Topband: Using shielded CAT5 data cable as feedline for active antenna; benefits of multi-turn K9AY loop/SAL/etc?

Tom W8JI w8ji at
Sat Jan 2 23:19:23 EST 2016

> 2.  He cites experimental data showing that coplanar crossed loops and 
> multi-turn quad loops both offer very significant improvement in the 
> recovered signalcompared with a single loop.  See 
> <> to 
> check whether I got this right. Anyway, it occurred to me to ask if anyone 
> has ever tried multiturn K9AY, SAL or flag/pennant receiving antennas, and 
> did you see something similar?

Be careful in what you might think the data means. The measurements are for 
an unmatched system, and apply to broadband untuned loops with "low 
impedance" loads.

 In a case like that, the parallel wires reduce the impedance primarily by 
reducing reactance. It is no different than a thicker conductor, which would 
reduce reactance and increase current in the simple circuit.

This does not necessarily mean the loop would have a higher SNR, that would 
depend on how the amplifier "likes" the lower impedance and if external 
noise is limiting the system.

It does not mean more directivity. An even larger improvement in sensitivity 
would come from cancelling reactance.

If  a small terminated loop had increased conductor size it would have more 
sensitivity, which means increased signal and noise pickup, because the 
termination and source resistances would decrease.

You can see this effect by modeling an EWE antenna, or any small loop.  As 
the conductor is made thicker the optimum termination resistance decreases. 
This increases sensitivity, because radiation resistance remains the same 
and loss resistances decrease. You can pick up a few dB in sensitivity in 
certain cases.

If the amplifier or receive system is affecting S/N in a significant manner, 
it would improve S/N. If external noise is the significant factor in 
sensitivity, then it would pretty much do nothing.

This effect occurs in all sorts of lossy antennas. For example, if you 
paralleled two close-spaced Beverages (making them act like a single very 
wide conductor) sensitivity increases. This does not mean S/N ratio 
increases, because signal and noise from the antenna would increase at the 
same rate. It just means the level of signal and noise from the antenna is a 
bit higher.

If receive amplifier or system internal noise is helping set noise floor, 
then it would help S/N.

73 Tom 

More information about the Topband mailing list