Ross Archer wrote:

"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 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

After reading your original post again, I see that you were using
'stress' to mean any signal level that causes the front-end of the
receiver to operate outside it's design limits, particularly with regard
to linearity.
I too am skeptical of the value of impedance matching or antenna tuning
if the receiver is already getting the signal levels it needs to operate
above the local noise floor. However one of the benefits of impedance
matching for certain antenna types like the inverted-L is you can also
achieve an effective RF ground for the coax shield when the matching
transformer (unun or balun) is located close to the ground. This allows
for a short (effective) ground connection from the common end of the
transformer windings to ground.


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