http://elecraft.85.s1.nabble.com/75ohm-Impedance-question-K3S-and-Antenna-side-tp7659364p7659437.html
equations. He is WRONG!
> Requested technical comment from a long established coaxial cable
> manufacturer on this topic.
>
> This was the response:
>
> The Vp is not measured at any frequency and is independent of
> frequency. The only variable in play is the dielectric constant. The Vp
> represents the speed at which a signal transmits along the cable as a %
> of the speed of air. Air/vacuum will be the fasted medium to use to
> transmit an RF signal. The thing is, air is usually not practical since
> there is nothing to support the center conductor, can be easily crushed
> and is susceptible to the ingress of moisture.
>
> We use foamed dielectrics and expanded tapes to get as close to air
> as possible. Vp will vary based on the specific dielectric used. The
> vast majority of our cables range in velocity of propagation from 76% to
> 86%. For a given cable, the Vp will likely not vary more that +/- 1%
> over it’s length but as I mentioned earlier, it would be best to use
> +/-2% for planning purposes.
>
>
> End response
>
>
> Regards,
>
> Mark, K1RX
>
>
>> On Apr 1, 2020, at 1:43 PM, Jim Brown <
[hidden email]> wrote:
>>
>> 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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