Topband: Another source of RFI solved

[NR1DX]

I moved to a new QTH in early December. For a number of reasons one of the
first orders of business was setting up an ethernet here at the house
connecting among other things the computer in the shack to a server else
where in the house. To that end I purchased the following: "EtherFast
10/100 auto sensing 5 port workgroup hub" by LINKSYS.

A few weeks later I finally got my 80/160M antennas up and much to my
dismay there was a an S9+ buzz centered around 1815 and 3510. What was at
first odd is that while it was broad band in nature (covering 1800-1830) it
was definitely RF in nature and not power line breakdown Although the buzzy
nature of the thing sure looked like power line. I lived with it for a
while using the noise blanker and an external noise canceller to tune it
out, but for anyone who has tried to use a noise blanker and a noise
canceler during a contest will tell you, that can get very frustrating.

Finally after putting up a beverage last week and still seeing the noise I
started in earnest to search it. I remembered that at one point I had been
moving stuff around in the shack and that when I had the ethernet line
disconnected that the noise was less (but not dramatically). Anyway
unplugging the power supply for the ethernet hub killed the noise. The hub
has a 5V (1.5 AMP) "wall knob" power supply. I tried unsuccessfully putting
ferrites on all the leads coming and going from the HUB and the DC side of
the supply. None of this made the slightest dent in the level of the noise.
Because of the wall knob style construction there was no easy way to put a
filter on the 110V side of the  supply which I now suspected to be the
source. What also momentarily confused the issue was that with the wall
knob plugged in to an outlet but not connected to the ethernet hub there
was no RFI. Another odd thing about this wall knob was that it ran
uncharacteristically cool and was very light weight. Replacing the wall
knob with a conventional (transformer based) power supply immediately
solved the problem. 

A post mortem (sawing open the case) on the wall knob revealed a small
switching supply, which apparently is reasonably quiet with NO LOAD (thus
allowing it to pass FCC certification as a stand alone unit?), but which
under the 1 amp load of the ethernet HUB was saturating switching
transistors and generating all kind of garbage. 

The clues in hind sight were.
1) The noise only affected 80 and 160 (not a hint of it from 7mhz up)
2) While sounding like 60 Hz line noise it still had a very frequency
noticeable pattern appearing in some parts of the band not in others. Also
the noise spectrum was about 20-30 KHZ wide and recurred every 60HZ !
3) The wall knob was very light weight ( no significant transformers
inside) which points to a switching type supply.

Hope this helps others faced with a similar problem.
Dave 
NR1DX




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