On Sun, 17 Jul 2005 13:08:44 -0700, Frank Gilliland
wrote:


4) Here is the weird one. The SWR without the amp on is 1.1, but
when I turn the amp on the SWR goes to 3.x:1 or higher, and I'm
talking about the SWR displayed on a meter AFTER the amp. Not the SWR
on the input side of the amp.

Hmm. This one stumped me, but some other smart fellows suggested the
amp has parasitic oscillations and the frequency (or frequencies) of
the oscillation(s) are outside the bandwidth of the antenna. I can
take another amp with two 3-500z tubes, and put it on the same antenna
with no substantial increase in SWR from the antenna. The Palomar
Elite 300 is only good for about 100 watts. The two 3-500s are good
for 1300 watts. The antenna is rated at 5000 watts.

I'm still working on the fix for this one. Tuned input and output
circuits would probably fix it, but there is not alot of space inside
the amp to work with.


The reason for this problem is obvious -- the output impedance of the
amp is not even close to 50 ohms. It's that simple.


Actually the other guys were correct. I ran an experiment using a
Barker and Williamson low pass filter with a 32 MHz cutoff.

With the set-up below the SWR went high when the amp was turned on.

RADIO===AMP===SWR METER====ANTENNA

With the set-up below the SWR stayed at 1.1:1 with and without the amp
on.

RADIO===AMP===LOW PASS FILTER===SWR METER===ANTENNA

Also the output of the amp showed about 110 watts without the low pass
filter installed, but with the low pass filter installed and the watt
meter connected after the low pass filter the output showed about 75
watts. That means about 30 watts was being transmitted above 30 MHz
even through the fundamental was at 28 MHz.

I doubled the value of the capacitors going form the transistor
collector to ground, and the SWR dropped to 1.5:1 with the amp on and
1.1:1 with the amp off. Then I add 470 pf capacitors from the
transistor base to ground and the SWR with the amp dropped to 1.3:1
with the amp on.

Re-inserting the low pass filter in between the amp and the SWR meter
showed 1.1:1 with and without the amp on, so some harminoc content was
still be transmitted but to a lesser extent.

A lack of working space inside the amp case made it difficult to
install a pi-network on the input and output side of the transistor
finals.

RESOLUTION: The SWR increased was caused by harmonics above the
fundamental frequency, and they were outside the bandwidth of the
antenna.

Lessons learned....

1) Just because your amp shows 100 watts output does not mean that 100
watts is being transmitted on the frequency you are on. It may be
less, and if the amp is poorly designed it could be alot less.

2) If your SWR goes up when you turn an amp on it could be because
your amp has increased the harmonic output of your transmitted in a
non-linear fashion.

3) If installing a low pass filter between your amp and the watt meter
causes the wattage displayed on the watt meter to drop it is probably
because you amp is wasting power on frequencies far above your main
fundamental frequency.


4) If you go to the trouble of re-designing the Palomar Elite 300 amp
and clean up all the dirty output your real output power on the
frequency you are listening to will only be about 70 watts RMS tops,
and even then you are over driving the amp.

...and lastly...

The Palomar Elite 300 is a poorly designed piece of crap. Don't buy
one.