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 <j...@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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