[Elecraft] K3 - Numeric SWR Display

[Terry Conboy]

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" <w5wvo at cybermesa.net>
>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