Topband: Loop antennas

Fri Dec 23 06:23:54 EST 2005

From: "john devoldere, ON4UN" <john.devoldere at telenet.be>
To: "'Don Moth'" <w2mpk at dreamscape.com>, <Topband at contesting.com>
Subject: RE: Topband: Loop antennas
Date: Fri, 23 Dec 2005 10:20:33 -0000

On 160 you hardly, if ever, need gain (take de isotropic antenna as a
reference). As I thought I explained in great detail in the last issue of my
book (edition4), receiving on top band is a question of eliminating the
noise (in most cases not just "a" noise) that comes from all around or from
over a wide angle (like if one lives near a city, or maybe if you live on
the coast -in which case the direction towards the coast is quieter than
towards inland-). You can only do this by using an antenna with a good RDF
(receiving directivity factor) or DMF (Directivity Merit Figure). What's
that? See book pages 7-9 and 7-10. That's why Beverages (and broadside
phased 2 el vert. arrays, or 8-circle) score very well. When long enough
Beverages have a narrow forward "cone", and attenuate noise coming from
outside this cone. With a so-called magnetic loop, this cone looks rather
like an over-inflated Michelin tire, with one deep null. As Tom explained,
this is a good antenna for hunting one particular noise source, because of
its null, and because it is small and portable.
Coming back to the gain issue, I use 12 Beverages, between 500 and 1000 ft
long, and in one direction (US) I have an end-fire phase couple (each
element approx 550 ft long. End-fire offset is 100 ft, side stacking
distance is 13 ft. This antenna must have a gain of approx -13 dBi (did not
really calculate it), and I NEVER lack sensitivity. As a matter of fact,
under most circumstances (99% of the time) I switch the preamp in my ORION
off, and have the RF gain turned down to around 70 or 80 %!
"General" noise on 160 is, unless you live in the middle of the desert, or
in a place like Tom, W8JI, with no neighbors or villages within 10 miles, of
such a level that the signal to noise ratio (and that is the only thing that
counts) is NOT determined by the sensitivity of the receiver nor the output
(gain) of the receiving antenna, but by the ability of the antenna to cancel
the "general" noise (some of it actually comes in via ionospheric
propagation).
Since using end fire phased beverages, I have the feeling that ON AVAERAGE I
have improved my signal to noise ratio by at least 3 dB, that means that
signals now are 3 dB stronger in or above the noise. Note that such a phase
Beverage has a DMF of approx 30 dB and a single one only approx 20 dB! If
you noise source is predominantly coming from the back you can gain up to
10dB! The F/B of the end-fire phased arrays is a startling 40 or 50 dB on
signals coming from YU, 9A, HA, UB5 etc... Amazing. And that's where I used
to have most of my problems with in contests, when trying to work the weaker
US stations. In only wished I had enough room to have two such systems
broadside (1/2 wave lateral spacing), like Tom has.
One more thing, getting out on transmit from a city lot does not seem to be
the problem. You can shunt feed the tower, or use a sloper or put up an
inverted L. Hearing from such a location is the challenge. There is not
much, if anything one can do, from inside a city. I live 7 miles from the
city, in a suburban area, which must be al least 15 dB worse than Tom's
location, but even in a city and even from a place like mine you can work DX
on the low bands, maybe with a little more patience, and it also requires
knowledge and understanding. With my book I, in the first place, have always
wanted to share my knowledge and understanding. This is also what I
appreciate so much about my friend Tom, W8JI, he is often like a preacher
man, always teaching (in de good sense of the word) his fellow hams what it
really is all about.

I hope this helps to shed some light on the real issue (by the way, all of
this is in the book issue 4, but you must of course take the time to read it
and understand it, hi)
73
John, On4UN