Hmmmm.... I'm seeing great variability in the Vf does/does not vary with F.
This link says yes:
https://owenduffy.net/transmissionline/concept/mvf/index.htm
Other's say no...
My "gut" says yes.
Yea! Now I have something to research in order to figure out how/why!
My favorite kind of puzzle! I'm guessing that not only can this be
demonstrated mathematically one way or another, but also experimentally
using say, my VNWA, sig generator, et al.
73,
______________________
Clay Autery, KY5G
(318) 518-1389
On 04/01/20 16:45, Mpridesti via Elecraft 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 <[email protected]> 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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