In order to guarantee the ability to obtain a 1:1 SWR, you need to be
able to adjust at least two independent or partially independent things.
This can be two variable lumped components; any two of the following:
stub length, position, and Z0(*); and so forth. Adjusting, say, a stub
length and the value of a capacitor across the input end of the stub
won't do it because they're basically adjusting the same thing and
therefore aren't independent. If you've only been varying one item,
you'll get a perfect match only if lucky.

If you're as close as 1.7:1, it's likely as easy or easier to find the
solution by trial and error rather than calculation.

The reason that two adjustments are required is that 1:1 SWR requires a
particular value of R and a zero value of X. If you vary only one item,
it'll change only R, or only X, or, more likely, both R and X but in
some fixed relationship.

(*) Because you can practically adjust Z0 over only a relatively narrow
range, this is a good method only for fine tuning or as a way to set the
range of other adjustments.

Roy Lewallen, W7EL

Jimmie D wrote:
"Cecil Moore" wrote in message
...
Jimmie D wrote:
Cecil, I got the antenna up Sunday, for now the stub is straight out from
the antenna. Is it normal for the 50 ohm point to be a little hard to
find? About the best VSWR I can get now is 1.7:1 This is pretty close to
the point where my radio starts limiting power. I m thinking either the
stub or the antenna may be a little long is there an easy way to tell or
is easier just to try something and see what happens? Checking the VSWR
at differerent freqs doesnt help much. Maybe a slight indication that it
is too long.
You've discovered the problem with SWR meters. They
don't read phase. If you were using an antenna analyzer,
the solution would probably be obvious. With only an
SWR meter, your tuning algorithm either converges or
diverges but you may not know which. The easy way to
tune the antenna is to use an antenna analyzer to
determine where the impedance is purely resistive and
then adjust the stub tap point to 50 ohms. An antenna
analyzer is a good investment.
--
73, Cecil http://www.w5dxp.com

I think before I start cutting I will try adding a little series capacitance
or inductance to the antenna and see how it reacts to that. My work week
just started so it may be a week or so before I get back at it again.