If you have 1:1 SWR at the transmitter, you must have a 50+j0 load at
the other end of a 50 ohm coax, regardless of the line length. Using
a Smith Chart will just show a dot at the center. (Of course, if you
have a very lossy line, it could be a little off 50 ohms at the
load.) This also assumes that you have a reasonable balun at the
antenna, so the outside shield of the coax isn't a coupled part of
the antenna that radiates.
As for the MFJ-259B, it can be a great tool, but it can also give
some pretty crazy readings, especially in the presence of other
nearby transmitters, even when well out of band. I see postings
almost every week on the Topband list about strange feed impedances
of an inverted-L that are probably getting a 100 milliwatt signal
from the local AM station.
73, Terry N6RY
At 03:52 PM 2008-09-17, Bill W5WVO wrote:
I should add that you CAN adjust an antenna for minimum SWR at the
transmitter, but when you do that, you are in all likehood
including some non-zero feedpoint reactance in the net impedance,
and this is being observed through the length of your transmission
line, which then becomes part of the overall load the transmitter
sees. If you subsequently change the length of the transmission
line, you will no longer have Z=50 ohms at the transmitter. If you
tune the antenna for TRUE resonance (zero reactance, R=50), then
you can put any length of transmission line on it that you want
to, and it will behave just the same.
I know we all got along without the MFJ-259B for years, just going
by guess and by gosh (or sweating over Smith charts), but now that
we can actually tell what is happening in an antenna so easily,
it's crazy not to use one. Beg, borrow, steal, or if necessary buy
one, and learn how to use it. You won't regret it! Greatest thing
since CW killed King Spark. :-)
Bill W5WVO
----- Original Message -----
From: "Bill W5WVO" <[EMAIL PROTECTED]>
Sent: Wednesday, September 17, 2008 4:39 PM
Subject: Re: [Elecraft] K3 - Numeric SWR Display
> Actually, looking at transmission line SWR at the transmitter is a
> poor way to adjust an antenna. To get it right, you need to use a
> complex impedance analyzer like the now-ubiquitous MFJ-259B (or
> equivalant instrument), and put it as close to the antenna
> feedpoint as possible. Adjust as close as you can get for Xc=0,
> Xl=0, R=50. You can't do that with an SWR bridge! :-)
>
> Bill W5WVO
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