Topband: Magnetic Loops

[Andy Ikin]

Richard Karlquist wrote on May 21.


> It is important to understand that these loops have
> a circumference of 10 feet, which they are limited
> to because the manufacturers want to offer a bandwidth
> up to 30 MHz.  Also, they are untuned, again because
> the manufacturers want to offer a broadband solution.
> Very little signal is available from such a small untuned
> antenna, and hence these antennas require a high gain
> preamp.  Depending on your location, the amplifier
> noise may be the limiting factor on sensitivity.
> One of the commercial loops claims to have 0.5 dB noise
> figure, however I measured 3 dB on a sample of one.
> These issues are simply physics, no matter who builds
> the loop.  IMHO, a so called "Moebius" configuration
> doesn't fundamentally change anything with respect
> to these tradeoffs.  As the inventor of that design
> states, it is only an advantage for its intended purpose
> of EMP weapons testing.

Rick,



The most important limiting factor in regard to sensitivity with a small 
wideband loop is the loop/amplifier mis-match over several octaves. Any 
mis-match is equivalent to putting an attenuator in front of the amplifier. 
Hence, the amplifier noise figure is not a significant factor.

That is why Wellbrook use an amplifier that copies the loop element 
reactance over at least 5 octaves.



Where the small loop size is a limiting factor for Topband and MW Dxing, 
then the large Aperture ALA100 offers significantly better performance, when 
used either as a single or Phase array antenna.



73



Andrew Ikin  G8LUG