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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