Topband: The WD8DSB mini-flag antenna

[Richard (Rick) Karlquist]

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