference) mitigation.
You want them with a hole just big enough for the coaxial cable to fit through and around 25mm (1") long. They will be about 12-14mm in outside diameter.
Ask for help at your friendly electronics store. You want something roughly equivalent to a Fair-rite Type 43 ferrite mix.
You can fix them in place with adhesive to stop them moving around.

Reason: this will act as a Choke Balun to "balance" your (unbalanced) coaxial cable to the balanced feedpoint at the centre of the antenna.

I have deliberately made this a bit wordy to help you understand what you are doing. You can Google any of the key words you don't understand.

Hope this helps.

Cheers...Bob VK2ZRE