LF: Re: Cardioid RX antenna for LF...

["James Moritz" <james.moritz@btopenworld.com>]

Dear Stefan, LF Group,

I have used terminated loops of the K9AY and EWE type on LF and MF - they 
work well down to VLF also. They offer a simple way of obtaining a cardioid 
pattern; a null of 20 - 30dB can be obtained if the terminating resistor is 
carefully adjusted - this means it must be possible to distinguish between 
QRM and general band noise so that the null can be detected. The physical 
design is not critical - changing the shape or size of the loop does not 
matter much as far as directivity is concerned, although the terminating 
resistance will have to be adjusted for the best null, and the signal level 
will be greater for bigger sized loops. I found values near 200R were right 
in the VLF - MF range, which is a bit lower than the values given for HF 
use. The resistor value also seems to depend somewhat on the antenna 
environment.
The main disadvantages are that the output signal level is quite low, so a 
low noise preamp will be needed with almost all  receivers, and that these 
antennas take up quite a lot of space. If space is restricted, the same 
result can be achieved by combining the outputs of small loop and E-field 
antennas using amplitude and phase adjusting networks to achieve a cardiod 
pattern, but the terminated loops need less electronics, and require minimal 
adjustment over quite a wide bandwidth.
Cheers, Jim Moritz
73 de M0BMU