Certainly true that the coax VF is part of the tuned network,
There are two possible issues going on here:
While you may adjust by the ratio of the velocity factor to get the
inside-the-coax tuning back to where it was, you will also at the same
time be changing the length of the shield on the OUTSIDE, which at the
moment is the same as it was. I'm not sure what the shortening of that
might do, but you can bet it won't be constant in its contribution to
overall resonance. It may mean you have don't have to take away as
much, or need to take away more.
You probably need to do it a little bit at a time, to make sure you
don't overshoot and ruin the coax.
Maybe change it 1/3 of the amount indicated by the velocity factors
and see how much it changes. Use that to estimate the final amount.
----- Original Message -----
From: "Pete Smith" <firstname.lastname@example.org>
Sent: Sunday, March 09, 2003 11:36 AM
Subject: [TowerTalk] Effect of feedline on apparent resonance
> Recently, I set about replacing the feedlines on my K3LR-type 80m
> array. Each feeder in the original design is 87 feet of RG-8X. I
> and received some heavy-duty "RG-8X type" coax and made up the
> feedlines. When I replaced the original feedlines on two of the
> that make up the array, I was surprised to see the apparent resonant
> frequency of each of them jump upward ~100 kHz.
> Looking into the matter, I discovered that the "RG-8 type" coax has
> velocity factor of .72, while that of Belden RG-8X is .8. This
> me that since the dipoles are somewhat mismatched to the coax, and
> different velocity factor yields a different electrical length than
> this is transforming the load impedance differently and making the
> resonance appear to have shifted.
> Am I on the right track?
> If I cut the new coax to the same electrical length as before
(.72/.8 x 87
> feet) should I see the apparent resonant frequency return more or
> what it was before?
> 73, Pete N4ZR
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