Re: Is RG-59 ok for feedline?

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From: John F. Woods (jfw@jfwhome.funhouse.com)
Date: Fri Sep 29 1995 - 11:22:04 EDT


> When I first became a ham, 20 years ago, we always used RG-59 for dipoles
> and RG-58 for verticals. If I remember right, a straight dipole is
> actually about 72 ohms impedance. Then, as the dipole elements change
> angles, the impedance lowers. A Vertical antenna with radials at 90
> degrees is actually about 34 ohms impedance.

There's two issues. Probably the most important one is that, as you point
out, modern transistor rigs are designed for a fixed 50-ohm load, which implies
50 ohm coax coming out of the rig (though 75-ohm coax is only 1.5:1 SWR!).
Next, the impedance of a dipole *in free space* is 72 ohms; closer than about
4 wavelengths to a ground plane (like the ground) the impedance changes,
starting out around 20 ohms for around 1/8 wave above ground, peaking around
100 ohms at some height I've forgotten, and then oscillating around 72 ohms
with lessening amplitude until you get sufficiently far away. (There's a
diagram of this in the ARRL Handbook.) The usual rule of thumb is that a
dipole at "typical" amateur elevations is actually around 50 ohms, but I
think that's just an excuse to validate calling it 50 ohms; that's the most
height-sensitive portion of the curve, and I don't recall the requisite height
for exactly (or even nearly) 50 ohms being any more obviously typical than
any other impedance (in fact, for 10 meters, I think well above 72 ohms is
not out of the question for many perfectly reasonable installations!).

72 ohm coax feeding a "50" ohm antenna to a "50 ohm" rig might end up
presenting the rig with an embarassingly reactive load; if you can get your
dipole WAY up in the air, 72 ohm coax to a 72 ohm antenna from a "50 ohm"
rig will be a better match than many people achieve using 50 ohm coax and hope.

72, John, WB7EEL


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