[Towertalk] Newbie Question

[Pete Smith]

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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