|
|
Last month, I began looking for ways of lowering the noise floor at my QTH. Talking with several area amateur radio operators, I learned that many of us were experiencing daytime noise floors that exceeded S-6 and sometimes increased to as high as S-9. I was looking for antenna ideas that may help attenuate the noise floor without compromising received signal strength too much.
Magnetic loops popped up early in my research and I learned that a magnetic loop is a high-Q tuned antenna that also possesses a narrow pass band that usually runs between 15 - 30 Hz. This high-Q and narrow pass band is beneficial as it significantly reduces off-frequency noise and typically receives less noise than a conventional antenna. The loop is a tuned circuit and it will act as a pre-selector for your receiver. The received signal that you are really interested in, but usually buried in the noise - will be more readable.
I thought, "Wow! This could be my answer!"
During my research, I found many Internet sites that proved to be helpful and educational. I can't take credit for the design of the loops I constructed as I used the website of Steve Yates, AA5TB, as my primary source of information. I encourage you to review his site at: http://www.qsl.net/aa5tb/loop.html
WARNING: Even at low RF levels the loop can exhibit several hundred volts and produce a large magnetic field. At higher RF levels, several thousands volts my be present on this antenna. Exercise caution when building and using this antenna. Build this antenna at your own risk.
Building a magnetic loop antenna feels more like a small plumbing project as the principle building materials consists of copper water pipe and fittings that must be soldered ("sweated") together.
Here are the parts and tools list to build a magnetic loop:
Miscellaneous Items:
Required Tools:
I built two loops. One loop is 38" in diameter and uses 8 pieces of copper pipe cut to 13.5" long. It tunes from 10 to 40 meters. The second loop is 64" in diameter and uses 8 pieces of copper pipe cut to 24" long and it tunes from 10 to 80 meters.
Construction of the loop is straightforward. Cut the 10 feet sections of copper pipe into 8 equal lengths. Clean the ends of the copper pipes and the 45-degree fittings with sand paper and alcohol. Remove the bar code stickers from the copper while you are at it. Lay the copper tubes and connectors on the ground so that they form an octagon. One of these copper tubes will be the top of your loop and is where your air variable capacitor will be mounted. Measure a tube and indicate the exact center. If your tube is 13.5 inches long, then the exact center will be 6.75 inches from each end.
Mark the center of the tube with a marker. Measure ½ inch from each side of this mark, and mark the tube again. Now cut this tube of the two marks you just made. This will leave two tubes that are 6.25" long. This is the top tube where you will connect your capacitor. Cut a 2x4 into 8 - six inch pieces. Flux both the copper tubes and join the tubes with the 45-degree connectors. You will now have an octagon that has a 1-inch opening on one side. Place a 2x4x6 under each side of the octagon, raising the loop off of the ground. Make a few more checks of your loop insuring that each joint is firmly seated inside of each 45-degree connector and that your loop looks like a nice shaped octagon.
Looking cool already, huh?
With your propane torch and your rosin core solder, solder the pipes and joints together. Be patient and don't rush the soldering. After you are finished, give your loop a good 10 to 20 minutes to cool down. Cooper is a great conductor of heat and it is amazing how long the loop will stay warm.
While you loop is cooling, you can construct the Faraday Loop. The Faraday Loop is not physically connected to the cooper loop. It interacts and couples with the copper loop through magnetic flux. I use RG-58 coax to construct the loop. This diameter of this loop should be exactly 20% of the diameter of the larger loop. If your copper loop is 38" in diameter, then your Faraday Loop will be 7 5/8" in diameter.
The drawing will give you an idea how to construct the Faraday loop. At the top part of the loop, the inner conductor is attached to the outer conductor. At the bottom of the loop, the outer conductors are soldered together. Using electrical tape, tape up the connections neatly and you'll have a completed Faraday Loop.
Your copper loop should be cool by now, so now you can attach the capacitor. I used a two gang, air variable capacitor rated at about 500 volts. It came out of an old broadcast band receiver. You can use much more expensive vacuum variable or butterfly capacitors if you wish. Plan to spend $100 to $300 if you go this route. Since I'm using less than 5 watts and just wanted to get the antenna on the air to try it out, I used a capacitor from my junk box.
Connect and solder one side of the copper loop to one gang and the other side of the copper loop to the other gang. Install a large knob of some sort on the capacitor shaft for tuning ease.
Using electrical tape or plastic wire ties, attached the Faraday Loop to the bottom of the copper loop.
You can construct a really nice loop stand from PVC pipe or you can use a wood ladder to support your loop. I used a wood ladder as a temporary support.
For cosmetic purposes, I painted my 38" loop with black hammer finish paint. Without a protective coating, the copper pipe will oxidize and turn green. The hammer paint does a great job in not only protecting the copper, but it also gives your loop a really cool looking finish. Hammer finish paint is available at Wal-Mart in the spray paint section.
You must have an antenna analyzer. Forget about trying to tune the loop without one. I used an MFJ antenna analyzer. I connected the analyzer to the Faraday Loop, set my analyzer for the proper frequency and adjust the variable capacitor on the loop. You will find the tuning to be sharp and be warned, your body will interact with the loop.
Through experience, you will find how much your body detunes the loop and you can eventually compensate for this effect. I'm able to get 1.2:1 or better with about 40 Hz bandwidth at 2:1 or better.
My noise floor dropped 3 to 5 S-units while received signals only dropped 1 S-Unit as compared to a center fed zepp antenna.
What a difference!
Signals that were in the noise popped out of the noise using the loop. On 20 meters using the 38" loop I'm able to work most stations that are heard. On 40 meters, my signals reports were equal to or slightly less than reports using my center fed zepp. The loop does a great job in reducing noise, increasing my listening comfort and bringing out signals buried in the noise.
 |
| Figure 1. Inverted-L at 40 feet compared to a magnetic loop at 2 feet above the ground |
 |
| Figure 2. 40 meter dipole antenna a 25 feet. My S-meter was indicating S-7 of noise |
 |
| Figure 3. Same signal as in Figure 2. Receiver was switched over to a 62" magnetic loop mounted 18 inches about the ground. Noise is now down to S-3. |
I built a larger loop with a 62" diameter for use on 40 and 80 meters. It tunes 1:1 SWR on 40 meters. Signal reports are better than my zepp when stations are more than 750 miles away. It seems that the loop has a much lower angle of radiation than the zepp.
The loop is directional--acting like two co-phased verticals. If I want to work the west coast, I make sure that the loop is positioned with the vertical sections pointing East - West ... the loop is broadside to the North - South.
  |
Copyright © 2004 Jeff Imel, K9ESE All Rights Reserved |
 |
