Tam/WB2TT wrote:
"Tam/WB2TT" wrote in message
...

Meanwhile, I measured the impedance of a Drake 100W dummy load with and
without the HPF. All readings without the HPF are within Drake spec.

FREQ NO HPF With HPF

4 Mhz 47j2 33j13
7MHz 47j2 55j3
14MHz 47j1 50j7
28MHz 48j2 47j3
50MHz 49j2 54j11
144MHz 53j11 74j36

Everything was connected with UHF adapters, and no coax was used. Ignore the
VHF readings,. as the filter was not built that carefully. Capacitors are
mica (actual values 1000, 560, 1000), and inductors appear to be 68-2
(2.2uH).

If I get a chance later today, I will rewire it into the Pi configuration
with the same inductors.

That's about what I'd expect. The increasing X with frequency is
consistent with a small amount of series stray inductance which is
unavoidable in the physical construction. It can be minimized, of
course, by careful construction.

A typical homebrew HF filter will begin becoming poor at VHF and above
due to series self inductance of the capacitors and shunt self
capacitance of the inductors, plus other effects. Component selection
and layout can help a lot, but it might be necessary to cascade a
VHF/UHF filter with the HF filter if very wideband rejection is
necessary. A network analyzer or spectrum analyzer with tracking
generator or noise generator are invaluable in solving those problems.
Of course, the more stuff you put in the path, the more you're likely to
disturb the measurement.

Roy Lewallen, W7EL