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 Colorado.  The floating loop was very symmetrical,

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 noon 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

different locations.


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