Bill Janssen wrote:
Joeseph wrote:
This is a very screwy problem. First lets get the set-up of my
radio equipment;
The amp puts out about 100 watts on HF.
RADIO-----2 TRANSISTOR AMP-----SWR METER------ ANTENNA
Note the SWR meter comes AFTER the 2 transistor amp.
Ok, when I check the SWR on my antenna it shows 1.1. When I turn the
amp on it shows over 3.x SWR. The antenna is rated at 5 KW. The amp
only puts out about 100 watts.
Hmmm.....Ok lets see what happens when I use my 1 KW oil filled dummy
load.
RADIO----- 2 TRANSISTOR AMP----SWR METER-------1 KW DUMMY LOAD
Same thing. The SWR is about 1.5 in to the dummy load, but as soon as
I turn the amp on the SWR meter shows 3.x plus.
Ok, Maybe my SWR meter is screwy, so lets use my Dentron MT3000
tuner with a built in 200 watt dry dummy load and built in SWR meter.
RADIO ----2 TRANSISTOR AMP----DENTRON MT3000
Same thing. The SWR is low without the amp on and goes high when the
amp gets turned on.
Ok, Lets see what happens when I use my Drake L-4B amp instead of the
2 transistor amp.
RADIO-----Drake L-4B-----SWR METER-------Dummy load
Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.
The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.
I've duplicated this scenario with three different SWR meters, and
different patch cables. Lets me say for the record the SWR meter
comes after the amp, so I'm not looking at a high SWR on the input
side of the amp. This is on the output side of the amp.
?????
Check the frequency (or frequencies) coming out of the amplifier. I will
bet
that what comes out is not equal to what goes in. The antenna is not a
good match for those other frequencies.
Bill k7NOM
What Bill is trying to tell you is that your amplifier is probably
oscillating or putting out way bad harmonics. That certainly fits the
symptoms with the antenna attached; it's a bit surprising with your
dummy load unless the dummy load is only good to two meters (chances are
your oscillation is higher than 2M, and harmonics certainly would be).
You could also see odd results if the SWR meters aren't good beyond 2M,
but I don't know which direction they tend to go with way high frequencies.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com