[TowerTalk] Amplified Receiving Loop Size

[Lux, Jim]

On 1/31/23 5:28 PM, Tony wrote:
> All:
>
> I've been experimenting with an amplified receiving loop for 160/80 
> and trying to come up with a loop size that gives the best s/n ratio. 
> I found that smaller loops in the 1.5 to 2 foot diameter range seem to 
> have a more pronounced nulls compared to larger loops, but they lack 
> signal strength. It appears the best compromise is somewhere between 
> 2.5 and 3 feet.


have you changed the number of turns?

And

Have you tried a ferrite core (loopstick)?


A larger loop is more likely to be affected by surroundings (it's 
physically closer to them) - I would think that the "free space" pattern 
would be essentially the same.

So then it's down to how many volts do you get out of the loop vs what's 
the input impedance of your preamp, and what are its noise properties.

Don't forget that amplifiers have both current noise and voltage noise, 
so the Z of the antenna can turn one into the other.  And ultimately, 
what you have is a voltage divider (or a current shunt), so Vin = Vant * 
Zin/(Zin+Zant) + en + in*Zant, where en is the voltage noise, and in is 
the current noise, and you're doing a Thevenin.  You can do it as Norton 
sources with currents, too.

Non-resonant small Low frequency antennas tend to be very empirical in 
terms of design process.  You try different things and see what works.  
Measuring noise and Zs is a pain.

If null depth is what you're looking for, then some sort of dual antenna 
with an adjustable combiner is probably a useful strategy. For N 
antennas being combined, you can get N-1 nulls.