>From: "Steve Thomas" <steve@n6st.org>
To: <towertalk@contesting.com>
>Date: Thu, 3 Sep 1998 21:57:01 -0700
>
>Amen Steve! It is also interesting to contemplate what happens on
>the receive side. The 20:1 SWR doesn't do any good for the
>receive signal either.
>
>73, Steve Thomas, N6ST
>
Steve(s),
Nobody has been advocating operating at a 20:1 SWR at the end of
a lossy transmission line. The 20:1 SWR was mentioned by someone
(I forget who) merely to illustrate the point that the SWR in and
of itself is not what causes the losses. It is the loss in the
line.
He offered an example of a broadcast station operating an
antenna at a 20:1 SWR successfully. At 600 KHz (or wherever
they were in the BC band), the losses in the airline they were
using probably amounted to very nearly zero. A reasonable guess
might be on the order of .001 dB/100ft.
So for that particular case, would you rather have your receiver
tied to a tuner that looses 0.5 dB plus the 0.001 dB (we'll
assume 100 ft distance here) airline and a 20:1 mismatched
antenna or to 100 ft of 1dB/100ft coax and the same antenna with
a perfect 1:1 match?
I'll bet that you couldn't tell the difference.
For the above example:
Coax & perfect match: Total system loss = 1.0 dB
Tuner with airline and 20:1 mismatch: Total system loss = 0.51 dB
Most Ham uses of tuner and mismatched antennas don't involve a
mismatch anywhere near that extreme. And while the line
typically has more loss than that cited above, it is usually low
enough that combined with a lower mismatch ratioo than 20:1, it
will not usually make the increased system losses _significant_.
Note that in the above example, the mismatch caused the line to
exhibit losses 10 times higher than would have been the case if
the line were driving a matched load. But this was still not
enough to be _significant_. This is particularly true when
operating 1/2 wave long or longer antennas via open line (and
even with coax most of the time) on the lower 3 bands.
73, Eric N7CL
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