Sunday, January 8, 2017

Squashing One Low Band Noise Source

Out in the country it is pretty quiet. I mean this in the usual sense and in regard to radio noise. The hearing has gotten increasingly difficult in urban and suburban areas, and even is some low density rural areas. Rare is the ham of my acquaintance who does not have a story of QRN woe and their efforts to track down and tame the beast.

I did my best to include a spectrum scan for every property I visited while shopping for a house last year. Even so there is noise. Here is a list of what I am dealing with:
  • Ethernet noise from the long Cat 5e cable for my terrestrial wireless internet connection. It was very bad with my temporary inverted vee propped up on the balcony and adjacent to the internet dish and cable but is now much reduced due to the distance of the yagis from the Ethernet cable. It had strongly affected 10 through 40 meters. Now it is merely a nuisance on 15 and 40, and select frequencies, depending on yagi direction.
  • The long high-voltage power line distribution is on rare occasions active during adverse weather. It is not a significant concern currently. The drop from the transformer (70 meters from the house, near the road) is underground, which helps with noise suppression.
  • Low level rapid impulse noise evident on 80 and 160 meters. Since I had not yet put up a low bands antenna it was impossible to predict whether it would become a problem. The noise does not seem strong but would cover up the weakest signals. It is managable with a noise blanker, although that is typically associated with IMD and therefore an imperfect solution. 
I originally assumed the impulse noise was coming from the electric fences to control the cows on the adjacent farms. This was even though the pulse rate is more rapid than I expected from that source. My inexperience with rural life and electric fences made me unsure of my suspicions.

There is a plan in place to reduce the Ethernet noise, so while not solved there is hope. In any case it is not hampering my activity to any significant extent since putting up the tower. It is the prospect of soon getting on the low bands that drove me to track down where where the impulse noise was coming from. So I dug around and improvised a noise detection tool.

That radio is pretty much an antique, dating back to when Pearson was our Prime Minister and I was very young. It was the very first radio that was my very own. It isn't much of a radio yet despite being a half century old works well. Or at least it did after the liberal application of contact cleaner to the volume control and the AM-FM slide switch.

What makes this radio so useful is that it is very easy to carry around and it has a ferrite rod antenna for the AM broadcast band. Ferrite rod antennas in these old radios have a strong null broadside and a strong peak on axis to the rod. It is not the ideal tool for direction finding, however I have it and it works well enough for my purpose.

I walked around the house looking for noise with the tuner set to the high end of the band, which is less than 200 kHz from the bottom end of 160 meters. This is always the best first step since it is not only convenient it is good practice to ensure that your own house is clean before seeking others to blame.

As expected there was strong fundamental induction noise when approaching house wiring and appliances, especially those with a large metal enclosure. Of greater interest was electronics of all types, including smoke and CO detectors, HRV, heat pump, LED light bulbs and so forth. These all came up mostly clean with my simple test equipment: the LED bulbs with integrated electronics are imperfect but not too awful. To be doubly sure I turned off the breakers to branch circuits servicing potential culprits and listened to 80 meters.

Finally I picked up impulse noise as I approached a bedroom that was devoid of furnishing since I was in the midst of repairing the drywall in preparation for repainting. I didn't expect that, and I only went there to be comprehensive in covering the entire house. As I entered the bedroom it was immediately evident where the noise was coming from.

For some unknown reason the ceiling light fixture was controlled by a dimmer switch. The dimmer has slider and an on-off switch. I know that dimmers are a common source of noise but assumed not in this case since there is a switch and the switch was most definitely turned to off. I cut the branch circuit and the noise on my portable radio vanished. I listened to 80 meters on the transceiver and it was perfectly quiet. Success!

I replaced the dimmer with a conventional switch and reactivated the branch circuit. All remained quiet. It was a switch I was intending to replace after painting so all I did was to advance the work by a couple of weeks.

I can now put up an 80 meter antenna with added confidence that I will not be plagued with noise. In time I have more ambitious low band plans, and antennas much further from the house and noise sources. If it had instead turned out to be one of my neighbours the solution would have proved more difficult. With a sigh of relief I can get on with antenna work.

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