I've been on a common mode choke (CMC) 'march' for about 6-months with
good HP equipment to measure my 'creations'. My set of HF wires consists
of a 450-foot long doublet fed with open wire. I ran another test
yesterday, just for grins on 40-meters. I'm in a rural area with the
closest sparse population about 2.5 miles to my north. Nothing to the E,
W, or S. Without the CMC in place the waterfall on the Icom 7300 showed no
signals, just noise. With any of the CMCs I've built in place between the
open wire feeders and the input of the L-Network, all kinds of signals on
the waterfall. In both cases the L-network was adjusted for best match,
1:1 SWR. My all new appliances have Chinese SMPSs in them with the
accompanying RFI which induce CM noise on the open wire feeders (with Love,
from China). The CMCs take care of that - completely!
Dave - WØLEV
On Tue, Jan 19, 2021 at 9:28 PM K9MA <k9ma@sdellington.us> wrote:
> 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@sdellington.us
>
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>
--
*Dave - WØLEV*
*Just Let Darwin Work*
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