Doug's Shielded Loop Antenna Page


I've been reading about antennas for MW BCB DX'ing recently. Most articles and web sites recommended loop antennas, and some of those described shielded loops which reduce noise pickup.

After I ran across the page at www.beradio.com/features/tech/loop.htm. So I tried it.[However, the link is no longer any good. I looked around their revised web site and thru the article archives list but couldn't find this article any more. On October 7, 2000 I scanned my printout of it and posted it at ..\WebjumpSiteRoot\radio\loop_ant\index.htm]

But then on December 2, 2002 I received an email from Chriss Scherer, CSRE, editor, Radio magazine
http://www.beradio.com and he gave me the new direct link, which is

http://notebook.beradio.com/ar/radio_shieldedloop_am_antenna/index.htm




This article describes a loop made with seven feet of six-conductor shielded cable formed into a circle and with the conductors connected in series to form a 6-turn loop. I had just picked up some scrap 3-pair Belden #9513 cable from work. My piece was about12 feet long, though. But I figured it would just resonate lower in frequency and I could trim it if necessary. That got me going thru the literature on Grid/Drain Dip Oscillators. Today I put the loop of cable on a wooden frame and tried it out. I've added a picture of this to my web site as SHLDED_LOOP_01.JPG . It's not very big or easy to see, but you can tell how simple it is.


IT'S AMAZING!!!

It really does reduce noise. When I just touch the "hot" lead from the loop the receiver noise increases markedly. There are lots of really powerful noises and buzzing signals. Rotating the loop will null them to near inaudibility! I tried the nulling on two strong broadcast signals, too. I'm only using an old Radio Shack DX-200, but it does have an S-meter. Both stations read S9 +10dB when I aimed the loop at them. The lowest reading with the rather narrow null aimed at the station read only S2. I tried it on the second station, in another direction, because the first one surprised me so much. I also got about the same reductions on the noise signals.

And it was really cheap! Of course not everyone can just pick up scrap3-pair shielded cable, but in a couple of ARRL publications like their "Antenna Book" 14th and 15th (but not the 16th) editions there is an article by Doug DeMaw describing a very similar method using RG-59 co-ax cable. It's called the 4T-ES for four turns, electrostatically shielded. He uses a tuning capacitor, though. I didn't because the Beradio article (which says it's quoting a Motorola C-Quam AM Stereo Bulletin) didn't include one. I've just got 12 feet of cable, two sticks that used to be a wooden pallet, masking tape holding the cable on the sticks, and a C-clamp holding the two sticks together at their centers. The clamp was the only thing that cost any money, and a nail would work just as well. Later I added a yard-sale 50-cent Christmas tree stand to hold it upright, and a lazy susan bearing with a piece of plywood on top for the tree stand to rest on.


It's really worth trying!

The antenna is about two or three feet above ground level, in my walkout basement. The other antenna I'd been using is 40 or 50 feet of wire strung around the basement. I've also strung 50 or 75 feet of wire outside thru some trees and that antenna has about the same noise level as the indoor random wire. I haven't compared them for radio station signal strengths yet.

And I haven't gotten my test equipment together and working to check the resonant frequency of the antenna yet either. I've been researching dip meter projects in order to build a modern one, but haven't started anything yet. The loop is actually an "untuned" one anyway, and doesn't really need to be tested for resonant frequency.

How I made it:

I first found the center of my piece of cable and marked it. About 1/2 inch each way from the mark I carefully cut thru the cable jacket and shield and removed the shield there. There MUST NOT be a continuous loop of shield conductor! It would act as a secondary of a transformer, but short circuited. All energy picked up the the loop, which is the primary winding, would be absorbed in the shorted-turn secondary and there would be nothing left for the receiver to detect. I connected the original ends of the shield together and connected them the the receiver ground.

This (mine) is an untuned, broadband loop. Loops often do not work as well at higher frequencies, so be warned. Although a transmitter-hunting book I have does have a plan for a 10-meter band (30 mHz.) shielded loop made out of co-ax cable. All loop antennas work with ground-wave signals, which reach the antenna horizontally. The skywaves reflected down from cannot give a good directional indication. Because they're not coming from the actual direction of the transmitter. So loops often don't work too well at night. The beamwidth of the null on mine was surprisingly narrow and deep. Turning the antenna about 10 degrees could take the signal from full strength, down thru a 30 dB or greater null and back up to full strength. That means you need a fairly good way to carefully rotate it in order for it to do a good job of reducing interference or locating a transmitter.

 


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