[TowerTalk] Foam cables and velocity factor
btippett at alum.mit.edu
Fri Nov 11 20:06:50 EST 2005
>Can the true VF be determined from the data I
Vf = actual length / 268.75' (for 1.83 MHz).
268.75' is the free space 1/2 wavelength at
1.83 MHz using the formula 983.56/f * 0.5
for 1/2 wl (983.56 from ARRL Antenna Book).
What you did sounds complicated to me. I
short the far end and tune for minimum R. I
believe you get sharper nulls this way. Also
you can calculate Vf after your first attempt (as
in the example above) and come very close (inches)
to your final length. Let's try it.
>I whacked off a 222 foot chunk AND...
>My first scan showed a nice, sharp VSWR dip at 1643 KHz
Thus Vf = 222 / ([983.56/1.643] * 0.5) = 0.7417 and
therefore 1/2 wl at 1.83 MHz should be 268.75' * 0.7417
= 199' 4"
How close did I come? Check it using your next data point:
>I cut the ten feet and reswept the feedline. The
center frequency moved up precisely 80 KHz to 1723.
222' - 10' = 212' 212/.7417 = 285.83' in free space
or 571.66' for one wavelength. 983.56/571.66 = 1.7205 MHz,
which is close to your 1.723 MHz measurement.
I would have made one cut to 200' and checked
once more before removing the final few inches.
73, Bill W4ZV
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