I have a 6 meter beam with "plumbers delight" construction. I have not
been successful in making a 1/2 wave coax balun (T-match) get a proper
impedance match at 50.125 MHz center frequency. The driven element is
0.625" Outside diameter. The T-bars have adjustable shorting straps for
a max length of 14", with 3" spacing below the driven element.

The balun is made of RG-142 Teflon-coax, and is 81 inches long. The
velocity factor is 0.695, so the length is:

1/2 WaveLength (meters) = (150 * V)/F where F is in MHz.

Thus, the half-wave coax would be (150*0.695)/50.125 in meters.
Multiply by 39.37 inches per meter to get 81.8 inches for the coax length.

Any ideas why this did not do the job? I checked the coax for
opens/shorts and moved the T-bar shorting straps to no avail. The best
SWR was no better than about 3:1, even after adjusting the length of the
driven element.

I tested the driven element (both by itself and installed on the beam)
mounted on a temporary mast at about 10 feet to avoid most of the ground
effect.

Thanks,
Steve Bell KJ7OG