Topband: The WD8DSB mini-flag antenna

[n4is at comcast.net]

Hi Rich

" The problem here is that any loop antenna inherently has a noise
temperature of 300K, because it is lossy, and because it "sees"
the earth"

Would you elaborate on that? Small loop 1/10 of wave length works very
different from a loop or large loop. I experimented with a large number of
small loops in phase on 1.8 MHz.  The frequency is very important because
almost everything we know is different around 1.8 Mhz.

Local noise is the most misunderstood figure. On top-band during a winter
Sunday morning the noise can be low as 70K. On topband the only source of
nom mam made  noise atmospheric noise. No atmospheric activity means no
noise. Yes it is that low.  I am using horizontal phased loops for almost 15
years. It needs to be high for 160m, my is at 120 FT, it is large 50 Ft boom
and loops are 24 x 12, all fiberglass. The gain is around -53db. The
preamplifier required  a minimum sensitivity  is near 1 db NF, for that
winter Sunday morning on 160m.

I tested almost all available preamplifier, and the NF at 1.8 MHz is very
different from NF measured at 10 MHz. The best I found was the NORTON with
1.5 to 2 db NF. I developed my own preamplifier , the same I build in 2010
still in use here, using 6 BF981 and  large Q input filter. It measured .7
db NF including the input tuning filter loss.  It does make a difference on
signals at noise level. I built one for NX4D and Doug still use it.  Making
it short, I  heard 316 countries (CW) and worked 305 since I built my N4IS
preamp on 0ct 2010 (city lot). Doug worked 311 from a 1/5 acre lot using a
pair of phase loops. A/B tests with NORTON preamps show not possible to copy
the weak signals I did copy with my 6xBF981.

Implementation is a different ball game. The .7 db NF preamp needs 3 level
of shield including a magnetic shield with steel, just aluminum is not
enough, and no open shield at all, a 1 mm gap will ruin your system. You can
find information about my preamp on google or WWROF archives.

A friend told my 2010 seems to be a very long time ago. Sure It does.

73's
N4IS
JC




-----Original Message-----
From: Topband <topband-bounces+n4is=comcast.net at contesting.com> On Behalf Of
Richard (Rick) Karlquist
Sent: Wednesday, February 24, 2021 12:50 PM
To: GEORGE WALLNER <aa7jv at atlanticbb.net>; Don Kirk <wd8dsb at gmail.com>;
topband at contesting.com
Subject: Re: Topband: The WD8DSB mini-flag antenna



On 2/24/2021 6:32 AM, GEORGE WALLNER wrote:
> Don,
> I put that note out because friends were asking if with a 
> "ultra-low-noise" pre-amp they could use it for DX. Unfortunately, 
> that's not the case. (I have tried.) George, AA7JV/C6AGU
> 

The problem here is that any loop antenna inherently has a noise temperature
of 300K, because it is lossy, and because it "sees"
the earth.  Therefore, a 0.5 dB NF amplifier adds 0.5 dB of noise, etc.  As
opposed to the 0.5 dB NF amplifier on an EME array that is something like 10
dB better than a 3 dB NF amplifier because of how noise temperature works.
Also, for NF's below 3 dB or so, the source impedance is very critical in
order to actually achieve the specified NF.  This is impossible in an
untuned wide band loop antenna.  So in practice, a NF of around 3 dB is the
best you can do, and even that may be optimistic.

BTW, the multiple turns on the loop do not increase the available received
power or SNR in any way.  They simply increase the source impedance.  It is
analogous to a folded dipole.

There are two tools that will reliably increase sensitivity:

1.  Make the loop area larger.

2.  Change to a tuned loop.

73
Rick N6RK
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