Dear Uwe, LF Group,
It certainly would be worth investigating where the apparent second harmonic
distortion is coming from - assuming a reasonably balanced push-pull PA
design, even-order distortion products should be at very low levels. I would
suggest using the 'scope to check the drain voltage waveforms of the
IRFP450s (using x10 probes). If the waveforms are significantly different at
each drain, this would indicate some kind of component or construction
fault. In an ideal class D design, the drain voltage should be a "half-wave
rectified sine" waveform, but the actual drain waveforms can be quite
frightening in some amateur class D designs!
It is surprising that quite a lot of higher harmonics seem to be getting
through the low pass filter onto the feeder. One thing that does occur is
resonance at various frequencies in the LF/MF range. At the working
frequency the antenna and loading coil is normally series resonant. As the
frequency rises, the antenna + loading coil impedance becomes inductive, but
this can then resonate with the capacitance of the coax feeder to become
parallel resonant. At higher frequencies, the feeder itself could resonate -
90m of coax would be lambda/4 somewhere around 550kHz. So the impedance at
the input to the feeder can vary very widely at quite low harmonic
frequencies, which might lead to reduced attenuation of harmonics by the
low-pass filter. It would be interesting to see what happened if you changed
the length of the feeder, by adding another length of coax, for example.
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: 16 June 2006 01:21
To: [email protected]
Subject: LF: PAs
Hi all,
I`m somewhat surprised / disappointed:
I tested the TX aerial with the ROPEX (157W) and with the homebrew PA
(reduced
to 150W).
ROPEX: phase of V and A on the feeder cable OK.
Homebrew: phase of V and A = chaotic (but it works).
see oszilloscope pics.
the feeder is a 90m long koax cable RG213U.
if connecting a 50 Ohm dummy instead of the aerial both TXs show the same in
phase of V and A.
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