Received-SPF: neutral (gmail.com: 193.82.116.20 is neither permitted nor denied by best guess record for domain of owner-rsgb_lf_group@blacksheep.org)
From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Fri, 16 Jun 2006 13:31:38 +0100
Message-ID: <000001c69140$d0843ea0$e6a4c593@RD40002>
MIME-Version: 1.0
Importance: Normal
In-Reply-To: <20060616022030.coqaz7zeo7y8wgw8@webmail.versatel.de>
Subject: LF: RE: PAs
Content-Type: text/plain;
	charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Status: O

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: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of
uwe-jannsen@versanet.de
Sent: 16 June 2006 01:21
To: rsgb_lf_group@blacksheep.org
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 =3D 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.