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
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