Sunday, November 3, 2019

HF phasing noise eliminators, or "wow ok this mostly works"

I have a long standing issue with noise from a neighbour down the road that generates super wideband noise on the HF and low VHF bands. It's leaking out into the powerlines. It's pretty amazing.

Ok, so whilst the FCC goes and figures this out with the house in question, what can I do?

Well, one thing is to use a HF phasing noise eliminator. It's basically a pair of amplifiers and a phase inverting mixer. The "noise" antenna is a low gain antenna that's picking up the noise you want to filter out, but not sensitive enough to actually act as a longer distance HF antenna. You adjust the gain of your main input antenna and the gain of the noise antenna to be mostly the same level and then adjust the phase delay to cancel out the local noise source.

I bought a kit from Russia ( and .. well, it worked. Pretty well. I used a discone antenna with a vertical radiator for 25/50MHz operation as my "very deaf" noise reception antenna and mixed in the signal from my HF antennas and .. well, it worked fine on 7MHz and 3.5MHz. I think it's a bit too deaf for 3.5MHz operation though; I bet it'd work better on higher frequencies because my noise antenna can start picking up more of the local area noise.

What about 1.8MHz? The 160m band?

Well, this is where the fun begins. The short version is - all I heard was lots of AM radio stations where they shouldn't be. The long version is - I am surrounded by super loud (>10kW) AM radio stations that you definitely need a bandpass filter for. The HF radios I have are good enough to filter them out, but the noise eliminator? It's doing all its mixing before there's any band filtering, so it's also mixing in all the AM broadcast band crap.

So this points out another couple things I need to do - I need to add a little bandpass filter on the noise antenna, and I need a proper HF preselector (ie, a narrow adjustable bandpass filter) on the main radio receiver. That way the transistors doing amplification/mixing in this phaser doesn't get swamped by crap that you're not trying to receive.

(For those who are radio inclined - I'm a few miles from 1100KHz (KFAX) which is a 50KW AM radio station - which shows up as S9+30 or more here. I have a bunch of others that show up as s9+20; so I have a lot of AM noise here..)

1 comment:

  1. Any chance that neighborly noise could be something like this?

    Relatedly…I never realized light sources could generate so much RFI…