Lowering my noise floor and stopping RFI getting back into the shack
This weeks project has been a concerted effort to reduce both the noise floor for HF receive and at the same time make further efforts to cut down the level of common mode RFI coming back into the shack along the coax.
First of all I decided that after putting it off for many months I would build the GM3SEK mains filter (details here) to clean up any noise coming through the 240v mains power supply. I’ve been putting off this in the past because it’s playing with 240v mains power and I'm not that confident of working at those voltages given that you can kill yourself or properly blow things up.
However, after taking a “brave pill” and watching a pile of good videos on YouTube that take you through the build step by step (I followed this one) and after a few hours of messing about I ended up with a working mains filter box that (a) worked and (b) I was quite proud of!
Plugging it in and not getting any smoke of explosions I spent a few hours testing and have seen a small improvement in the overall level of noise across the HF bands. Nothing dramatic but then again I don’t think that my 240 mains supply was particularly noisy in the first place so there wasn't much to improve.
If anyone wants to make one of these filters, here’s a list of part numbers and descriptions for the bits you will need and expect to budget around about £100.
Type 43 Large Fair-Rite x1 P/N 0431177081
Oval Fair-rite x2 P/N 2643167851
Power Line Filter, Chassis, General Purpose, 250 V, 16 A, Single Phase, 1 Stage x1 P/N FN2030-16-06
IP65 ABS Enclosure - 170x170x75mm x1 P/N GR17012
3 Core 2mm2 cable 2.5m length
Bulkhead glands x2 P/N PP01645
I recommend having a go for no other reason that I really enjoyed the time it took to make it and you might well get a reduction in your noise floor which is always a good thing.
Next up I had to tackle the the massive problem of RFI getting into the shack along the coax and knocking just about every electronic gadget out, even my weather station went crazy! Every time I hit TX my computer would crash, every USB device would disconnect and I would need to reboot everting and start again. I used to have a I:1 unun choke balun located between the antenna and where the coax came into the shack and it really helped but a week ago it failed so I have had to order a new LDG 1:1 unun replacement and while waiting for that replacement I have just not been able to transmit any signals at any power level.
Instead of mounting it half way along the coax at the place where the coax enters the shack (the bedroom window!) like before, I decoded to relocate it to the antenna itself which seemed to be the general recommendation. Once the L bracket arrived long with a small G-clamp I made up a short coax patch lead and ventured out to fit it all. At the same time I carried out an overall inspection and service of the antenna ready for the oncoming autumn/winter seasons. Just a bit of a clean, oil the joints and tighten the guy ropes.
Once the 1:1 unun was fitted I carried out testing and as expected, it has slightly adversely impacted the SWR levels across the HF bands (Baby Loop antennas shouldn’t be fitted with coiled coax) but at the same time it has almost completely eliminated any RFI coming back down the coax and now when I hit TX everything in the shack continued to work perfectly which is a total win! The only RFI I’m getting now is a small background hiss on my speakers when TX in 40 meters at 100watts and the antenna loop is pointing at the shack.
All in all a very enjoyable weekend and a VERY successful weekend.