Hello Jim,
It was nice to hear from you. I have really enjoyed and utilized your “A Ham's
Guide to RFI, Ferrites, Baluns, and Audio Interfacing”. It is a great resource.
Thanks for making that available.
Re. your questions.
In my case the 137 kHz antenna base loading coil is outside but the wire from
the bottom of the coil (and also a wire to the ground system) passes thru the
shack wall to a transmission line toroid stepup transformer. I was measuring
the waveform with an oscilloscope probe connected to the transformer input that
the 6 ft coax from the amplifier connects to. Alternatively, a dummy load was
connected to that coax and the probe was on the dummy load. In the latter case
the coax was not connected to the antenna ground - I see your point, that could
make a difference.
Re. the “balun”. I wound 5T bifilar around a toroid. I am not sure what the
composition was, but that gave 290 uH inductance for 250 Ohm X on 137 kHz. It
showed a nice 1:1 SWR when a 50 Ohm resistor was connected across the output
(the end of the bifilar transmission line). The same type of transformer
“balun” is used at the input of my amplifier to go from the unbalanced coax
input to the balanced mosfet gate circuit. My reasoning for trying the balun
was twofold: first some mosfet PP amp circuits that I have seen incorporate
such a balun after the output transformer; second the distorted waveform that I
was seeing was asymmetrical which made me think that the balanced mosfet output
was being unbalanced by connecting the unbalanced coax to the output
transformer secondary.
My previous post reported that the use of a Pi form LPF cured the distortion
problem. Both the input and the output of the LPF are now nice looking
sinewaves on the scope.
73,
Roger
> On Jun 2, 2018, at 1:37 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> 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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