Jim,
Lots of possible issues on this one.
At 10m, many of the HF type characteristics begin to break down and you
are nearly into the VHF region anyhow. Line lengths, etc can become more
important.
Hams have a real misconception about SWR. It stands for Voltage Standing
Wave Ratio. You probably new that, but what does it mean? Well,
basically, the forward and reflected voltages set up a "standing wave"
that if you could look at it would look like it is standing still.
To best illustrate this, take a piece of string, tie it to a chair or
some other object. Give yourself about 5 to 10 feet of string. Stretch
it somewhat taught and give it a good single flip of the rist. You'll
see a wave pulse propagate down the string, hit the terminator (chair,
fence post, whatever) and then come back to you. Now keep moving your
end of the string up and down at a steady pace. Notice what happens.
You are forming a standing wave that actually looks stationary. You will
notice peaks and nulls on that wave. Depeding on the frequency of your
pulses, the number of peaks and nulls will be different. Also, with a
longer or shorter piece of string relative to your frequency, things will
change also.
Now take and apply this to RF. Your VSWR is actually not constant along
the coax but will have peaks and nulls all along the coax.
Also take this into account: Your Bird coupler is going to have some
VSWR to it (nothing is perfectly 1:1), depending on the match that your
antenna tuner gives, the input to it may not be quite 50 Ohms either but
may have a significant reactive component. This would cause your meter
on the amp to possibly display something other than a perfect match.
Lastly, many hams think that if they tune their tuner, that their antenna
is now a 50 Ohm antenna. WRONG! The only thing a tuner does is to
essentially act as a buffer between your exciter and your load. It will
not improve a bad antenna. If the antenna has a 15:1 VSWR without the
tuner, it still has it with the tuner. You just don't see it.
Given all this, if you reflect 50 watts while running 2KW out of the amp,
that's not much. That's a mere 2.5% of your power. 2.5% reflected power
is about a 16 dB return loss or an SWR of about 1.38:1. Not really much
to worry about. Your tuner probably has worse than a 16 dB input match
when tuned. In industry anything better than 15 dB return loss is
generally considered a 50 Ohm load!
Also, double check your MFJ-259B manual to make sure it accurately reads
highly reactive loads such as your antenna. The original 259 was not
acurate with highly reactive loads. Hopefully they fixed that.
Be careful though as Dan said about damaging your 259 through that
switch. If the switch has 30 dB of isolation (generally that's a pretty
decent number) with 2000 Watts into it you could have problems. 2000
Watts is 63 dBm. Take away the 30 dB of isolation and the power hitting
the MFJ-259B is 33 dBm. That's 2 watts! If your switch only has 20 dB
of isolation, you would be dumping 20 Watts of power into your toy! BE
CAREFUL!
Anyhow, I wouldn't worry about your variances in SWR readings. It's
really just to give you an idea of what kind of power is coming back at
your rig. Personally, I'd match it so that the PA sees the best possible
match. This means that your tuner and whatever you have on the output of
your PA are optimally matched. Who cares what the SWR meter in the tuner
sees. The output of the tuner is going to have a high VSWR anyhow!
Matching this way will cause you to put the maximum available power into
the tuner instead of reflecting 2.5% of it back to the amp.
SWR is a fun topic, ain't it?
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
"A life lived in fear is a life half lived."
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