At 11:16 AM 7/18/02 -0700, Troy Flowers wrote:
>Ok I'm a relative newbie so I'll ask also. I ran 1/2" Heliax up my tower
>for HF. But 75 ohm CATV line can be used also? Being a cheapskate I'd much
>rather spend nothing so explain. My radio/amp wants 50 ohm but the feed
>line is 75 ohm and the antenna is some range also based upon freq. Are you
>supposed to do a complex analysis of this or is it safe to take a simplistic
>view of it (in terms of station performance)?
Well, here's what I did. RG-213 from the antennas to the antenna
switch. At the antenna switch (at the bottom of the tower) I switched to
75-ohm CATV, using a multiple of a half wavelength at the lowest frequency
of interest, measured at the highest frequency of interest, using an
antenna analyzer. A pigtail of RG-11 at the tower bottom was used to make
trimming easier. At the house end, where the prescribed length of CATV
hardline ended, I connected 50-ohm cable to go the rest of the way into the
shack.
I was warned that the 1/2-wavelength rule is good theory but wouldn't hold
up well at higher frequencies, particularly 10 meters. It is true that
there are all sorts of minor impedance transformations going on along the
transmission line, which produce some odd results (for example the 20m
resonance moves up 200 kHz as measured in the shack compared to at the
bottom of the tower), the bottom line(s) are just fine, as follows:
10m 28.0 - 1.6, 28.8 - 1.5
15m 21.0 - 1.0 21.4 - 1.5
20m 14.0 - 1.4 14.3 - 1.2
40m 7.070 - 1.0, swr bandwidth (< 2:1) 150 kHz
80m 3.520 - 1.0, swr bandwidth 70 kHz
160m 1.830 - 1.0
Feedline loss measures right on the predicted value, as well as I can tell
with my ham-level instrumentation.
I don't think I was just lucky, but maybe I was. Comments?
73, Pete N4ZR
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