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October 3rd 03, 05:38 PM

On 3 Oct 2003 07:54:50 -0700, (Richard) wrote:

What would be the best way to adjust a quarter wave of 75 Ohm twin
line using an antenna analyser? I just connected mine across the
analyser (open circuit) and looked for the freq where a short showed -
is that likely to be accurate? At 10.1 MHZ I ended up with a 15.7 foot
line which seems a bit short (low velocity factor).

73

Richard
G3CWI

Hi Richard,

I believe your data may be correct. Electrical length Lamda at 10.1 MHz is 97.24
feet, and Lambda/4 is 24.35 feet. And 15.7/24.35 = 0.645 for the VF. Although
this VF is in the vicinity of what you'd find in coax, if there's a lot of
dielectric surrounding the conductors, this value could be correct.

Suggest you check the impedance at 20.2 MHz to see if the input impedance goes
to a very high value, as one would expect for an open-circuited half wave. In
addition, if you record the actual resistance values for both measurements and
take the square root of the product you will have the Zo of the line. If it
comes out close to 75 ohms it will prove the measurements are valid.

Hope this helps,

Walt Maxwell, W2DU