The K1YPP Magnetic Loop Antenna

Note to my readers of this blog: I’ve decided to expand the blog to include various other interests I have. Up until now everything that I’ve posted has been strictly about writing and author related topics. Since I have varied interests, such as ham radio, bicycling and hiking, for example, I’ll be including topics related to those fields. This post is about an antenna that I have been researching for several years for amateur radio (also known as “ham radio”). There is a Youtube video of this project at: https://www.youtube.com/watch?v=B-tYtbTts0s

I’ve been living in a home owner’s association (HOA) for some time now and have been on the air as an amateur radio operator. There are challenges, but one can get on the air in an HOA.

I’ve been experimenting with Magnetic Loop Antennas, (MLA’s) for years. They are small, light-weight, and have excellent local electrical noise rejection. For years I had avoided building one because most of the designs involved a daunting array of expensive vacuum capacitors, fancy welding and brazing, special low-resistance materials, etc.

My new antenna would have to meet three requirements:

1. Cost $50.00 or less (excluding remote tuning devices, such as motors, hydraulics, etc.).

2. Cover 40, 30 and 20 meters.

3. Have a minimum power limit of 30 watts ( for CW and PSK31).

The primary challenge is to keep resistive losses to a minimum. Many MLA designs use large diameter copper pipes and silver-soldered joints. Builders favor vacuum capacitors or butterfly capacitors that have no electrical contacts or brushes because the contacts will overheat.

Based on the calculations on Steve Yates’ (AA5TB) web page Small Transmitting Loop Antennas, I settled on a four-foot diameter for the desired frequencies. My initial prototypes used folded aluminum foil on PVC pipe, but the foil was too difficult to work with, especially if outdoors in a windy location. I then tried aluminum weather-seal tape and found success.

Parts before assembly. The PVC-to-CPVC adapters are not shown.

I formed the loop with 12 feet of the flexible ¾ inch Sharkbite PVC and used two straight pieces of a larger ¾ inch diameter PVC (Lowes # 23990) to form the capacitor. ¾ Inch copper tubing fits nicely inside this particular PVC tubing, without too much “slop.” I used a continuous piece of aluminum foil tape (2 inch width) spiral wrapped around the loop and down along the two tubes that formed the “capacitor.” The tape I used was made by Nashua Corp, and has a sticky back with a blue plastic cover over the glue. I didn’t remove the plastic, it made construction easier and less messy. The only soldering is the SO-239 coaxial connector. In the first prototype, I wrapped folded aluminum foil in a spiral around the loop and then around the vertical tubes, but the spiral proved to be problematic for the capacitor. The contacts between the spiral as it wrapped down the vertical tube provided contact resistance and the resulting high currents affected the SWR. I found a solution by leaving a gap in the spiral wraps so that the foil did not overlap or touch.

The capacitor is actually a form of “Butterfly” capacitor, formed by the outer foil on one tube, the “trombone” copper tubing, and the foil around adjacent tube. When purchasing the vertical tubing, make certain the 3/4 inch copper tubing will fit inside the plastic tubing. There are many variations so careful selection is necessary to insure the copper will slide inside the plastic tubing. To adapt the CPVC “TEE” fitting to the vertical tubes, I used a PVC-to-CPVC adapter (Lowes #65322) with a small section of Sharkbite tubing inside. The parts are all held together with some self-tapping screws, glue isn’t needed. With the dimensions used, the capacitor measures about 200 µF per foot, giving a total of about 100 µF/foot for the two legs being in series. The 40 M band needs about 160 µF of capacitance, so the legs have to be about 22 inches. The good news is, since the capacitor goes through two pieces of plastic tubing, the voltage rating doubles. PVC is rated at about 140 kV/CM, so this configuration should be good for about 20 kV.

Tuning the capacitor is a matter of sliding the “U” shaped copper tubing “trombone” up or down in the capacitor to achieve resonance.

I made a small feeder loop to couple to the main loop. I soldered a SO-239 coaxial connector to common household #12 electrical wire, about a foot in diameter. I then attached the small loop at the top of the MLA and coupled it tightly to the main loop. Using an antenna analyzer, I adjusted for best SWR. This proved easier than anticipated. A loop about one-fourth to one-fifth the diameter of the main loop worked best.

Construction

Simply cut the plastic pieces to length and plug them together. You can also use duct tape during the wrapping process to hold the aluminum in place. To form the U-shaped “trombone,” notch the copper pipe with a hacksaw or “crush” it in a vice, and then bend it to ninety degrees. For a more “finished” look, one can heat the copper to red-hot, then let it cool slowly. Then, fill it with sand or salt, then bend it over a round surface. Look around on Youtube for various methods.

Wrap the foil starting at the bottom of one capacitor tube, follow up around the loop and then down the other capacitor tube. Keep a small gap between windings on the capacitor tube, about a 1/4” is sufficient.

After wrapping the foil, mount the small loop with duct tape or tie wraps, and then insert the “trombone” copper tubing. The antenna is now ready for testing.

Testing

With an antenna analyzer

I used an MFJ antenna analyzer for the testing phase. If one is not available, see the instructions below for testing with a transmitter and receiver.

To test, push the “trombone” all the way into the capacitor tubes. Search above and below the expected frequency to find a dramatic dip in SWR. The coupling loop may need adjustment to get the lowest SWR. If the dip is below the 40 M band, withdraw the “trombone” until it resonates at the bottom of the band. If it is too high in frequency with the “trombone” inserted all the way, then there is not enough capacitance. Either the aluminum is not tightly wrapped around the capacitor tubes or the plastic tubes are too short.

Once you are satisfied with the adjustment, mark the copper tubing with a grease pencil to show where the 40 M band is.

Next, pull the “trombone” out a few inches and then find the resonant location for the 30 M band. Finally, do the same for the 20 M band. If your main loop is about 44 inches in diameter, there is a good chance it will work on 17 meters as well.

Using a transmitter and receiver

Use a VERY low power transmitter and an SWR meter. Caution: the currents and voltages around the capacitor section of the antenna are extreme. Even a few watts will cause serious RF burns. Be careful.

Move the “trombone” in fully and then tune around with the receiver until noting a marked increase in noise. This is where the antenna is resonant. Slide the “trombone” until it is resonant at the desired portion of the 40 M band and then apply a small amount of power from the transmitter and check the SWR. If the small loop needs adjusting, turn off the transmitter and cautiously adjust until the antenna is working as desired. Mark the copper tubing to indicate the band.

On The Air

Once you have tuned the antenna to satisfaction, mount it where no one can touch it when transmitting. With a loop diameter of 48 inches, the bandwidth is about 129 kHz on 40 M, and 293 kHz on 20 M. This is wider than many of the commercial MLA’s, but I was more concerned about efficiency and bandwidth. A larger loop is more efficient. A smaller loop can be better at rejecting local noise, but I found the combination here to be more than adequate. A wider bandwidth eliminates constantly re-tuning the antenna.

At a future date, I will post something on how to build a stepper motor driver for this antenna. Look around the Internet for existing designs.

As for maximum power, the antenna has consistently handled 100 watts on CW without heating or voltage breakdown. Since I only tested with 100 watts, it isn’t clear what the upper limit might be.

As for performance, that is a difficult thing to judge. My experience indicates that this antenna on 40 Meters performs almost as well as a G5RV at 35 feet, and as well as the G5RV on the higher bands.

Did the antenna meet the design goals?

My original design goals were modest: the antenna needed to cost under $50.00, operate on three bands, and handle 30 watts. Yes. In fact, it exceeded all the goals, it:

1. Costs under $20.00 to build (not counting a remote tuning mechanism).

2. Operates on 40, 30 and 20 M. One version, a slightly smaller loop (44 inches), also functions on 17 M.

3. Runs well at 100 watts. The upper power limit is yet unproven.

This antenna is impressive. Over the last several months, I have worked many DX stations and maintained reliable communications in long-winded rag-chewing sessions. Not only is it a pleasure to use, it has been a rewarding homebrewing experience.

A 20 meter through 10 meter version of this antenna is possible. I have one where the main loop is 32 inches in diameter (81 cm) and the capacitor tubes are 20 inches (51 cm). The capacitor tubes can use PVC pipe, such as the straight version of the 3/4 inch Sharkbite product and 1/2 inch copper tubing.

73 de K1YPP

Partial parts list

Note: Lowes will ship to store if ordered on line
Item	Quantity	Length	Comments
Charlotte Pipe ¾ inch x 10 ft, 200PSI PVC pipe	1	10 feet	Lowes #23990
Sharkbite PEX flexible tubing	–	13 feet	Cut short pieces to connect Tee’s Lowes #998500
¾ inch copper tubing, type M	1	5 feet	Confirm it fits in capacitor PVC tubing, Lowes #23792
¾ inch CPVC Tee	2	–	Sharkbite should fit snugly, Lowes #23760
¾ inch CPVC to PVC adapter	2	–	Couples Tee to capacitor PVC, Lowes #65322
Aluminum tape, 2 inch x 10 yards	1	30 feet	Plastic backed sticky tape Nashua Corp, #322