Topband: Using shielded CAT5 data cable as feedline foractive antenna; benefits of multi-turn K9AY loop/SAL/etc?
w8ji at w8ji.com
Mon Jan 4 17:22:07 EST 2016
> Sounds like you, Tom, and LZ1AQ are saying the same: in order for loops to
> be effective (low SNR and high signal levels), they must have large area
> and low reactance (inductance to be exact). Parallel loops or fat conduits
> increase the signal levels, while the CP configuration and other similar
> measures are aimed at lowering the loop's inductance. All this of course
> is paired with a designed-for-purpose amp that does match the low loop
> Rudy N2WQ
There are a dozen ways to say the same thing, but the physical area of a
loop (when it is very small) determines the maximum energy extracted. This
is why small transmitting loops are all pretty much single turn and single
The sensitivity and what configuration produces maximum sensitivity has a
great deal to do with the load placed on the loop and how the loop matches
the load. For example, if the loop has a high impedance amplifier or
matching system terminating the loop, it might be more sensitive with the
extra turns in series rather than parallel.
Then we have things that are called loops and look like loops, but really
function in a different mode than a small loop. All of the small
unidirectional loops act like pairs of small verticals that are phased. This
includes the EWE right through the flag or pennant. They ideally have
uniform current, which is made uniform by the terminating resistance which
terminates the wire in its surge impedance, but the vertical or sloped ends
are what we want to act like the antenna.
It is pretty risky to generalize across everything, but what it all boils
down to is the multiple wires can be used to improve the matching or reduce
the losses. Which is more effective depends on the exact antenna and the
things we have terminating the antenna.
I wouldn't count on a system of more series turns, more parallel turns, or a
thicker conductor, offering improved S/N or performance without know the
specific system, the external noise, and the internal noise. Pretty much
everything "loop" I have played with gets into propagated noise without
multiple wires or a thick element.
The thickest element I have used was old flexible copper waveguide from a BD
station, I think it was maybe four to six inches and oval. I've also used
ribbon cable in small loops, but as a series connected group. For all of my
directional loops, I never used more than a single turn because they all
occupied enough area to get into external noise.
If there was any magic in this, it would have been used 40 years ago. :)
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