On 5/26/2018 9:10 AM, Roger Graves wrote:
2) The output waveform is close to a sinewave (with no LPF) when driving a 50
Ohm dummy load. However, when driving my antenna, especially on 137 kHz, the
waveform is distorted. On 137 kHz the antenna load is 50 Ohm and close to
resistive (at 137 kHz) as best as I can measure.
Exactly how did you measure it? If, for example, the coax is grounded
at the feedpoint and grounded in the shack, did you measure it with
those connections in place?
I tried inserting a 1:1 balun at the output of the output transformer, which
had no effect.
Define what you mean by "balun." The word is used to define at least ten
very different things.
A LPF cleans up what comes out of the filter nicely. However, the input to the
filter (amp output) still looks distorted in the same way. I am using a T
format LPF which I believe is the correct configuration for the amp output
stage which is (correct me if I am wrong) voltage feed b/c of the center tapped
choke DC connection. The input drive to the amplifier looks like a nice
sinewave in all conditions.
Do you see a problem operating the amp into a T (inductor input and output) LPF with
the distorted output (voltage) waveform? It seems to be running reasonably cool so I
don’t see an overheating problem. I would prefer to see a clean output from the
amplifier and would like to operate with linear modes occasionally so the distorted
waveform leads me worry about IMD (I have not run IMD tests).
A properly operating filter removes the harmonic at the output, but,
depending on the form of the filter, the harmonic may still be present
at the input. As Manfred has noted, a Pi section (or Pi-L) is commonly
used in power amps.
On 5/27/2018 11:58 AM, Manfred Mornhinweg wrote:
I tried inserting a 1:1 balun at the output of the
output transformer, which had no effect.
No surprise. A balun shouldn't be frequency-selective.
WRONG -- again, it depends on what you mean by the word "balun," which
is used to describe at least ten very different things. A properly
designed common mode choke wound on a ferrite core, commonly called a
"current balun," SHOULD be VERY frequency sensitive, in that it should
present a very high resistive impedance at the frequency of interest. To
accomplish that, the number of turns and the core material must be
carefully chosen to put the self resonance at the frequency of
interest. I've recently studied and found suitable designs for the new
480 kHz band, but not for 137 kHz.
The need for resistive choking Z is simple and fundamental, but not as
well known as it should be. A choke whose impedance is primarily
reactive, whether L or C, can cancel the common mode impedance of the
feedline that is of opposite sign. The common mode Z of the line is
simply it's behavior as an antenna, and it's electrical length is that
of the conductor(s) with their outer insulation, typically 0.98 -0.99,
depending their diameter. If the Rs of the choke is small as compared
to Xs, cancellation causes common mode current in increase rather than
decrease.
73, Jim K9YC
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