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[TowerTalk] Effect of feedline on apparent resonance

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Subject: [TowerTalk] Effect of feedline on apparent resonance
From: (realex)
Date: Mon Mar 10 07:28:10 2003
Hi Pete,
Ive had experience with cables listed as something or the other "type" cable
have found that quite often physical appearance is the only thing the "type"
has in common with the original.  Sounds like your new cable is neither 50
ohm nor
is VF what you think it is.
I realize most of us do not have access to something like a network
analyzer, but if you
can, place a known mismatch on the line...say 2 50 ohm loads in parallel for
25 ohm/ 2:1 mismatch... and determine the frequency where the 25 ohm Z
repeats itself.  Thats your half wavelength frequency and you can determine
VF by calculation.
Also, from your comment below, I presume you mean the frequency with the
best SWR went
up.   SWR has nothing to do with antenna resonance.

73, Bob, W5AH

-----Original Message-----

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 ordered
and received some heavy-duty "RG-8X type" coax and made up the replacement
feedlines.  When I replaced the original feedlines on two of the dipoles
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 a
velocity factor of .72, while that of Belden RG-8X is .8.  This suggests to
me that since the dipoles are somewhat mismatched to the coax, and the
different velocity factor yields a different electrical length than before,
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 less to
what it was before?

73, Pete N4ZR
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