Magnetic Loop Antenna for Restricted Areas
In the past few years there have been several articles in the ham magazines about building multi-turn loops using small diameter wire and regular tuning capacitors. There was even one where the author said that a loop made from one turn of wire, using standard tuning capacitors and of small diameter, was all the average ham needed to have to work the world. Wrong!
Welcome to the world of misconceptions and misinformation. It isn’t that simple. If it were, we would all have loops up instead of anything else. The misconception here is that the authors of such articles are not considering that with the state-of-the-art receivers now available, anything that can radiate will be heard somewhere. As far as a one-turn loop consisting of a short wire is concerned, an efficient radiator it is not. There is too much loss in the capacitor and resistance in the circuit, even though it is very short. Yes it will radiate, but I would be willing to bet the coax feeding the antenna is radiating a lot too.
As an example, consider a loop made from a 10-ft. length of 1/2 inch copper pipe. The following values are what will be found with an input of 120 watts, on 20 meters.
Radiation resistance = .2695Ω
Loss resistance = .0745Ω
Q = 383
Current = 18.6A
Voltage = 9.741 kV
Now that is a lot of current and voltage on an antenna. As you can see the Q is very high, which is why these values are so high. Continuing on, the bandwidth is 36.556 kHz. Now these figures were obtained from a special program written to calculate the needed values for construction of a loop. Running this program using a piece of wire as a small loop, results in the program saying this particular loop is too inefficient to calculate the values and it stopped. So, the use of wire is definitely out in the construction of a loop. It is just too small in diameter, which makes the loss resistance excessive. Besides, the large amount of current flowing in this antenna indicates the need for large conductors.
In the March 1968 QST (See Below), there was an article on constructing a loop design and the results obtained when it was put on the air. The results were miserable since the persons that built the loop didn’t have enough information to do it correctly. So the antenna did not work well at all. There was simply too much resistance in the circuit for it to work correctly. The loop was made up using pop rivets to connect the flattened ends of aluminum tubing that formed the loop. This type of joint resulted in a loss resistance that went to the moon. It cannot be done that way, nor can it be done using the tubing obtained from a muffler shop. Copper tubing is the best way to go, unless you have a budget for silver or gold! Thus, even though the only two loop antennas on the commercial market are made of aluminum, copper is still much better because of its lower resistance.
The other problem is the tuning capacitor. Unless you have a vacuum variable, you will have a capacitor that will have a lot of internal loss due to the way it is constructed. If the stator plates of a conventional air variable are not welded to the supporting rods holding the stator plates, and the plates in the rotor are not welded to the rotor shaft, there is resistance present in the capacitor. The wipers will definitely contribute to the losses even more. Both the MFJ and the AEA loops all have welded connections in the capacitor and to the loop, which lower the losses greatly.
There is a way around the “lossy” capacitor, and that is the use of a split stator capacitor. These capacitors have no wiper loss when used in a loop as the two stators are connected to each end of the loop. The rotor is used to couple these sections to each other. This effectively places the two sections in series, but it also increases the voltage rating of the capacitor. However, the plates still must be welded together in order to lower the losses even more. Connecting to the loop must be done with solder and by using copper or brass strap. Then solder to the ends of the loop and capacitor to help ensure very low loss connections. Suitable brass straps can be fabricated from brass stock found in many hobby shops. These are just a few of the factors that diminish the efficiency of most small transmitting loops.
Now, for more facts. The original loop article was published in the August 1967 edition (Sorry Cannot locate) of Electronics and showed both the loop and the tuning box used. This antenna was designed to be carried into the jungles of Vietnam and assembled there and used to overcome some of the communications problems present in the lower HF bands. Since this loop was built for military use, no expense was spared. The antenna had gold-plated joints which made a big difference in resistance. Most do not have that sort of luxury and must make do with less-expensive solutions.
The loop is the ideal antenna for the ham in restricted spaces. The 10ft. loop, described in the previous example, will tune from 10 MHz to 29.5 MHz with no gaps in the coverage. A 40ft. length of copper tubing will make a loop to cover 80-30 meters and will be less than 15 feet in diameter. In addition, these antennas can be mounted right on the ground, no tower needed. Also, the loop radiates a polarization component that is parallel to a line tangent to the loop at that point. So if the loop is mounted horizontally, parallel to the surface of the earth, there will be no vertical component because there will be no vertical polarization component existing outside of the plane of the loop.
If the loop is mounted vertically, perpendicular to the face of the earth, the loop radiates at both vertical and horizontal angles, giving the benefits of both vertical and horizontal antennas. In this mode of operation, height above ground is not important and in fact the loop works better at ground level than elevated to any degree of height. However, if the loop is mounted horizontally, then the performance becomes height dependent, and the higher the better. Also, this mode of operation eliminates the directivity that the vertically polarized loop has due to the figure 8 pattern produced by the vertical loop. This can be used to advantage, as the loop can often be rotated slightly to place an interfering signal in the null of the pattern while keeping a decent signal level at the other end of the circuit.
One big difference between the loop and conventional antennas is that it is a magnetic-field antenna. What this means is that the loop is more sensitive to the radiated electromagnetic field from an antenna than it is to the electrostatic field. Now most people are not aware of the fact that an antenna radiates two fields, the electromagnetic and the electrostatic field. Another fact that many people may not know is that static, and most other noise sources we deal with, consist of 75% electrostatic energy and 25% electromagnetic energy. Therefore, a small transmitting loop will be quieter by an enormous amount over a conventional antenna due to the loop’s preference for the electromagnetic field. All magnetic antennas possess this characteristic which is a big improvement over a dipole or other type antenna. Not only does the loop receive the electromagnetic field better, it transmits the same field more efficiently. All of which adds up to an improved signal on both transmit and receive.
As a point of interest, back in 1968, I was living in Richardson, Texas and got on 75 meters every evening on 3.805 MHz for a few hours of “intellectual discussion.” One evening, one of the guys came on with an outstanding signal for 100 watts. He didn’t suffer much QSB during the evening, while the other guys were bouncing up and down like a golf ball on a concrete floor. After about a month, someone asked him what he was using for an antenna. He replied that he was using a loop. Then when asked how big it was, he said it was about 12 feet in diameter and was made out of 3-inch heliax salvaged from a TV station. No one believed him until one of the guys drove 100 miles just to see that loop. It was leaning up against the side of the house and was orientated east and west, which was why it put such a good signal into my antenna. I was due west of him as were most of the other hams.
The visiting ham reported that he could not hear any of the static that plagues 75 meters at night during the summer and signals just seemed to pop out of the speaker like a 2 meter fm signal. I don’t remember about the ultimate outcome of the loop and the ham that was using it because it was nearly 30 years ago. But I do remember that his signal was solid with very little QSB and was consistently at the 20-30 dB level every night. It cut through the noise like nothing outside of a 20 kW broadcast station could do.
Now, the bad news. At the lower end of the tuning range, the loop will suffer from an extremely narrow bandwidth. In some cases, it will be as low as 2 kHz on the lower bands of a small loop. This means if you QSY 20 kHz, re-tuning is required. This is due to the loop being a very high Q-tuned circuit. This requires remote motor tuning, unless you build a loop for only a narrow segment of the band you like. In the case of the loop example above, it will cover 600 kHz of the ten-meter band with little problem, but only 13.1 Hz of the 30-meter band. A semi-fixed capacitor would allow you to set the tuning for one part of the band while remote tuning will allow the loop to be placed in a favorable location, out of sight, such as in the attic of a wooden frame house. It will also remove the intense field further away from electronic equipment, such as TV sets and other home entertainment equipment. This will lessen the chance that interference will occur. If you must hide the loop in an attic, the efficiency will be enhanced with a ground plane equal to 2 times the diameter of the loop. So, a 3ft. loop would need a 6ft. diameter ground screen under it. This could be made out of copper window screen, not galvanized wire because it is too lossy. The object is to give the loop a reflecting ground plane, not a lossy one.
This antenna will not function well if it is covered by a metallic roof or surrounded by metallic structures such as may be inside an apartment. The best way to use it in this case is to stick it out the window as far as you can on a non-conducting pole and anchor it down. If you operate from an apartment, it will be necessary to experiment to find out if mounting the loop vertically or horizontally works best. Vertical polarization is usually best, but sometimes the loop will couple into the metallic frame of the building and make it hard to tune.
If the apparent complexity and other factors seem a bit fearsome to you, consider the cost of a tower, antenna supports and the antenna itself. The loop simply can be placed on the ground, supported by PVC pipe a few inches off of the ground and it will work very well. If you have a problem with restrictions on antennas where you live, the loop just might be the answer. It can be disguised by draping it with artificial plastic vines as long as the vines don’t have any wires in them and can be part of a garden area. Just don’t let vines grow on it. While there are many ways to conceal a loop, just remember it must be kept as far as possible from metallic objects, not unlike any other transmitting antenna.
This has been a brief description of the loop, what makes it tick and why others have not had very good luck with them. The best and ONLY source of information for amateur usage of the high-efficiency small transmitting loop is the Loop Book found for sale in the Shopping Shack. This book was written specifically to show anyone how to construct a small transmitting loop and have it work well. This book, together with the Transmitting Loop Computer Program, you will have enough information to design, build and tune up such a small transmitting loop. The program is entirely menu driven and with the required inputs, it will calculate the math necessary to build the loop antenna along with the detailed instructions in the book. Check it out, it is better than not being on the air.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
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Last Updated : 12th March 2024