"William Mutch" wrote in message
l.edu...
I used to think that passive antenna tuners or matching networks
were worthless, but over the weekend I got some real SWL evidence to the
contrary.
My prime RCVR is a Sat800 refurb hooked to 156 feet of AWG 16 hard
drawn stranded, oriented NNW/SSE up about 40 feet between two large
trees and fed off center at the 1/3 // 2/3 point with twin RG6U which
comes down to a 4:1 balun outdoors at a decent ground (copper ground
pipe filled with copper sulfate, steel well casing and cast iron soil
pipe bonded together with coax braid.) Coax from the balun goes into the
house to the RCVR.
One thing about the Sat800 has always bothered me...there's no way
to turn the AGC *off*, so the noise floor is always rushing up to blast
you in the headphones when you tune between stations. I'd previously
not been able to receive anything but a couple of local aero beacons on
longwave, nor anything but the Christian megawatt at 3200 on the 90
meter band. I didn't care much about the longwave since the very useful
aviation weather Elmira NY on 385 khz was replaced by better service on
162.40 Mhz vhf.
Last week, just for fun I made a Hi-Q parallel resonant tank for
longwave from a 385 pf variable cap and a 3.850 mh inductor.
(82 turns
#24 awg wire on a 1.3" audio toroid core) This tunes 520 khz down to 170
khz with a pronounced peak.

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.