[Lf] [Fwd: LF: Noise Cancelling RX antennas]

[Andre Kesteloot]

James Moritz wrote:

> Dear LF Group,
>
> As I mentioned a few weeks ago, I have been having problems with local
> mains-generated QRM from an unknown source which made it impossible to copy
> the weaker signals on the band. Over the weekend I tried out a noise
> cancelling system which seems to more or less solve the problem.
>
> I first tried using various antennas I already had as "signal" and "noise"
> antennas, with the HF type phasing circuits that have been around for a
> while. I found that it was possible to null the noise, but the bandwidth of
> the null was at best a few 100 Hz; changing frequency or re-tuning the
> antenna meant complete re-adjustment of the phasing network was required,
> which was not easy because the noise level varies rapidly and  makes it
> hard to tell if the null has been found. So the system was not very practical.
>
> Thinking about it, the problem compared to HF is that the LF antennas and
> phasing networks are relatively very narrow band, and so phase and
> amplitude of the signals changes rapidly with frequency. In order to get a
> null, the noise signals from the 2 antennas must be equal in amplitude and
> 180degrees out of phase when the signals are combined, therefore a null is
> obtained only over a narrow bandwidth. I decided the solution would be to
> use two identical, matched, loop antennas with fairly wide bandwidth, in
> combination with differential phase shift networks (similar to those used
> for the LO in a phasing SSB exciter), so that the whole system has a
> reasonably constant relative phase over a considerable bandwidth.
>
> I made 2 tuned loop antennas with 2m sides from 15mm copper water pipe.
> These have low noise pre-amps and a bandwidth of about 30kHz, and by
> themselves have more than adequate sensitivity to hear the external noise
> in a quiet band. One is located at the end of my garden nearest the noise
> source, while the "signal" antenna is positioned at the opposite end of the
> garden. Both loops are oriented to null the Loran signal form Lessay, which
> at the same time gives quite good Europe/NA coverage at my QTH. The signals
> are each passed through RC phase shift networks tuned by a dual gang pot,
> arranged so that as the phase shift of one increases, the other decreases.
> The phase shift networks are buffered to prevent loading effects. The
> relative phase can be varied over a range of about +/-120degrees, and a
> transformer and switch gives a further 0/180degree shift means any phase
> shift can be nulled. The "noise" antenna signal also passes through a
> variable attenuator, and the two signals are combined in a transformer and
> fed to the RX.
>
> In operation, once the null is found it seems to be quite stable, and
> covers the whole amateur band without adjustment. While operating, there
> was no noticeable interaction with the TX vertical, although one of the
> loops is directly under it (the TX ant is de-tuned on receive). Without
> noise cancelling, the normal mains noise is about 10-15dB above the natural
> band noise, with peaks up to 30 - 40 dB. After nulling, the normal mains
> noise is inaudible, while the peaks can still be heard but are obviously
> much weaker. The system is still in the breadboard phase, but is certainly
> a huge improvement over the situation before - it is entertaining to switch
> the noise antenna on and off and hear the change in noise level!
>
> Cheers, Jim Moritz
> 73 de M0BMU