The so called (poorly) "magnetic loop" only works when it IS small -
.1 wavelength maximum but that is not the optimal size.. According to
several texts that should be .037 wavel. which is the 5 ft diameter
loop.
But I think there remains confusion over the term LOOP. The magnetic
loop is a tiny antenna used primarily to null out strong, highly
localized single-point noise. Secondarily, you get a general receiving
antenna with very poor characterics starting with very low sensitivity
and no directivity vertically - so it won't discriminate between high
angle atmospheric noise and low angle DX despite nulling out local crud
noise if you can. It always work well for local work, but is truly an
antenna of last resort for DX. I've worked VK with it from noisy NNJ
but the conditions were exceptional at those times. However, without
that magnetic loop those days I could not work VK. None the less it is
a VERY POOR receiveing antenna for DX.
The terminated loops (diamonds, flag, pennant, K9AY) and entirely
different. They are also larger (although there is a mini diamond Flag
of 5 ft dimensions). The nulling they provide are not nearly as useful
for nulling out local noise crud - like a bad insulator on a power
pole, light dimmer, etc.BUT that null is broader and much more
particular to a certain arrival angle. That null is used to increase
the S/N ratio but the directional characteristics of the cardiod
pattern and the overall pattern has fairly low angle sensitivity. This
is a good DX antenna but is much larger than a magnetic loop. That
said you can build a half size Flag (7 by 15 ft) or even a mini diamond
config flag of 5 ft per side and still get the same cardiod pattern of
the full sized models (15 by 30) but with much lower sensitivity.
When these antennas get smaller and their outputs become much lower the
chances of feedline pickup and pattern distorion come into play and
that can ruin everything but those conditions can be avoided. ON4UN's
book remains the "Bible" and lots of other on line resources out there
with this stuff -- including the w8ji.com site.
73
Pete W2PM
-----Original Message-----
From: Rick Karlquist <richard@karlquist.com>
To: RICHARD SOLOMON <w1ksz@q.com>
Cc: TopBand List <topband@contesting.com>
Sent: Fri, Jan 1, 2010 10:46 pm
Subject: Re: Topband: Loop Antenna Questions
I found that a 40 foot perimeter square loop, roughly
equivalent to your 12 foot diameter round loop had
an inferior null to a loop half as big. It did of cource
produce about 10 dB more signal (and noise, same SNR).
Rick N6RK
RICHARD SOLOMON wrote:
>
> I have an NZ4O(KN4LF) Loop Antenna for receiving now. I was wondering
> what folks thought about using a "bigger" Loop ??
> By that I mean around 12' diameter instead of the 8' diameter I have
now.
>
> Would I gain anything by doing this ?
>
> Tnx es HNY, Dick, W1KSZ
>
> _______________________________________________
> "160-meters is a band for men, not for sissies!" - SM5EDX
>
>
_______________________________________________
"160-meters is a band for men, not for sissies!" - SM5EDX
_______________________________________________
"160-meters is a band for men, not for sissies!" - SM5EDX
|