[Elecraft] VF Varies With Frequency

[Jim Brown]

On 4/1/2020 7:58 AM, Don Wilhelm wrote:
> When dealing with a length of transmission line, the use of the '468' 
> factor should not be used - compute the actual wavelength and then apply 
> the velocity factor.

There is yet another variable -- VF varies with frequency. At low 
frequencies, it is lower (slower), increasing until it converges to the 
published value at VHF. For this reason, matching sections and stubs 
must be measured at or near the operating frequency with an analyzer or 
as a stub placed in line with a generator and receiver. They should be 
cut long, then trimmed so that the null in that generator/receiver 
circuit is heard, or the analzyer reads a short or open.

How much is this variation? For typical transmission lines, it's on the 
order of 1% from 80M, a bit more for 160M as compared to the published 
value. If what you're building is a stub to kill harmonics, it's the 
difference between the CW and phone bands on 80M.

73, Jim K9YC