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[TowerTalk] Effect of feedline on apparent resonance

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Subject: [TowerTalk] Effect of feedline on apparent resonance
From: (Pete Smith)
Date: Sun Mar 9 11:42:18 2003
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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