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Which
loop is best: Kiwa or Quantum
Mark Connelly, USA, AM DX Newsflash,
March 15 2001
The loop experiments at East Harwich did produce some interesting
results. Mainly I was looking at both absolute weak signal audibility
(signal to amp. noise ratio) and raw S-meter readings. The former
is of prime concern with a good communications receiver; the latter
is more important with cheaper receivers (especially portables)
where the receiver's own lack of sensitivity is the biggest problem.
I did give a bit of attention to ease of nulling, and achievable
null depth.
I have some additional tests that I want to do before publishing
the article. One problem I ran across was, even near midday, really
weak high band stations (above 1300 kHz) exhibited some skip-related
strength variation over time. That's what you get from trying to
do these types of tests during the winter. In any event, the high-band
usable sensitivity differences aren't as great as the low-band differences.
Generally the "old Quantum" had about 6-10 dB higher S-meter
reading for a given head than the "new Quantum" in non-regeneration
mode. I guess the new Quantum with your transformer update would
perform about the same as the old Quantum I have. That being said,
the old Quantum had a noisefloor of S9 on the R8A set to PREAMP
ON, versus a noisefloor of S8 for the new Quantum. The usable sensitivity,
therefore, was no different for a given head with either base. The
Kiwa Loop produced about an S4 noisefloor (amp. noise) when peaked
on a no-signal frequency. The signal output of the Kiwa was lower
by something like 10-15 dB versus the Quantum (normal head, old
base) and more like 25 dB less versus the Quantum with the larger
head (the one you'd sent me a long time ago, presumably the same
one you're providing with your longwave-capable QX+). Large head
versus smaller head on a given Quantum base gave about 16 dB of
gain on the low end of the dial (530) and about 8 dB of improvement
up around 1600 kHz. That's usable gain, as the amplifier noisefloor
is the same with either head.
On both the "new Quantum" and the Kiwa, regeneration increased output
by as much as 15 dB before received audio got excessively muddy.
Amp. noise also goes up during regeneration, so weak signal sensitivity
really doesn't change much. Using single sideband and passband tuning
on receive lets you squeeze a little more out of the regen. before
audio quality goes into the dumper (or before oscillation breaks
out). Actually, from my experiences, regeneration has the greatest
benefit when the receiver isn't, by itself, up to snuff for split
DXing ... e.g. Sangean portables, car radios, etc. Only once in
a while will regeneration improve reception on a receiver that already
has a good choice of IF bandwidths, PBT, etc.
So who's better, Kiwa or Quantum?
A good sensitivity test is Turks and Caicos on 530 kHz at a distance
of about 1400 miles on pure midday groundwave. At south-facing beaches
in Dennisport, etc. it is strong enough to be a hair over the noisefloor
on my Ford Taurus car radio (and easily copyable there on the Drake
R8A with any good loop, a 100' wire, or an MFJ-1024 active whip).
At the E. Harwich house with something like 3-4 miles of sandy pinelands
on the bearing to T&C-530 before hitting the water, the signal level
is about 6-10 dB weaker than at the beach sites. The car radio doesn't
receive the 530 groundwave at the house. It's still easily heard
on a 100' sloper to the Drake R8A. Using the normal Quantum head
(with either base) produced a signal that was barely over the amplifier
noise threshold. The Kiwa Loop produced a cleaner (though "S-meter
weaker") signal, perhaps an S-unit over its S4 noise deck. The winner
turned out to be the Quantum (again, either base) with the larger
ferrite head. It had a narrow, but noticeable, hearability edge
over the Kiwa on the Turks & Caicos - 530 signal there. Weak signals
higher in the band were pretty much a dead heat between the "big
head" Quantum and the Kiwa in terms of usable signal-to-noise. With
cheapo portables, the Quantum would be preferable because of higher
"S-meter output". On a good radio like a Drake or AOR, it wouldn't
matter. The normal head Quantum lagged some, thereby validating
a "size does matter" principle. With really weak signals, amplifier
gain cannot make up for lack of ferrite, even with the best noise-figure
FET's you can find. It should be stated that, especially at night,
over 95% of what you can hear with a Kiwa or large-head Quantum
can also be heard with a Quantum having the standard size head.
I think that the large-head version of the Quantum Loop may come
fairly close to the performance of the old Gordon Nelson NRC 6 ft.
FET Altazimuth Loop in terms of digging out the weak ones. You'd
have to be way out in the middle of nowhere to see much difference.
The 100' sloper at E. Harwich, when fed through an MWT-3 regenerative
preselector (gain about 40 dB), could still pull up a couple of
signals none of the loops could find, but you're talking about 1/10
of 1% of anything of interest unless you're DXing from Antarctica
or somewhere comparably remote.
Nulling is best with the Kiwa, second best with a normal
head Quantum, and third with the large head Quantum. In all cases,
if there's something on a channel at a right angle to a dominant
and within 30-40 dB of its strength, it can be made audible without
much difficulty with any of these loops.
I did notice some hand capacitance effect. Instead of causing imprecise
peaking, the main thing it did was to couple in some "hand signal"
to make a tight null less deep (if I was peaking a subdominant's
signal). A bit irritating, but not a big deal. |
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antennX