Low power 20-meter Magnetic Loop Antenna

Design: There is nothing original about this 20 meter magnetic loop antenna project. I used the calculator at http://www.66pacific.com/calculators/small_tx_loop_calc.aspx to compute dimensions. According to this convenient on-line calculator, 1-inch copper pipe is slightly more efficient than 3/4 inch. However, the latter is easier to work with, and 3/4 inch fittings are more commonly available than larger diameter ones.   

cut pieces   
Acquiring Supplies: For this type of project, it is generally possible to use materials that are either on hand or readily available from a nearby hardware supply. There is no exact parts list. Copper pipe can be purchased from Home Depot or Lowes or any plumbing supply source. That is the easy part. Of course, constructing an antenna is not the same as plumbing. It is not advisable to explain the project to the store assistant who offers to help with locating bits and pieces.

    The photo (left) shows cut pieces for the main loop. Each segment was 23-1/2 inches, making the loop roughly 16 feet in circumference. The exact length is not critical. Some cut pieces in the photo already have 45-degree elbows attached. To me they are 135-degree elbows, but in plumbing they are called 45-degrees. That goes along with some other anti-intuitive things about plumbing that I’ll skip for now.

mock up   Putting it together: The first step was to layout and join all the pieces on the garage floor, which is a reasonably flat surface, although there was barely room between the grill and the sawhorse to lay the thing out. My original plan was to solder most of the joints, leaving a couple of them unsoldered for easy disassembly. I had envisioned the possibility of using this antenna in portable operation. Disillusionment did not strike home until later.

Before soldering elbows I fine-sanded the pipe ends. Low resistance connections are particularly important in magnetic loop antennas. Each piece of pipe was then secured in a vice (but not overtightened) for soldering the elbow. It is a very bad idea to touch the pipe shortly after soldering it. Soldering or ‘sweating’ copper joints requires a blow torch. It isn’t the same as soldering a resistor or capacitor to a circuit board, but not difficult to do. Many Internet pages explain or demonstrate how to do this. After soldering one elbow on each piece, and waiting for it to cool down, I reattached the piece to the big loop to make sure it was still flat. In retrospect it couldn’t have been anything but flat with just one end of each joint soldered. Concern for flatness comes into play when two or more pieces are soldered together.

small loop   The small loop: The driven element in a magnetic loop antenna is a smaller loop about 1/5 the circumference of the main loop. I used 1/4 inch copper tubing because there was some left over from a previous project that also had nothing to do with plumbing. The photo shows the small loop after it was secured to the mounting frame, a six foot length of 1 x 4. Ring terminals that fit snugly over the 1/4 inch tubing were soldered to the ends, and connected to an SO-239 socket.

variable capacitor   Tuning: This was to be a QRP antenna, handling no more than about 15 watts, so I purchased a butterfly variable capacitor MFJ-23 that has a high enough voltage rating for the purpose. In the photo the capacitor is shown bottom side up (mounting tabs at the ends). This one component substantially increases the cost of the project, in fact more than doubles it! Alternative strategies for tuning are possible, though perhaps not as easy to implement.

    The butterfly capacitor is a bit clunky and I was not able to scrounge an enclosure of suitable size, so with the help of a friend who has a band saw, and knows how to cut straight lines, I made a birdhouse-like enclosure of Plexiglas.

capacitor unplugged   Final assembly and initial measurements: In the end all elbows except one were soldered. The antenna can be broken into halves (approximately). Also to facilitate breakdown a couple of connectors were inserted in the capacitor hookup. The capacitor can be unplugged, as shown in the photo. This photo (right) does not show a fixed capacitor that was added later. For initial testing the wooden mounting frame was clamped in the vice. I don’t have a good photo of the test setup, as the background is cluttered.

    As a first step the antenna was connected to a receiver and tuned for maximum signal in the CW part of the
20-meter band (around 14.06 MHz). Then it was disconnected from the receiver and connected to an antenna analyzer and fine-tuning. These initial tests revealed that tuning was terribly non-linear and too sensitive at the frequency of interest (nearly open capacitor). This made me think to reduce the maximum capacitance by adding a capacitor in series with the butterfly (68 pF, 3 KV). With this configuration tuning was less touchy, and the range of tunable frequencies included both the 30-meter and 20-meter bands (roughly 10 to 15 MHz).  

first contacts
   On the Air testing: [December 2016] — My wife N4EFS enjoys working digital modes and volunteered to try the loop on the air. We live in an HOA-restricted neighborhood. Our imitation ‘Charleston’ style house has an upstairs porch or balcony, facing west. Actually it faces the neighbor’s house, but the direction is west. Using my patented Harbor Freight antenna rotator (photo) it is possible, of course, to aim the loop in any direction. 

first JT-65 contact on the loop was with Canada’s Yukon Territory (mid-afternoon Eastern US time). That contact was followed by several others ranging from the east to west coasts of Canada and the US. She couldn’t get Europe or South America, however, even after turning the loop in what we thought to be appropriate directions. Her subjective impression was that the antenna received as well or better than the vertical (less noise), but did not transmit as well.

    The next morning she worked Australia on the loop, again at 15 watts, but this time using JT-9. This result was satisfying, although we do seem to have a pipeline to Australia on both 20 and 40 meters with the vertical antenna as well. Soon after completing this contact she thought of a more objective test, namely to check PSK Reporter after test transmissions using each antenna. This test was informative. The spotter detected the loop all over North America and displayed one spot in England. At the same power, frequency and time of day (mid-morning) the vertical was spotted in several European countries and South America as well as North America.

   Miscellaneous: The junk box contained a plastic rod of the same diameter as the butterfly capacitor’s tuning shaft so it was possible to extend the shaft by joining the two plastic pieces with heat shrink tubing. I also added a Vernier gear-reduction knob to reduce the sensitivity of tuning.  

    Far as I know nothing about the project is critical. The web calculator can be used in a trial-and-error fashion to match available materials to practical dimensions for the frequency of interest. The loop has
no right-side up or upside-down orientation, if Google images can be believed. However, I know very little about antenna design—it is not my field. If in doubt, trust the ARRL Antenna Book or an equivalent authority!

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