Single Turn Loop Antennas
Table of Contents
- What are we talking about...
So enough theory, what kind of loops are we describing here. As you know, if you have read the literature or taken a casual tour of the internet searching for information about loops, there are "alotta" loops out there!
Here is the typical "specimen" that I will be describing:
- a single turn, full wave length wire loop antenna cut for the
design frequency (1005/Mhz in feet),
- opened up to its maximum diameter (maximum spatial interior),
- mounted either vertically, horizontally or diagonally,
- fed with either open feed or coaxial line driven through a tuner,
or a matching network, from the transmitter, and not operated
below the design frequency.
- Loop Shapes and Sizes
Like many things in life, loops come in all "shapes and sizes!" There are square loops, round loops, diamond loops, triangular loops, rectangular loops, and even polygonal loops: in short, almost any kind of shape you can imagine! Sizes can vary from a few feet across to hundreds of feet across.
But let's focus on the "normal" everyday practical loop designs, and these reduce to really just a few:
- the square or diamond,
- the rectangle,
- the triangle or delta-shaped loops.
These are the ones that can be conveniently suspended with pole supports and ropes. The delta and the diamond require only one central mast, while the square or rectangle need two points of vertical suspension. A plus for the delta and the diamonds is that the guying ropes can be part of the antenna itself, in other words, the wires can be attached to the sloping sides of the guy ropes. (And, they need to be ropes so that they don't electrically couple into the antenna itself...)
- Loop Directivity
What do you think of when someone mentions the loop antenna? I often think of the small loops on vhf beams, whether square or diamond in shape. Larger loops are rarely seen these days in full view since they usually blend in so well with their surroundings. (When was the last time you stumbled across a 160m horizontal loop?)
These observations could lead you to wonder why loops are in either the vertical or horizontal position. And the answer is, of course, directivity. A horizontal loop is similar to a vertical radiator in that its propagation is omni-directional. This is a nice feature of the horizontal loop. Remember, a horizontal dipole has a figure eight radiation pattern. It may not do too well off the very ends of the wire. A horizontal loop, though, has 360 degree coverage!
What about the vertical loop? Working at the design frequency, the radiation direction is axially through the "hole" in the loop, or at 90 degrees to the plane of the loop. Here is a case where the vertical loop and the horizontal dipole share a common directivity characteristic, the classic figure eight pattern, regardless of loop polarization! (Don't confuse signal directivity with wave polarization when considering the many combinations!)
So, a fixed-position vertical single loop will typically be working an axis, such as North/South or East/West directions. If rotated physically through 90 degrees, 360 degree signal coverage will be attained. (And, the vertical loop may either be vertically or horizontally polarized.)
- Multi-Band Operation
Perhaps the single most compelling reason to operate an HF loop antenna is its capability to work more than one band. Hams have been searching for that "universal," one-size-fits-all, antenna since Marconi first heard his brother's signal shot over a century ago! There is no such "Eureka" antenna! But, the loop comes pretty close, about as close as we are going to get, given the state of our radio engineering.
To achieve efficiency across the bandspread, the loop really needs to be fed with open wire line, ladder or windowed. A tuner box is a must! And you will probably need a balun, either a one-to-one or a four-to-one. If your tuner box has a balun built-in, then you can feed the ladder line directly from the box. Otherwise, RG-8 is a good choice for about the first 20-30 feet before switching to the balun to convert to the open line. (This setup is analogous to feeding a universal dipole which also may be cut for the lowest useable frequency and may be "resonated" through its upper harmonics to achieve multi-band operation.)
In a similar fashion, the loop may be resonated through at least 5 octaves worth of harmonics, and perhaps more, depending on its fundamental frequency! In general, the impedance "mis-matches" tend to be lower for the loop than the dipole, one reason among many that I prefer the loop. In fact, if you operate with lower power, you may be able to feed the loop with low-loss coax directly without "horrible" losses, although open line is usually preferred on HF.
The trade-off for the loop, and there is always a trade-off, is its relatively narrow tuning range, which means you will need to get pretty good at tuner "knob-twiddling." Perhaps, the last "bastion" of hands-on amateur radio!
- Loop gains...
Contratry to popular rumor, single turn loops do have gain! It is about 1.9 db on average. But here is an interesting case where the shape can influence the gain! The perfect loop or the circle has a gain approaching 2.0 db over a dipole. As that "donut" hole gets out of round, the gain drops off. So a flattened-out rectangle might not have any gain at all, while a square or diamond shape might be very close to 1.9. A delta loop design might be around 1.5 to 1.7 db gain, depending on how large the "interior" space is. (The more space, the more gain.)
Another very interesting phenomenon is that gain can increase with an increase in harmonic frequency and signal directivity can be effected too! Lets say, you are working a vertical diamond-shaped 40m loop in the 20m band. Normally the gain would be about 1.9 db through the "hole" of the loop. But, at this first harmonic, you might see a gain of 3-4 db off the sides of the loop! At 15m operation, this might approach up to 5-6 db off the sides... And, you don't loose the lesser "through-the-hole" propagation either. Your end result is a clover leaf pattern, but one that is not perfectly symmetrical.
Doug DeMaw once related an intriguing story about his 80m vertically sloping delta loop out performing his tribander on 15m. The normal propagation for the loop was in the North/South direction, but his 15m transmission was East/West, off the sides of the loop, not through it. He also credits this unexpected outcome to the very low angle of radiation of a vertically oriented loop, which is typically anywhere from 10-15 degrees. That is a low take-off angle and bodes very well for "big time" DX! (Above reference from the ARRL publication, a 1985 edition, entitled "W1FB's Antenna Notebook", written by Doug DeMaw, W1FB.)
What about increased gain for horizontal loops at the higher harmonic frequencies? Good question, and I must confess, I am not really sure. I typically operated my 40m horizontal loop at 20m, 15m, and 10m and experienced plenty of DX, but this does not constitute "proof." If I had to guess, I would say, yes, it does offer gain. However, in all my perusing of the literature, I have never read anything confirming this!
You may orient or position your loop in the horizontal, sloping, or vertical planes, or anywhere in between, or any combination thereof. (I once built a "horizontal" loop that was a diamond in shape. Part of it was sloping at a 45 degree angle and the other part was horizontal. It worked fine. I made many DX contacts with that "strange" looking antenna! Of course, 15m was very "hot" at the time.)
Typically, though, you should orient your loop antenna all in one plane. It just makes it easier to support or guy it in position. However, if you have space restrictions and you need to experiment with shapes, you may be "advancing" the science of Antenna Engineering. (So, don't hold back; give it a try!) In the words of Doug DeMaw, "... you never know how good an antenna is until you try it." (W1FB was a leading recent pioneer and educator in the field of Amateur Radio as well as an accomplished RF engineer.)
- Polarity: Horizontal or Vertical?
It should be noted that horizontal loops can only transmit in the horizontal plane. And that vertical loops can be fed in such a way as to either effect horizontal polarization or vertical polarization depending on how (i.e., where) they are fed. If a vertically oriented antenna is fed at either the bottom or the top, the polarization will be horizontal; if fed on the side or bottom corner, it will be vertical. If you are going to the trouble of putting up a large vertical loop, be sure to feed it to take advantage of the low angle of radiation mentioned above! If a delta, then feed it at one of the bottom corners. If a diamond, then feed it a mid "corner." If a square, then at the center of one of the vertical sides...
If it's simplicity you want, probably the simplest, all-purpose loop is the "The Loop Skywire" as noted in the ARRL Antenna Handbook, the 1997 Edition, pages 5-15 thru 5-18. It is horizontally oriented and requires four (4) vertical supports, poles or trees. I once built one that was cut for 40m and only up about 25 feet suspended from the branches of four trees. It was fed with a 50 coax cable and worked very well for many years as a general receiving antenna and DX "chaser," even working an Island off Northern Japan! (So even the simplest of loops can belie amazing transmission capabilities!) You might wonder how a horizontally polarized wire antenna can reach half way around the world. It just goes to show that you don't always need a vertical wave for the long haul! (Sometimes horizontal waves can have low angles of radiation too!)
- Polarity Re-Visited for Diagonal Loops
What about loops that are mounted in the 45 degree sloping orientation? These pose good research questions! It is well known that sloping dipoles have both horizontal and vertical components in their radiation patterns. How about loops, do they follow the same principle since the feedpoint influences the polarity of vertical loops? This is a very good question. It gets even more convoluted! Suppose you have a sloping diamond, or delta, loop and feed it at its base or top. If it were strictly vertical, this would produce horizontal waves. But since it is sloping, will it re-enforce the horizontal and possibly introduce some vertical waves? The same case can be made for a vertically feed (at the sides) sloping loop, will it emphasize the vertical, but still retain some horizontal components? And if so, how much?
This is why we (Hams) find antennas so fascinating! As simple as loops might look, there is a lot more to them than meets the eye! Perhaps the asking of these basic polarization questions will provoke some thought about the intricacies of loop antennas!
- The "Care and Feeding" of Your Loop
The nominal impedance for a typical single turn loop antenna is about 102 Ohms. It may vary slightly with height, orientation, or even location, but generally it tends to be a stable and reliable measure. This is an "odd" value and does not interface conveniently with standard cables. So feeding the loop can require extra care. What are your choices? Well, you can ignore the "slight" mismatch and use 50 Ohm coax as is suggested in the construction of the "Sky Wire" antenna. You will certainly need a "tuner box" or a transmatcher of some kind to match the transmitter to the coaxial line. But, you will be able to work all bands from the design, or cut frequency, right up to 10m.
NOTE : You cannot work a loop below its design frequency. For example, you cannot tune or work a 40m loop on 80m without terribly, and usually very unacceptably, high levels of SWR!
A neat trick that I have used in the past is to take two pieces of equal length coax and solder the shields together at the ends, using the center conductors as though they were twin lead or ladder line. This raises the impedance value of the line to exactly 102 ohms ( 51 + 51 ). Now you have a perfect match to the loop at the design frequency! If you move up to harmonic frequencies, then this no longer applies since the 102 is only good for the design (lowest useable) frequency. Is it worth the extra length of coax? Well, you have to decide... If you were building a 10m single band antenna, this might be a perfect solution! And, your coax is impervious to nearby metal objects such as gutter pipes and stack vents... (NOTE: you still need a tuner, or tuner balun combination, between your 50 Ohm transmitter coax and your "shielded twin lead" line.)
Another interesting "experiment," one which I have not done, is to take three (3) equal lengths of indoor, light-weight TV 300 Ohm ribbon line and solder the end pairs together in parallel, resolving to an impedance of 100 Ohms! Here is another way to achieve a dedicated, and nearly perfect, feed line match at the fundamental frequency. But, you probably will still need a coax-balun combination coming from your tuner box, and you will certainly loose the line's match as you move up through the loop's harmonic frequencies. Additionally, you should tape the parallel lines together every foot or so to keep the material together properly. Trade-offs, trade-offs; that is what it's all about ... (And, even though the current-carrying capacity of the line has been tripled, via three parallel conductors, you should stay below 100 watts on the TX's with this lighter ribbon line for safety reasons.)
Suppose you want to maximize power to the antenna. Then you need to feed it with open wire line (>450 Ohm), ladder line (450 Ohm), heavy duty TV twin lead line (300 Ohm), or even hardline if you can get it. Since loops have an inherent mismatch to the line, using open feed makes a great deal of sense since you will be minimizing your losses. If you can't bring open lines directly out of your tuner box, you may use low loss coax for a short distance, under 20 feet, and then convert to open line via a balun. This is often the best compromise between ease of use and loss of power.
Speaking of a tuner, the single turn loop places very little electrical "stress" on a transmatch or tuner box. You won't see high voltage "nodes" appearing on the line anywhere for the loop antenna. Recall it is a current "device," and the current is every where the same along the wire. The range of impedance variation is much narrower than say a collinear dipole for harmonic operation, so is the corresponding voltage range. You won't get arcing when you try to tune your 40m loop on 10m! This means the components that make up the tuner box do not need to be as rugged as for other types of electric field antennas, and could potentially be a good candidate for a homebrew project!
- Loops, and the Effects of RF Ground
One of the advantages of loop antennas is an almost complete lack of the need for an RF ground. Since the loop is a "current device," it has very little electric field interaction between the magnetic element and earth. This is not to say that there is no interaction, since it is obvious that a low horizontal loop will certainly have some small capacitance between the antenna and ground, but it is negligible and has a minimal influence on the overall behavior of the antenna at the fundamental frequency. (In a similar fashion, a vertical (single turn) loop will have almost no capacitance between its current element and earth.)
What does this mean for the station operator? It means a very simplified ground system! Many electric field antennas (long wires, and especially verticals) often require extensive ground radials or very conductive earth grounds. The loop lets you bypass all this fuss about ground! So, if you live in a rocky, sandy, dry location, or in an "elevated" urban location, the loop may be your best choice! Of course, you still must have a ground for safety reasons, for lighting strikes, or other electrical surges; but it can be a single point ground with just enough depth to ensure reasonable conductivity. Grounds are often a crucial, but over-looked, component in the optimum performance of electric field antennas. With a loop, this is one less concern you have to worry about. You get maximum performance with a loop every time regardless of location!
- Location, or where to put it
Single turn loops have a major "flaw" that often puts antenna enthusiasts off; they take up huge amounts of space! If you have the room, I can't see why a loop wouldn't be your first choice. But, most of us are limited in our urban or suburban lots, and we cannot wire them right up to our neighbors' property lines, although some have done this with "mixed" results. So we either have to use smaller loops or be very inventive. I heard a story about a man who "looped" his house around the second floor outside. It is claimed that RF (electric field) is at a minimum inside the loop due to cancellations, but I am not fully convinced of this. So, I don't recommend doing this since the magnetic field would be at a maximum!. (But, those who have done this, think it is a great and safe idea. Proceed with caution... )
If you happen to have a tower, a vertically oriented, lower corner-fed, 40m delta loop might be just what you are looking for. However, just because it is a loop, doesn't mean you are free from coupling problems! Normally, loops are impervious to their surroundings. Objects like tree branches, etc, are usually no problem for the loop. It just doesn't "see" them. However, there are cases where loops can couple into nearby metallic objects, especially if these objects are resonant near the operating frequency and somehow decide to "join" the loop either electrically or magnetically.
A while back, I received an e-mail from VE7TK, formerly VE7ASR, asking about his 40m delta loop. He had the apex suspended from the top of his nearly 50 foot tower in his back yard. After many impedance measurements with an antenna analyzer, he repeatedly found the loop to have a "mysteriously" low impedance of only 25 Ohms, a far cry from the typical 102 Ohms! I was, and continue to be, baffled by this phenomenon. My best "guess" to date, is that the tower is coupling into the loop, acting like a vertical antenna. I know that a typical impedance for this type of vertical is only about 25 Ohms. But how is it making this coupling? We are still trying to figure this one out! Nonetheless, it is a very useable and tunable antenna, continuing to provide many fine QSO's!
Another interesting story comes from W4MOS in Florida. He had been using a dipole for his 75 Meter phone nets and was wondering if a loop would improve his "reach." He had a nearly 50 foot tower with two tall, over 25 foot, flanking trees on either side, suggesting a possible delta loop for 80m. Somehow, despite lots of scrapes and bruises from climbing his "lower corner" trees, he managed to string his 80 meter loop with a feed from the top of the tower; and has not looked back since! Despite the horizontal polarity, he reported very good signal reports up to 600 miles, much better than with his dipole... I tried to persuade him to feed from a lower corner, but his logistics and "comfort level" dictated the tower top feed. (The antenna slopes at about a 30 degree angle and should generate some vertically polarized components as well.) This shows that not only can you fit a 40m loop onto your lot from the "standard" 50 ft tower, but with a little persistence and flexibility, you can push it to 80m! It is doable!
Less problematic supports, which most hams can utilize, are tree branches, or poles that they have put up themselves. Probably the "best pick" for a general high-performance loop system would be the 40m delta loop suspended from a tree branch or pole, or both. If you want to be sure that you are not coupling into anything metal, you could use PVC pipe with wooden "stiffeners" placed inside the pipe. On occasion, I have lodged 20-30 feet of PVC pipe up in a tree, extending the pipe over the top of the tree. Often this can give you enough height for your larger loop antennas, such as the 40m. And, as mentioned above, you can change the equilateral triangular shape a bit without sacrificing too much gain if you find you can't fit it in properly. You should also be at least 5-6 feet away from the ground at the bottom of the delta. If not, you can slope it away up to 45 degrees and this will get you away from ground level and still retain your vertical polarization.
- The "Q" of a loop
The "Q" or sharpness of tuning of a typical loop is quite pronounced. This is sometimes considered to be another "flaw" in loop operations. If you find yourself roving all over the bands, then you will need to retune the loop quite often. One way to reduce the "Q" is to use relatively thin wire. This tends to flatten out the response. But, if you make it too thin, you may have trouble with ice loading snapping your antenna! And, you may be limiting the power output needlessly.
At the opposite extreme are wires that are too thick! These would make a very efficient radiator, but the retuning could become troublesome for general coverage work. However, some loops are made of copper tubing, and sometimes fairly large diameter tubing! These loops have very narrow bandwidths! Time to get out that automatic antenna tuner! Sharpness of tuning, though, is not all bad. It can help your receiver's front end in the elimination of unwanted adjacent signals, making your receiver more selective. (As I have mentioned before, you have to decide what this particular antenna is going to be doing at your station. There is no "universal" antenna; they all have dedicated applications and it is up to you to choose the best "fit.")
- "... One good turn deserves another ..."
(You knew that was coming... :) This brief article has described the single turn magnetic loop antenna. It can be thought of as the simplest form of loop. There are, though, multi-turn loops, their biggest advantage being their smaller size. If you really want to use a loop, even inside, you might consider one of these multi-turn types. A quick entry from Google will certainly yield many sources of information. And, here is an article that caught my eye several years ago entitled "Honey, I shrunk the Antenna", written by Rod Newkirk, W9BRD, published in the July 1993 edition of QST.
You may also "compress" loops even more by using non-traditional engineering solutions. It is possible to fold back their sides, forming a kind of zig-zag, or pull the wires into the middle of the loop interior, forming a kind of "windmill" or "propeller" design. This can dramatically reduce the size of loop antennas without any compromise in performance! However, it is trickier to build, feed, and sometimes operate, than the normal full size loop, probably not a first-time antenna project. Here is an article that can point you in the right direction: entitled "The Pfeiffer Quad Antenna System", written by Andy Pfeiffer, K1KLO, published in the March 1994 edition of QST. (It should be noted that you probably give up multi-band operation with this class of loop.)
- Concluding Observations
If you have the space, loop antennas are a great "buy" for the time and money invested in them, repaying your efforts many times over! Whether you hoist up the basic "Skywire," or go for that big DX vertical delta, diamond, or square; loops deliver! I can personally attest to this. After my first few early dipoles, the next series of antennas that I built through my hamming "career," were mostly loops. I had, and continue to have, great success with loops of all "shapes and sizes!"
(For more hands-on information about calculating loop dimensions, see The Loop Calculator Page for practical construction details.)