Jim,
Thanks for this info. Good stuff.
FWIW, the lowest frequency I used was about 2 MHz. The 140 KHz was the
frequency difference (not the absolute frequency) between adjacent
points where the impedance came back around to the same zero reactance
crossing point on the Smith chart. According to the graph in your
paper, the difference between the Vf at 2 MHz and the asymptote would
be about 2%. That was plenty close for my purposes (I just wanted to
make sure the length of the subject reel was close to what was advertised).
73, Mike W4EF..............
On 1/12/2020 12:54 PM, Jim Brown wrote:
On 1/12/2020 11:07 AM, Michael Tope wrote:
The published velocity factor for this type of cable is 0.81 so,
Lp = 3514 ft * 0.81 = 2847 ft
This is very close to what the guy was advertising, so I knew I
wasn't being short changed.
BUT -- VF for ALL transmission lines varies with frequency, and is
typically about 5 percent slower in the range where you measured. The
published value is that to which it converges at VHF, and is what most
TDR instruments measure. The difference will make the line "measure"
that percentage longer than it really is using the technique of
looking for the first quarter-wave null.
See page 8 of http://k9yc.com/Coax-Stubs.pdf for an example of this
measurement made by finding nulls at harmonic frequencies and plotting
VF from those data. This can be done with instruments as simple as an
MFJ-259 -- you turn the dial to find the null(s), then write down the
frequency(ies) and compute VF.
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|