I apologize in advance. I tried to take this off reflector but
the message bounced. Please disregard this message unless you
are the intended recipient.
73, Eric N7CL
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Date: Thu, 27 Aug 1998 16:17:01 -0700
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From: Eric Gustafson <n7cl@sparx.mmsi.com>
To: sbest@cushcraft.com
In-reply-to: <9807279042.AA904250614@cushcraft.cushcraft.com>
(sbest@cushcraft.com)
Subject: Re: [TowerTalk] Mismatched Antennas and Tuners - Experimental Results
Reply-to: n7cl@mmsi.com
Steve,
This is off reflector. Apparently the others are tiring of this
discussion. I can't blame them much. I am too. But I am
curious.
Nobody is having any problems with the idea that it isn't a good
thing to have an antenna fed by lossy mismatched line. We all
get it.
The problem is with the incorrect sweeping generalization that an
antenna matched to 50 ohms and fed by coax is _always_ superior
to one fed using mismatched line and a tuner. This statement
simply isn't true. And Pete proposed a question that illustrated
this fact.
Here is the question he framed:
Pete Said:
"To ease my brain, let's consider a specific case. A 1/2 wave
dipole, at its resonant frequency, fed with 600-ohm open wire
line, matched to the transmitter with an appropriate LC circuit
(including balanced-to-unbalanced transformation) at the
transmitter end. Assume that the antenna has a feedpoint
impedance of 50 ohms resistive."
"Compare to the same antenna fed through a balun with 50-ohm coax,
matched to a 50-ohm transmitter. Will this second antenna
"significantly" outperform the first? By how much? Where do the
losses occur?"
And that is the one I analyzed. It is easy to show that IF THE
LINE LOSSES ARE LOW ENOUGH, then relatively high levels of
mismatch can be tolerated without SIGNIFICANT degradation of
system efficiency. In this specific case, it would actually be
very difficult to measure the field strength difference between
the RG213/balun case and the airline/tuner case. In practice it
might go either way. I don't consider such a difference
"significant".
Why do you insist on analyzing the case of a mismatch applied to
a LOSSY line when specifically asked about line that is
essentially lossless at the frequency of interest? Run your
analysis again with the same SWR assumptions but using 0.05
dB/100 ft for the matched line loss and see what reduction you
get.
Of course there are many reasons why a particular antenna system
might require the use of coax for the feedline. And in those
cases one should take all possible measures to be sure that the
load matches the line Zo. But there are also many useful
multiband antenna systems that use open wire feeders and operate
at moderate to high SWR with little or no loss of efficiency.
73, Eric N7CL
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