Hi Peter,
The scope of this is beyond what is possible in one page "reflector
posting", but I'd like to make an important point. I've used loop arrays on
160 for well over twenty years now, the first was around 1973 as a
"simplified take-off" of a military system design.
> At 11:06 1/10/1998 -0400, Paul W5DM wrote:
> >
> > Is it practical, or desirable, to match
> > the very low impedance of the loop to the coax?
>
> Yes. Every dB of improved matching is 1 dB lower system noise figure.
I'm sure you know that is only true when the antenna is being limited by
internal system noise, and not
external noise sources. Even a very small loop or vertical will be limited
by local atmospheric and terrestrial source noises, and at night by noise
propagated in from large distances.
Virtually none of my large loop arrays used resonant loops for a very good
reason.
Resonance should be avoided at all costs if the antenna element is
part of an array of antenna elements. Both phase and amplitude shifts
rapidly near resonance, causing individual elements to be impossible to
track over
the most minor frequency excursions or remain tracked with temperature or
climatic changes.
Even with a non-resonant loop, it was very easy to get the system limited
by external noise sources.
Any reasonably well designed preamplifier allows very small non-resonant
loop to noise limit from external noise on 160, except perhaps during
daylight when absorption is highest on quiet days.
Elements that are part of an array with very close spaced (s<1/8wl) and/ or
fairly small elements (verticals<.01 wl tall or loops<.05 wl circmfrn)
sometimes require more critically designed active matching systems, but the
point remains arrays of elements should never employ a resonant matching
system of any kind when the elements are used in
directional phased arrays.
> This is why high loss antennas like loops and active whips are rarely
seen
> at the highest performance receiving sites, at least for general
reception.
Not up here.
During the final days of critical "HF" communications, preferred antenna's
were aperiodic loop arrays (with non-resonant elements). Small loop arrays
were the directional systems of choice when **performance overrode price**
at
lower SW frequencies down through VLF, since they easily outperformed large
wire arrays by a considerable margin and could be used in multiple
directions with multiple receivers.
It's so easy to get an amplifier with NF<2 dB and an excellent intercept
point, conjugate matching the loop to the amplifier was totally
unnecessary.
In one practical 160 meter case, I used four 45 foot circumference loops
spaced 70 feet center to center in line, with NO impedance matching,
through 75 ohm cable into phasing lines and combiners. After phasing and
combining, I amplified with a single stage amplifier with push pull LT-1001
transistors (like a 2N5109). That antenna, in a quiet rural location was
limited by external noises and had good directivity from a few hundred kHz
to 4 MHz.
The array that allowed me to work JA and other Asians (like UA0 and JT1)
from Ohio while LORAN was still active used eight such elements, none of
which were resonant. The array would never have worked if I attempted to
"tune" the elements, phase shift and amplitude balance would have been "out
of control".
73 Tom
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