I love magnetic loops. They work well as an indoor antenna for an apartment. They have some interesting properties, and some little things to learn. I recently discovered the benefits of using a ground plane with one.
This began on the ElmerNet, which I run Wednesday nights at 07:00 PM on the WM3PEN repeater. Carl, N3ZZK, mentioned seeing more magnetic loops advertised in QST magazine. He mentioned a pretty wild looking one on DX Engineering. It has a microprocessor and interfaces with the radio’s data port. I noticed that the description mentioned that the loop comes with a mat made of metallic fabric to act as a ground plane This intrigued me.
For the non-hams, a ground plane acts as a substitute for the Earth, not exactly accessible from a second floor condo. A sheet of metal or a wire or series of wires perform the same function. The radio signals reflect off the ground plane, producing a mirror image of the antenna.
That weekend I got a chance to go up to the roof deck with my Elecraft KX3. I just brought my AX1, but I made a few contacts, and enjoyed the low noise environment. When I came back down and hooked everything back up to my Alpha Loop, the high noise level hit me like a wall, and a wave of depression followed. I had to do something about this horrible noise floor. I began searching, and saw a reference to using a ground plane under a magnetic loop.
I remembered Carl’s comment, and the magnetic loop which comes with a ground plane. I emailed Steve at Alpha Antenna, and he sent me a short article on his site which confirmed that using a ground plane boosted the signal by up to 5 dB. He also told me that it blocked noise on the other side of the plane. This excited me, because for me that means my downstairs neighbors and the street. The street has a trolley which they only run once a year to celebrate the Fourth of July, but they keep the power line active all year round.
. I called DX Engineering and asked if they would just sell me the ground plane mat, but they wouldn’t. I had to find a piece of metal suitable for my apartment. I asked my friend Meg and my brother. Both had pieces of metal. When I next saw Meg, we started brainstorming. I told her about the mat made of metallic fabric.
“Oh. Why don’t you just search for the fabric?” she said.
I went on Amazon and searched for Metallic fabric. I quickly found Faraday Fabric. People use it for making Faraday cages or RFID blocking accessories. It has metallic copper, metallic nickel composition, and polyester fiber. Perfect! They didn’t call it a ground plane, but I saw through the marketing. If it could block signals then it could act as a ground plane. I bought two sheets.
They arrived a few days later, and I eagerly lay them down. My friend Meg later helped me do it even better. I immediately noticed a drop in my noise floor by 1 S-unit, or 6 dB, maybe even a little more. On 20 meters I could copy signals down to S4 or S3. On 40 meters I could copy down to S6 or S5. The readability would often drop, but I still did it. I never gave out a signal report on 40 meters below S7, and I have the logs to prove it. I even saw a drop on 80 meters, the noisiest band. Before I would only hear S8 or S9 signals, and now I could hear down to S7. Amazing!
I wondered if I would see a similar increase in my effective radiated power. That weekend I played in the CQ WPX contest. We had bad conditions, but I worked 15 stations. I wanted one DX, and I worked a station in Germany, 4000 miles away. This happened on 20 meters using 5 Watts CW (Morse Code). We had to slow it down but we made it work! I can’t say that it has magically made me work everyone I hear, but I can hear more. Some better solar conditions would also help.
I would definitely recommend using a ground plane with a magnetic loop. Many people will use a metal screen or chicken wire, but this fabric installs easily and does a great job.