An Unusual Transmitting Loop Antenna
September 3, 2010 7 Comments
I live on the first floor of a concrete apartment building, so radio antennas are a challenge for me. I decided a while ago that a magnetic loop would be the best option for me. I built one out of coax, but after some testing I decided it was not efficient enough (I later found out that this conclusion was based on wrong data, but nonetheless that’s what I thought.
A few days later, I found a few meters of discarded hot-water pipe that looked like it could be used for an antenna. It’s a weird kind of pipe that I did not see before. It’s made of three layers glued to each other: polyethylene, aluminum, and polyethylene. It’s basically an aluminum pipe with a thick polyethylene coating on both sides. It’s flexible enough to bend by hand, but also stiff enough to retain a shape.Here is a diagram from the manufacturer’s web site:
A magnetic loop antenna consists of a loop conductor, which acts as an inductor, a capacitor to tune the loop, and some feed mechanism. Because the radiation resistance of small antennas is small, their efficiency depends on their Ohmic losses. These losses depend on the conductivity of the conductor (copper is better than aluminum), the diameter of the conductor (larger is better), and the resistance of any connections. The connections were going to be a problem: I knew that I could not solder a capacitor to an aluminum tube, and the loop needs a capacitor.
What I did eventually was to form a capacitor out of the tube itself. I cut a 4m piece of tube. One side remained flush. On the other side, I split the tube in half lengthwise with a saw, and removed the top half. I compressed the remaining half tube to give it a smaller diameter, and stuffed it into the other end of the loop. The surface of the half-open tube on one side of the loop forms a capacitor with the surface of the tube that it is stuffed into. By pulling out or pushing in the cut end I was able to vary the capacitance. The cut cut fits into the whole tube very snugly, which stiffens the entire structure and keeps the capacitance fixed. Here is a picture of what this capacitor looks like (you can also get a good idea for what this tube looks like from the picture):
I feed the loop with a coupling loop, which I made out of the same material. This was really not important; the coupling loop can be made of thin stiff wire, but the tube one looks good and it keeps its shape. The coax is connected to the coupling loop with lugs and screws. This is not a low-impedance point, so the resistance of the connection is not much of an issue. I used a ferrite choke on the coax just before the feed point to obtain a balanced feed. The following picture shows the whole loop (the big one), the coupling loop (the smaller one) and the feed point.
I tuned the loop in the balcony by pushing the cut tube in and out. I used a resistive SWR indicator to determine at what frequency it was tuned. When it was tuned, I pushed it on a wooden pole outside the balcony. The near end (the feed point) is about 0.5m out of the building, at a 45 degree angle (gravity decided on the angle). At this position the SWR was reasonable, less than 1:2. When it was closer to the building, the SWR was much higher. Here is the loop sticking out of the balcony. The boxes near the laptop inside are the radio.
The results were fantastic. In one afternoon, with a 16W on 20m PSK31, I was able to work stations from Azerbaijan to Britain, and PSK reporter shows that even people in the US heard my signal. The antenna is only about 4m above ground.
- This is basically a fixed-tune loop. It works well on one frequency, but it’s hard to tune it to another frequency.
- No electrical connections means there is no losses in the connections to worry about; this point and the previous ones form the basic trade-off.
- As in all magnetic loops, making them large improves the efficiency (up to a point). This one is made of aluminum, so this is even more important.
- The polyethylene might cause some losses, but I really don’t know a thing about this; if somebody does, I’ll be happy to know.
- The capacitor end of the loop is not water proof right now. Water probably won’t harm anything to the tube (it’s a water pipe, after all), but it would detune the loop and might cause the capacitor end to arc.
- Arcing is an issue even when the loop is dry. I don’t know what kind of voltage this capacitor can withstand, but it didn’t arc. If you build such a loop, keep in mind that the voltage across the capacitor depends on the frequency, size of the loop, and the power level; you can use loopcalc to estimate this voltage.
- A similar loop made of copper would be more efficient (you can try to isolate the capacitor plates with some kind of plastic tubing, coax jacket, etc.
The web is full of web sites on magnetic loops, and it’s worth the effort to read them if you build one. Good descriptions of home-made loops include Brian Levy’s, Alex Krist’s, Alexandre Grimberg’s, Julian Moss’, and Steve Yates’. Chris Trask’s web site contains an article on a small transmitting loop with a very clever matching network (and some complicated theory). His web site also contains a number of articles on receiving loops (he also wrote a pair of QEX articles on receiving loops a few years ago). Douglas Miron’s book, Small Antenna Design, contains a lot of theory, but also good intuitive explanations for why small loops tend to work better than small dipoles and small verticals. (His explanation is that the efficiency of all small antennas depends primarily on their Ohmic losses; loops need a capacitor to resonate while short dipoles/vecticals need an inductor, and capacitors are a lot less lossy than inductors.) The programs loopcalc and capcalc, available from Glenn KI6GD, are very useful for estimating the behaviors of magnetic loops and for computing the capacitance of simple plate capacitors.