"Cecil Moore" wrote in message
...

snip

The velocity factor of the twinlead might be as much
as 20% different from the copper tubes. If you use
the same dimensions, the resonant frequency will be
too low. Did you compensate for velocity factor?
Did you check the resonant frequency with something
like an MFJ-259B?

snip

Seems you need to make the adjustments using a
throw-away version of the antenna. Once you determine
the proper dimensions, you can build the final version.

I would start by shaving the insulation off both
stub wires so they can be soldered to the coax and
chopping off 10% of the length of the 1/2WL section.

Yes! All of the above!

Also, it has been my experience that the recommended quarter-inch gap cut in
one of the twinlead conductors [to create the stub section] is insufficient.
I make it at least a half-inch.