As if I don’t have enough balls in the air, I’m getting interested in magnetic loop antennas. This was prompted by a contact I made a month or so ago and a recent item on the KX3 Yahoo Group.
The message to the KX3 Yahoo Group asked, “Has anyone used a Wonder Wand or Wonder Loop (I don’t know if there is a difference) with the KX3?” After a little Googling, I found The G4ILO Wonder Loop. G4ILO describes the Wonder Loop this way,
The G4ILO Wonder Loop is an inexpensive, easy to make magnetic loop antenna that covers five HF bands, 40m through 15m. It is designed for portability, and can be assembled or disassembled in about a minute. When disassembled the antenna easily fits in a suitcase or backpack.
The Wonder Loop is the perfect antenna for picnic table or hotel room operation, or for home station use where outside antennas are not permitted and attic antennas not possible. It needs no supporting mast, and no counterpoises or ground connections are required. Tuning is quick and easy.
What follows is a really great description of the antenna and how he built it. I have all the parts for this antenna, so I just might give it a go at some point. I don’t have a high-voltage capacitor, so I’d be restricted to QRP operation, but that’s OK.
Helically-loaded magnetic loops
A couple of months ago, a guy I worked on 40m told me about the loop antennas designed by Rich, K8NDS. These are somewhat different than the typical magnetic loop antenna in that these typically consist of a flat, wide copper conductor, such as copper foil or copper flashing, helically wound over a PVC frame. According to one of Rich’s websites, his design allows you to achieve a higher inductance in a small form factor. Another of Rich’s websites, http://www.hlmagneticloopantennas.com/, shows some of his designs and discusses some practical considerations.
Rich’s designs are made for high-power operation. As such, they use vacuum variable capacitors. The second website above gives a lot of useful advice for finding and buying such a capacitor.
I think a loop antenna may be a solution to getting on 80m. I don’t have enough space for an 80m dipole in my backyard, but I could set up a decent-sized loop in my backyard. Another solution I’m considering is one of those 43-ft. verticals, but they can be pricey, and need radials. Instead of buying one of these verticals, I could put the money towards a quality vacuum variable capacitor.
Other resources
There are a bunch of other resources on the web, including:
- DIY Magnetic Loop Antennas by Owen, KF5CZO
- Small Transmitting Loop Antennas (Magnetic Loop Antennas) by Steve, AA5TB
- Magnetic Loop Antenna for 80 – 20 m by Frank, N4SPP
How often do you change frequency on 80m? If you stay in the CW or data segment, a loop might work. They are a hassle if you tune around.
Here is one quick way to get into mag loops. Get one of the MFJ mag loop tuners. Get some flexible copper tubing, like used to connect water to refrigerator ice-makers. Hammer the ends flat, drill holes, and connect it to the MFJ.
The MFJ instructions recommend loop circumferences. The best for 80m is 63 feet, but it probably works fine with 40 feet. That is about seven feet in diameter, so it might need some PVC support.
http://www.mfjenterprises.com/Downloads/index.php?productid=MFJ-935B&filename=MFJ-935B.pdf&company=mfj
I am a Ham in Tampa. I own and operate a roof cleaning company here, so I see all the compromise antennas in our deed restricted communities.
Of all the small compromise antennas, only the magnetic loop can approach the efficiency of a dipole. The problems have always been power handling and bandwidth.