"starman" wrote in message ...
Ross Archer wrote:

A disclaimer: I may be totally wrong about this, but I think what you built
is actually more like a sharp-tuned preselector than an antenna tuner,
because it's not resonating your antenna system or improving the impedance
mismatch, so much as it's favoring signals around the frequency of interest
over signals that are outside this range.

This would reduce the stress on your receiver's front end by attenuating
out-of-band signals. You can verify this (or disprove it) by comparing a
signal at say, 15 Mhz with and without your matching network installed. If
your HF signals drop in strength, this may explain why reception is so much
better with the parallel resonant circuit in place.

It's my understanding that a substantial antenna like yours will pick up
many volts of total RF at a very wide range of frequencies. These
components stress your front end, driving it into non-linearity and causing
mixing products, some of which will fall within the passband of the LF
signal you're trying to hear. This is experienced as a higher noise level.
The pre-selector knocks down these out-of-band signals, dramatically
reducing the amount of rmixing and thus reducing the background noise level.

I'm EXTREMELY skeptical of any claim that impedance matching by itself will
make any difference to HF reception once you have more than (say
arbitrarily) twenty feet of wire antenna, unless you're feeding a crystal
set. The reason why is, external noise is so high at HF that increasing
the efficiency of energy transfer increases the efficiency of noise transfer
just exactly as much as it increases the efficiency of signal transfer,
resulting in a net 0 dB change in signal-to-noise ratio. You get higher
S-meter readings, but no change in signal readability.

Preselection, on the other hand, should never hurt and would help in cases
where the receiver is overstressed by total signal levels.

This is one possible explanation for your results. Again, I'm not certain
this is correct. However, if you notice a big drop in signal level well up
the HF spectrum when your network is installed, this would be plausible at
least.

-- Ross

I coupled it very loosely (2 1/2 turns) to
the coax near the RCVR and was astounded at the difference in reception.
Around 0500Z I logged 15 different aero beacons at between 260 and 420
khz ! The tuned circuit didn't bring the signal levels up...it took
the noise floor DOWN ! Same thing happened when I picked up a used
Barker & Williamson AT-300 Tee section tuner at a hamfest last weekend.
The peaks are not as pronounced as the longwave tank, but the 90 meter
band yielded a half dozen African stations where previously I'd heard
nothing but QRMN.
On some frequencies the most dramatic improvement came where the
peak in signal strength and the peak in noise came at slightly different
settings of the tuner. This to me is evidence that nearby noise sources,
even when filtered out of the detector and audio in the receiver are
still affecting the AGC line, turning down the effective signal. Seems
like an active preselector will help if you can't get an antenna out in
the clear, but even if you can, some more selectivity *before* the front
end of the receiver can help. I'm a convert.

I thought your choice of the word "stress" (above) was curious. I've
never seen that word used in the context of overloading a receiver's
front-end. I'm sure you know that signal overloading doesn't actually
damage anything in the radio, so what does "stress" mean to you in this
case? Just curious.

Stress, an in overall signal levels taxing the ability of one or more of the
linear amplification stages to remain in their linear regions.

Even a little bit of non-linearity allows spurious mixing products (e.g.
intermod) to occur.

Anythinng that knocks down the total signal level, especially if it's only
signals you don't want to listen to, will reduce that "stress".

Yes, it is a curious usage, but I'm just copying a usage I saw elsewhere wrt.
dynamic range.

-- Ross






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