[Elecraft] VF Varies With Frequency

[Jim Brown]

Hi Mark,

Whoever you talked to needs to go back to school and study Maxwell's 
equations. He is WRONG!

http://k9yc.com/TransLines-LowFreq.pdf
http://k9yc.com/Coax-Stubs.pdf

73, Jim K9YC

On 4/1/2020 2:45 PM, Mpridesti wrote:
> 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 <jim at audiosystemsgroup.com> 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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