Everyone talks about ‘noise’ contribution, noise figure, IMD, and MDS. These are all on the minds of anyone that wants to receive signals at the lowest possible level. There are other factors, one be ‘noise’ factor in your receiver. But that is not what this article is about.
At this station WI2XJQ RX noise is in copious quantities, from all directions. So, exploration on how to reduce this noise and increase the possibilities of ‘hearing the weak ones’ is the goal.
Where we started is a receiver that had S6 to S9 noise and nulling it was quite a chore, even with an 8 foot receiving loop. So, lots of attenuation on the front of the receiver was used to get the noise down to an acceptable level to receive WSPR signals. Receiving loops, no matter the type are a great antenna for the 630 meter band. Others use probes, or verticals to receive and they work great as well. However, in the urban setting one needs to have the ability to ‘null’ noise around the neighborhood. So, this is where we will start.
Suffering from big noise issues is probably the norm for urban receiving on any band, but with the ‘loop’ antenna on can null much of the noise that ones receiver hears. So, we started out the morning turning the loop to null out the noise in the neighborhood. It turns out that most of the noise is coming from within my own house. WOW! That was totally unexpected. Finally determining the null of the noise, broadside to the loop; the reduction of noise was almost 7 S-units!. That is significant.
Next was to come inside and find the source of the noise, knowing full well that it would not be any one thing. A portable AM radio tuned off station so that you can hear the noise works very well. These noises being broad in nature can be detected quite easily using this method.
First was the notorious ‘wall wart’. I found 4 that were bad. I replaced them with known tested ones and that helped. But it did not seem to affect the real noise source. Checking computer power supplies determined the one computer has a power supply going bad. But it is not used much of the time so no worry there. TV’s, especially plasma TV’s are very noisy and you might find your neighbor has one. That one you can’t fix. Other things will also be contributors so search everything in the house.
What ended up being my largest noise contributor was my switching power supplies on my transmitter amplifiers. I found that by putting a 600VDc, .1uf capacitor on the output, almost totally eliminated the noises in my receiver.
So, in summary, check everything, you never know what will be a noise contributor. My noise has been reduced from S6 plus to S1 by this simple method. Good luck finding that pesky noise.