LF: Re: Loop RX Antennas

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

Dear Clemens, LF Group,

> So a RX  noise figure of about 15dB would be necessary to have a SNR 
> margin of 5-10dB.
> Good MW/LF receivers have such noise figures (at least I know one).
> 4x2m seems to turn out as the lowest reasonable limit with a sensitive RX.
I was wondering why your estimate is so pessimistic on the required size of 
loop - in practice it is fairly easy to hear the band noise with a 1m^2 loop 
at 136k. Part of the reason is assuming a 15dB RX noise figure; of course, 
if  using a suitable preamp, achieving < 1dB noise figure is usually 
possible, which reduces the loop size requirement. 15dB NF is a fairly 
typical number for a receiver with "good" sensitivity at LF - it amounts to 
sensitivity of about 0.4uV for 10dB SNR in an SSB bandwidth. Unfortunately, 
many receivers, particularly amateur HF transceivers with general coverage 
RX are much worse than this. My IC-718 is closer to 35dB noise figure at 
136k, which seems quite normal. I think poor RX sensitivity is frequently 
part of the reason for disappointing results with small antennas, with or 
without preamps.
Another aspect is what external noise level is it realistic to expect? 
Measurements here under quiet, daylight wintertime conditions indicate a 
field strength as low as 60nV/m per sqrt(Hz) could be reached when the 
man-made noise level is also low. The noise level rises typically 10 - 20dB 
at night. According to the formula in ITU-R "Radio noise" document P.372-10, 
this equates to an antenna "external noise figure" Fa of  88dB. The ITU 
noise curves allow for rather wide variations in possible noise levels at 
136k. The ITU-R curves suggest about 80dB median Fa on a winter morning, 
rising to over 100dB late at night. Since most long-distance propagation 
occurs after dark when noise levels are higher, the noise performance of an 
antenna for long-distance DX is probably less critical than for receiving 
intermediate-range signals via ground wave during the day.
Cheers, Jim Moritz
73 de M0BMU