[TowerTalk] Advice Wanted-Tuning A 10 Meter Telrex Beam

[Jim Brown]

On 7/2/2014 8:20 AM, Ray, W4BYG wrote:
> Additionally, I would connect the analyzer with a half wave length of coax
> instead of the 4 foot piece.  The half wave length will reflect what the
> actual impedance at the antenna feed is.  The 4 foot piece will only do that
> if/when the antenna impedance is actually at 50 ohms.  Remember to calculate
> the half wave length by the velocity factor of the cable you are using.

NEVER rely on a calculation for the electrical length of coax. Vf varies 
with frequency, and there is also variation as a result of manufacturing 
tolerances.  Start with a piece of line that is at least 10% longer than 
the computed length, connect it to your analyzer with a short at the far 
end, and look for the frequency where you measure a very small R and 
zero X. Now, trim the coax until that zero point is at the desired 
frequency.

You could, of course, make this measurement with an open circuit at the 
far end and look for a maximum value of R and zero X, but depending on 
the analyzer, the short may be more sensitive. You could also look for a 
short at half the operating frequency, but this would be subject to the 
variation in Vf with frequency.

Vf of any coax starts out much lower than the published value at audio 
frequencies, then rises to a value equal to the published value around 
50 MHz. In the range of 160M - 80M, the value is typically 1-2% less 
than the published value.  For more on this, see 
http://k9yc.com/Coax-Stubs.pdf

If you have an network analyzer or complex impedance analyzer that does 
swept Z measurements and can write a Touchstone file, you can export 
that file to Zplots and let Zplots compute and plot Zo and Vf vs 
frequency for your cable.  Zplots is an Excel spreadsheet developed by 
AC6LA. It's a free download. See ac6la.com.

73, Jim K9YC