Subject: Re: [loopantennas] I need a loop antenna for tracking down
power line noise on 15 meters
Date: January 19, 2013
I converted my HF loop from a shielded one to an unshielded floating
loop with magnetic coupling. Here is the picture of the HF loop and
Yachtboy general coverage HF RX. Also the pix shows 440 MHz yagi and
Yaesu VX5R I listen on 441.7 MHz AM mode to pinpoint the noise once
the loop gets me close to the source.
The non grounded and floating large 13 inch loop is just a piece of
RG8 coax with shield and center wired together and in series with a
360 pf variable cap. Wire could be used instead of the coax, but the
coax was already there and is stiff enough to not need extra
supports. The loop naturally tunes from 8 to 28 MHz. There is an
extra fixed mica cap in parallel with the variable cap that has about
250 pf and allows the loop to tune from about 6 to 9 MHz when clipped
across the variable capacitor. The 3,5 inch diameter (not critical)
magnetic pickup coil is shown on this picture:
Here is a picture of the connections between the large loop and the capacitor.
How well does this floating loop design work? Initial testing
revealed the loop tunes extremely sharp so the Q must be very
high. The antenna has excellent gain and the reception of shortwave
signals is much better than on the Yachtboy whip, but that's not
saying much, since the little whip was not all that good an
antenna. However you would not want to use this loop antenna for
general coverage reception because it must be retuned each time you
change receiver frequency. Note that you can pick up some signals on
the inner small loop when the large loop is not in resonance so it
should not be made too large.
After the loop was constructed this morning, I tuned up WWV on 10 and
15 MHz and the peaks and nulls were as expected with the direction of
WWV being toward
like the shielded loop was. However this loop is much easier to
construct and get working than the shielded loop and uses fewer
capacitors. The matching is achieved by the smaller loop being about
1/4th the size of the larger loop. The large loop is 13 inches
diameter and the small loop is 3.5 inches diameter.
At my friend with the RFI problem returned my call and I went
over to his house. He showed me what the noise sounded like on his
HF receiver which was a soft mushy sound, not one that the noise
blanker can take out. The noise was slightly louder on 24 MHz than
21 or 28. His HF beam gave the general direction as being north of
his house. He has distribution lines all over the place so the noise
could be from any number of locations.
We went outside and could hear nothing on the VX5R and UHF yagi. So
we fired up the HF loop and tuned around. At first we tried 21 MHz
but his neighbors TV was generating noise. The HF loop indicated it
was coming from his neighbors house. Then we went to 24 MHz and
still could hear the TV. We tuned up to 26 MHz and the power line
noise was heard as a light signal without the neighbor's TV
interfering. There was also a sharp noise we didn't hear on his
receiver in his house.
The loop indicated the noise was either north or south, but because
his beam said it was north, we started walking north. The HF loop
direction varied some as we walked along the street. There was a
distribution line about 150 feet to our left or to the west which was
also behind his house. We reached a point about a block north where
the sound of the sharp noise indicated it was to the west. He said
he was hearing that sound also on his vertical at this house but not
on his beam.
At this point we were one block north and then we walked west
following the location of the sharp noise on the HF loop. One block
north and one block west the exact location on the HF loop could not
be determined. However the UHF antenna on 440 pinpointed the sharp
noise as coming from the pole one block north and one block west of
his house. Returning back to the location one block north of his
house we could clearly hear the softer noise source was coming from a
pole one block north of this house. The sound was identical to what
his HF rig was hearing at his house. The loop helped us to get in
the general area and the UHF yagi pinpointed the two sources at two
So within a period of about 30 minutes we were able to pinpoint the
two noise sources. The softer sounding noise he was hearing on his
HF beam was on the pole one block due north and the sharper noise
source he was hearing on his vertical was one block north and one
block west. Going back to his house I checked the polarization of
the sharp noise signal when in his back yard where the vertical is
located and sure enough, the sharp noise source was almost completely
vertically polarized. This explains why he was not hearing the sharp
noise on his HF beam antenna.
My friend said that he would probably just sell his house and move
out in the country where there are fewer power lines. He and I had
struggled in the past on a similar noise problem and was becoming
frustrated with having ongoing power line noise problems.
The conclusion is that the new floating loop has excellent
performance and is easier to construct and tune than a shielded loop.
Good RFI hunting.
73 de k5gp, Gene