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. 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.