On Sat, 16 Sep 2006 07:53:03 +1200, armin.ficza@nokia.com wrote:
>I am just wondering, how do you handle the SWR issue with RG6? It is 75
>Ohm cable.
First, the nominal resonant Z of a dipole in free space is much closer to
75 ohms than 50 ohms. As a dipole gets closer to earth, its impedance tends
to get lower, thanks to coupling to the earth. That makes it tend to be
closer match to 50 than 75 ohms. Most practical 160 dipoles are low
dipoles, and thus closer to 50 ohms than 75 ohms.
Second, loss in transmission line due to a mismatch is negligible until the
mismatch gets to be very high -- at least 5:1. See the ARRL Antenna Book
and the ARRL Handbook. The worst case additional loss due to mismatch for a
1.5:1 VSWR (the mismatch of 50 ohm load to a 75 ohm cable) is 0.18 dB, no
matter how long the line or how lossy the cable is.
So, even though 50 ohm cable might be a closer match to your 160 dipole,
and 75 ohm cable is likely to be a better match to your 20/15/10 dipoles
(because they're higher in terms of wavelength), the mismatch either way
doesn't mean much.
The beauty of 75 ohm cable designed for the MATV/CATV industry is that it
is manufactured in FAR higher volume (probably 100x - 1,000x), and thus is
FAR less expensive for equal quality and equivalent low loss.
Jim Brown K9YC
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