On Mon, 25 Jul 2005 13:08:08 -0400, 800 Whiskeys
wrote in :

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.


No, the other guys were not correct, and you basically prove that fact
for yourself below.....


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.


No, that does -not- mean 30 watts were being lost to harmonics. What
you fail to realize is that a low pass filter is a pretty good
impedance matching device -regardless- of harmonics. You can shove
anywhere from 5 to 500 ohms into one end and the output will be pretty
close to 50 ohms (depending on the quality of the filter). Filters are
frequently used as a kludge just for that reason. Your 30 watts are
just being dissipated as heat or reflected back to the amp (since the
filter really wants a 50 ohm input). Put your SWR meter between the
amp and filter and you'll see what I mean.


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.


Which is basically proof that the problem is the output impedance of
the amp, not harmonics.


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.


If you want to measure the harmonic component of your output you need
to use a spectrum analyzer. There's really no other way.


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.


No need for a pi network -- just add a large variable cap across the
input and/or the output of the transformer. Quit thinking harmonics
and learn something about impedance.


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


That would be a 'conclusion', not a 'resolution'; either way it's
wrong.

Another point to consider is that if the amp -was- oscillating it
wouldn't permit -ANY- of the fundamental to be passed or amplified
because an oscillator is really a bandpass filter that works -too-
well. Your problem is not oscillation.

If you are getting harmonics then they are coming from one of two
places:

1) the exciter, in which case your filter should be between the
exciter and the amp. Actually, you need to fix the exciter because
something is probably broke; or

2) the amp, the cause being non-linearity of the amplification stage.
This is usually caused by incorrect bias, driving the transistors into
clipping, or a very poor power supply. IOW, something is seriously
wrong with the amp.

In either case, the use of a low pass filter is (as I stated before)
nothing more than a kludge. It masks problem but doesn't fix it. And
it also masks poor impedance matching (whether the matching is on the
inside or outside of the amp, or both). So a filter is -not- any sort
of definitive test for harmonic content.


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.


There's only one lesson to be learned he Don't allow yourself to be
educated in RF electronics by CB amp voodoo-techs.







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