Cecil and w4KAZ:

Thanks guys, for the replies. Yes, I used the antenna analyzer and
played with the driven element length, and the T-bar shorting straps to
get the "best" result at swr=3.

For additional checking, I removed the driven element, and replaced the
T-bars with a 200 ohm resistor. This should give 50 ohms at the
resonance of the 4:1 1/2 wave coax balun. Well, suprise, the coax did
not have the advertised velocity factor, so the resonance (as verified
by an MFJ 259 analyzer) was at 51.45 MHz, instead of the desired 50.125
MHz. This is a minor annoyance, since the SWR at 50.125 would still be
acceptable.

My next experiment: make the T-bars longer, since the driving point Z of
the driven element at the center is only about 20 Ohms. By tapping
farther out on the element, the differential Z would (hopefully)
approach the desired 200 OHms at resonance.

I will keep you posted.

Steve
KJ7OG
------------------------------

Cecil Moore wrote:
Steve B. wrote:

I am using a T-match, with the driven element NOT insulated at the
center, similar to the T-match used on the M-squared yagis for VHF. I
cannot get a match no matter how much I move the shorting straps on
the T-bars.


This is the kind of problem that is relatively easy to solve with an
antenna analyzer.