by Steve
4. November 2009 19:20
QRM, a nasty three letter word to anyone with a radio.
Ever since I moved to somewhere a little quieter (read not a suburban housing estate) I noticed that even with the loop between 250 KHz and around 600 KHz there was always this mush, no matter what I did or how I worked the antenna feeds or what options I choose as for feed line this mush was omnipresent. At times it would be evident higher up in frequency at other times nothing about 2 MHz at all. So having a little of the TADX bug inside me I decided it was time to swat this pesky noise once and for all! Armed with my Perseus (which is great for spotting noise) and a UPS I went for the big switch!
A recording of the the noise 390Khz_QRN.mp3 (469.39 kb)
Killing my own mains power removed the noise completely, A Ha! It was my QTH that housed the problem, in some ways great in others not so great! If it was somewhere else I could call on OFCOM and its team of QRM investigators to find it and cure it! However being my noise it was my problem, darn! So back to the QRM DFing, now I knew it was me I could turn on the houses’ electrical outlets one by one and see when it appeared, sure enough I discovered that once the lower floors “mains” was restored the noise returned.
I was glad it was not the central heating or anything like that! This now left me with basically the downstairs and the items plugged in or hard wired – not much really to “check”. Grabbing my trusty Yupiteru and tuning to where the noise was strongest 390 KHz I began to wonder around the house (-pretty glad my YL was not about as she always looks upon this hobby with some suspicion!) each time I got anywhere near anything electrical the noise would peak, so if I moved near a light switch, wall outlet or where the wiring runs in the walls the noise would appear. Which at least shows it was the house wiring that was radiating!
This meant it was findable … moving from item to item the kitchen appliances seemed to radiate the most – well! Now I was getting somewhere. The three items that had displays were the fridge, cooker and microwave; in turn I switched them off – voila! Switch the microwave off and the noise is gone! Switch it back on and a swish and the noise returns. Hmmm now this is not some relic from the 80’s this is a state of the art combo microwave from Panasonic, nothing cheap about it, metal cabinet, screwed together and with a bloody decent screen on the back of the unit, but even with all this as noisy as a junk yard dog on a good day!
Even moving the unit to other mains outlets did nothing, boy did this throw some junk up the house wiring. There is no local radiation as running it from a UPS there is only noise (the same noise) within a few inches of the mains lead and the back area where the PSU is located, so it is not the “item” that radiates but it is throwing QRM up the mains lead into the house wiring.
Anyway I have fired off an email to Panasonic detailing what I have found and I am awaiting a reply. Hopefully something will come of this, in the mean time I just leave it switched off until I need it.
Probably a good thing as I have read some reviews on web sites and someone noted that on standby they consume 75 watts! What the hell for, the front display? If it needed 75 watts to display the clock I would be able to see if from another town it would be that bright! Below are some screenshots of the noise so you get the idea of what it looked like on the Perseus spectrum and waterfall displays.
Narrowed in on the center where it was strongest.
A view of the lower part of the spectrum with the peak.
Zoomed in to see the noise in all its glory.
A waterfall view of the same spectrum
The audio component of the QRM from the microwave.
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