VK6JEN QRM hunting

Hello all,

I was inspired by the 16 August 2022 Tech talk by Jeff W4DD, regarding Mains Electricity Borne Radio Frequency Interference (AKA QRM). To this end I have assembled a small Direction Finding rig using a VHF air-band AM, a headset, a BNC to SO239 adapter, and a 3 element 70cm UHF Yagi-Uda antenna.  

Due to the gain pattern of the Yagi-Uda type antenna, it makes for an excellent direction finding antenna. There is high gain along the axis of the antenna, in an arc of 45 to 60ish degrees away from the feed point, with a strong null elsewhere. 

So I assembled the rig, set the receive volume on the radio to lowish, tune to 136.975 (an unused freq in my area), and went walking.  Because QRM is fairly broadband, the 70cm antenna and the 2.5m VHF radio will still receive the QRM. 

My technique was to walk through my house, and swing the antenna around, basically listening for the loudest noise. the Yagi-Uda, due to it's shape, acts just like a pointer arrow. the strongest source is where the thing is pointing. So walk, swing antenna, hear QRM, walk towards, repeat. 

I found that a Bluetooth speaker in my back verandah was the source of an awful racket on 7mhz 40m, and also on 3.6mhz 80m. The pulsing I could hear instantly vanished when i unplugged it. 

I also found that our central Aircon unit does a passable impression of a steam train. Not much I can do about it though. 

Jenny VK6JEN
Comments