[RFI] RFI Measurements

[K9MA]

I got to wondering just how bad the usual RFI is in my urban QTH, 
compared to a rural area. I've long been doing battle with the local 
power company and have had them fix many of the worst noise sources over 
the years. I'm surrounded by overhead 14 kV distribution lines, many of 
them very old. Currently, the noise level is about as low as it gets, so 
that's the baseline. The dominant source still appears to be power line 
noise, as opposed to nearby electronic devices in that it's broadband, 
with distinct 120 Hz harmonics in the AM audio.

To make the measurement, I built two small shielded loops. The 80 meter 
one is about 60 cm on a side (square), the 20 meter one about half that. 
Both are tuned and matched to 50 Ohm, using a method I published in QST 
long ago. I built a preamp with about 25 dB of gain. The receiver is an 
Alinco DJ-X3 in AM mode, which is quite sensitive. A set of HP step 
attenuators between the preamp and the receiver were adjusted for a just 
perceptible increase in the S-meter reading. In each location, I 
positioned the antenna for the worst case noise level.

The results: On 20 meters, the urban noise level was 3 dB higher that 
the rural one. On 80 meters, it was 11 dB higher. This explains why 
W0FLS and VE6WZ are always working DX I can't hear. On the other hand, 
it could be a lot worse. (An has been, when I've had a nearby source.) I 
do think this is about the best one can expect in a dense residential 
area with overhead distribution lines.

Note that these measurements are relative only. At the time of the 
measurements, The noise from my tribander at 70 feet was about -124 
dBm/Hz on 20 meters. On 80, it was about -107 dB/Hz on the shunt fed 
tower, but I'm not sure that means much, as I never use it for receiving.

In the future, I'll make the measurement in a few other rural locations.

73,

Scott K9MA

-- 
Scott  K9MA

k9ma at sdellington.us