[AMPS] Have Cake & Eat it too

[Jon Ogden]

Jonathan Kaplan wrote:

>>Also, you neglected to tell us what "low power at low SWR" means.  Is 
>>"low" 1000 Watts?  Or is "low" 100 Watts??
>>
>In this case, I'm getting readings of around 100w, which is barely more than
>the Ft-840 is putting out by itself.

OK,

Let's try to make some sense to this:

1.) Do you have a dummy load?  If so, try running the amp on the affected 
band into the dummy load.  Does all work fine then?
2.) What SWR do you get on your antenna when you run barefoot?  Is the 
antenna SWR good or bad?
3.) Does this phenomenon happen only on 15m?

4.) When you say you are tuning the amp for low SWR, what does this mean? 
 Now that I think about it:

An SWR meter is typically placed between a transceiver and an antenna 
tuner.  The antenna tuner is then adjusted so that the SWR that the 
transceiver sees is good - typically less than 2:1.  If the untuned SWR 
is 8:1, the tuner allows the transciver to see what appears to be a 50 
Ohm load.  HOWEVER:  THE SWR AFTER THE TUNER AND GOING TO THE ANTENNA IS 
STILL 8:1!  Tuners do nothing to "tune" your antenna.

Now for my question: When you tune your amp, where is your SWR/Power 
meter?  Is it after the amplifier?  Secondly, do you have antenna tuner 
between the amp and the antenna?  If you do not have the tuner, then 
tuning the amp should NOT change the SWR after the amplifier.  If you do 
have a tuner, then you need to tune that to your antenna prior to tuning 
the amp.  Adjusting the controls on the amp should have no effect on SWR 
whatsoever.

If the SWR is fine at low power and high at high power, then there is 
some sort of power saturation problem going on in either your antenna, 
feedline or both.  However, tuning the amp still would not cause your SWR 
to change.

Last thing I can think of:  If your SWR meter is based on relative power 
as some of the Radio Shack meters are, you need to re-adjust your forward 
power setting (on the meter) before reading SWR.  Relative power/SWR 
meters work by always setting the forward power reading to full scale on 
the meter and then switching to reading reflected power.  I am not sure 
if you have this sort of meter or not.

Anyhow, I don't fully understand your problem (as explained), but I have 
tried to think of every contingency that might cover what could be 
happening.

73,

Jon
KE9NA


--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!


Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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