Further analysis of my previously mention interference problems showed two things:
- It’s constant (I monitored it with SpectrumLab for a week) and seems to emulate a pattern of a person turning a heater on and off in the mornings and evenings; and
- This was not new, as I looked back to one of my 600m mailing list posts around the same time last year where I mistakenly assumed it was due to a change with my mini-whip.
So in a bid to fix it I started to look at Small Magnetic Loops (SMLs).
Using a suitable SML, in theory I should be able to use it’s strong null’s to block interference. So I went about a few ideas but in the end I basically built the amplifier as per PA1M’s page, however I simply re-used the power supply unit for my mini-whip and went with some different loop designs.
Now I’m sure you’ll agree from the image on the right, that the basic principle of a SML’s nulls is working for me. To explain, I started the capture with the loop oriented such that all was well, then in the middle you see when I rotated it such that the interference came back in. Then, at the end I’ve rotated it again to null out the interference.
Interestingly (albeit not surprisingly) the loops orientation is such that it’s radiation pattern is almost perpendicular to that of my dipole.
So, I’m mostly happy as a result.
However! The receive level is well down on the dipoled. For example, without interference and no pre-amp turned on my receiver used to sit at around S7 (and then s9 when the interference kicked in). However now, with the pre-amp on I struggle to see anything on the S meter. 😦
Building the above amp I did use a pair of 2n5109 transistors to ensure all would be well up on the 20m band. However, I fear the biasing network is not setting enough gain for these guys. So I really need to revisit this – or maybe just go and grab one of the commercial offerings.
Lastly, I originally attempted with a loop that was ~1m in diameter and found that on 20m I pretty well had no nulls. Then I remembered that a SML should be no more than 0.1 of the planned wavelength (so ~0.67m for 14.150MHz). So I setup with a 0.5m loop and that’s when I definitely saw the nulls I was after – albeit with degraded receive performance on the lower bands.
Anyway, when I get a chance soon I’ll provide some pics and hopefully also attempt to increase the gain.