(and an exploration into local noise sources)
After moving out of a tiny unit into a house I finally had the opportunity to throw a bit of wire into some trees and connect it to my radio. Easy interstate contacts were now available, but there was something nearby making obnoxious noise every 60kHz on HF:
Having turned off house power and confirmed the noise was still around, I assumed it was a neighbour’s solar inverter. Since I could do nothing about that, I bought a VK5TM noise canceler. As I pondered an enclosure for it I also remembered I should really build the FT-897 tuning aid I’d been thinking about for several years. Also, the FT-897’s upwards-firing speaker sucks.. so an external front-firing speaker was vital. Behold:
The front speaker is a Tang Band T2-2136SA from Parts Express. It’s overkill for listening to SSB but if I’m going to go to the effort I might as well make it nice. The sound of the ether has never before graced my ears with such fidelity.
The tuning aid (big red button, and grey adjustment knob) uses the CAT/Linear jack’s 13.8V and turns it into an adjustable negative ALC voltage using the venerable 555 timer as a charge pump:
This gives easily adjustable tuning power without having to menu-dive. An added benefit, checked using a direct-sampling RTL-SDR, is my radio tunes up on the suppressed carrier frequency – so I don’t even have to spin the dial away from a QSO before tuning!
The toggle switch powers the VK5TM noise canceler (also via the radio’s CAT/Linear jack). The addition of the switch turned out to be very worthwhile! The three black knobs drive gains and phases, with green and red LEDs as status indicators.
That just leaves the noise antenna.
I started with a PA0RDT mini-whip, my RTL-SDR and a laptop. It soon became apparent from wandering around the backyard that my noise source was actually coming from the HFC cable that brings me NBN service. So the mini-whip was duly installed next to the cable:
I also mumbled an apology to the neighbour, who didn’t hear it. But luckily they hadn’t heard my inner monologue regarding their solar panel inverter either.
This did a great job of canceling the spurs on 80m. But it didn’t pick up as much of the interference on 40m and almost none on 30m and above. So I made an active current transformer to clip around the coax:
This active pickup was excellent at picking up the interference, but it turns out that my HFC coax shield is also a brilliant receiving antenna on 40m! Not so good for noise canceling – the noise antenna should be good at picking up the noise and poor at picking up the desired signal.
Next step would be ferrites on the HFC to try and choke the interference, maybe with the noise canceler for fine tuning.
Is the VK5TM noise canceler worthwhile? If you are plagued by a single dominant noise source, absolutely! (and it’s a bargain). It made listening to many contacts a LOT more intelligible. But it can only be as good as your noise antenna so you need to invest some time into figuring out where your interference comes from.
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