My Skywire Loop Antenna
by Don Keith, N4KC
In we ham radio operators' continual quest for the perfect antenna system, we try some strange things at times, but often, the simplest is also the best. That is certainly the case with the basic "loop" antenna, an often misunderstood critter, but one that gives absolutely the most for each foot of wire of any antenna I have had occasion to play with. Many are leery. They are long, long, long...and copper is expensive, expensive, expensive. Who has room for over 500 feet of wire (in the case of a full-wavelength loop for 160 meters)? Won't that thing become the perfect conduit right into my house for lightning and other "wrath of God" type things? And will it work on more than one band? I don't want something that big in my yard if it only gets me a narrow swatch of one band.
First, let me reassure you that such an antenna does not necessarily take much room. One reason I went to one in the first place is because I didn't have room for a 260-foot-long dipole for 160 and I wanted to give the "top band" a try for the first time in my 45 years of being a ham. If you are talking 75 meters (and up if you want a multi-band antenna...more later on that), it's only about 65 feet on a side in a square arrangement. Yes, it could be a lightning attractor, but you should take precautions with any antennas, including this one. And, as noted, the skywire loop can be a fine, fine multi-band antenna that actually has impressive gain as you make it bigger or use it on higher and higher frequencies.
Okay, let's build one! Below is a rough diagram and some photos of my skywire horizontal loop antenna. It is roughly 475 feet long and fed with 450-ohm ladder line. It is a bit short--by about 100 feet--for being a full wavelength on 160 meters. That is for two reasons: I ran out of wire, and I ran out of space on my backyard lot. (The lengths of the rope supports are misleading in the diagram, by the way. The antenna is actually pulled snug up to each tree. If the copper stretches much, I will have a problem!)
Still, the antenna works quite well on all HF amateur bands except the bottom 500 kHz of 160 meters and a portion of 10 meters around 28.4 mHz. I have not yet looked at the SWR, but I don't really care. SWR is a grossly over-rated phenomenon...as well as being universally misunderstood. When fed with a very low-loss balanced feedline--such as the 450-ohm ladder line I chose--standing waves are almost all eventually radiated as useful RF energy and, so long as the system is tuned with a good quality matching device, the SWR--within reason--is no factor. See the W2DU web site for more on feedline loss, SWR, and related information. And for more on horizontal HF loop antennas, visit the W4RNL site. I waste some precious cyberspace on the subject with an article on this site, too.
This antenna, when fed with ladder line or one of its variations, is a true multi-band antenna. Yes, it could accidentally be perfectly resonant in some spots in the spectrum, and if that is the case, you could feed it with coax and run without a tuner of any kind and do fine. But if you intend to use it on some other frequencies in addition to those farflung resonant points, you will need to use very low-loss feedline and a matching device ("tuner") of some kind. A balun is also a possibility--a 1:1 current balun that will handle the power you intend to run. I am not using one at the moment and things seem to be working fine (no RF in the shack, no smoke from the antenna "tuner"), but I do plan on experimenting with one. There is a great article about baluns and their use with various antennas--including the non-resonant skywire loop--on the DX Engineering web site. Click on the "Choosing the correct balun" link near the picture of the balun on the page.
More on that later. First, let's talk about building this beast.
For best results, a loop should be as close to a circle as possible so as to have the maximum amount of square feet inside the perimeter of the antenna. A rectangle or square works almost as well, and are preferred by many because they only require four support points. However, you want to avoid a rectangle in which two of its sides are too close together. A long, skinny rectangle means that the antenna becomes more and more a folded dipole, with that type antenna's plus and minus points. As you see, my loop is a rather odd-shaped critter, and the truth is, unless you are setting poles or other structures to support the wire, yours may not resemble anything you studied in geometry. For some reason, Mother Nature was not thinking "skywire" when she placed the trees on my lot...and neither was I when we bought the place. I confess I even did some tree work (at considerable expense) to remove some that were in the way, but it was (in case the XYL reads this) work that needed to be done anyway. Of course, I made sure that the tree service did some trimming of lower branches that were in the way of my planned antenna. I even had them take some rope up the trees as they went and leave them secured. They were going up there anyway, so why not? At any rate, with the caveat mentioned already, it really should not matter what shape your loop eventually assumes. This does take some planning, some yard walking, and some measuring-tape dragging.
Let me interject here that it is possible to put a loop on many city lots that are far too small for the traditional dipole. As mentioned above, a full-wavelength, multi-band loop for 75 meters and higher frequencies would be about 260 feet long...but only 65 feet long on each side if it were arrayed as a square. Most of us have a 65-foot square somewhere on our acreage! Something that would work beautifully from 40 meters up would only be 32 feet on a side. See what I mean?
If you have room and enough wire for 1,060 feet of antenna (two-wavelengths for 160 meters), congratulate yourself and start hanging. If not, put up what you can manage. For best results, try to hang as much as you can. One wavelength--about 530 feet for 160--works very well for that band and all higher HF amateur frequencies--ten bands! Some even report decent performance by their skywire horizontal loop on 6 meters, but I would not plan on it.
Next consideration is the feed point. I will assume you will use ladder line, window line, or open wire feedline to get power from the transmitter to the antenna. In fact, unless you intend to use the antenna only where it is perfectly resonant and can use coaxial cable, I highly suggest using the open wire feeder. The Wireman, DX Engineering, W7FG, Radioworks, and a number of other suppliers offer various types at reasonable prices, or you can build your own quite simply.
There are several ways to make sure the feedpoint is both electrically and physically sound. Those same distributors each offer feed point supports that will do the job nicely, or you can make your own from plexiglas or other materials. I opted for one of the commercial ones and spent a grand total of $10 for it. Whatever you use here to help attach the feedline to the antenna should be crafted to take stress off both. The commercially made ones have a non-matallic "bolt" designed to go through the plastic insulation on window line and take the weight off the feedline and antenna wire. They work quite well.
Three views of the feed point of my skywire horizontal loop. Note the feed point support device that takes stress off both the feedline and antenna. Solder carefully!
It is a good idea to support the antenna at the feed point with a sturdy, UV-resistant rope. With whatever you use to attach the feedline to the antenna and with the feedline hanging down (as close to straight down as possible), you will have weight and stress at this point. Those distributors mentioned above sell rope designed for just such a use. It is worth a few cents more, believe me!
Be sure you use proper soldering techniques, with a good electrical and mechanical connection on both sides of the hanging device. Read up on the care and handling of ladder line, too. You don't want it to run too close to any metallic objects, such as a tower. Even a tree or roof tiles can de-tune the line if it they are wet. It is a good idea to twist the line every few feet, both to keep down wind movement and to discourage the stuff from coupling with something along the way.
There is some consideration on whether the feedpoint should be in the middle of a run or at a corner. It does make a bit of difference in your eventual radiation pattern but I doubt it is enough to become a determining factor. Feed it wherever it is the easiest and handiest. As you see in the photos and diagram, I'm at sort of the middle of a long run, but it's kind of a corner, too. Oh, well.
Next problem is how you are going to hang the long run of wire at each support point. I decided to use PVC elbows and T's, available at any Lowe's or Home Depot. I drilled a hole in the elbow and tied a short piece of rope into a small loop.
Support device I crafted for my antenna. I used PVC Ts and 45-degree elbows and a short loop of UV-resistant rope The object was to allow the antenna wire to move within the elbow/T when wind blew the trees around.
Then it is simply a matter of running the support ropes through the loop of rope on the elbow/T and tying it securely. I'm hoping the PVC will have less friction and won't wear out the insulation and copper wire. I like the idea of the rope being able to move within the holder as trees sway around the run of the antenna. You can probably use your imagination and come up with something as good or better.
At a couple of points, I inserted a bungee cord in the support line between the PVC elbow/T and the supporting limb. Again, I wanted to allow the antenna some "give" as the wind blew the trees around, yet maintain enough tension so the antenna would not sag.
Bungee cord hook-up I used at a couple of the tree-limb support points. The support ropes are actually tied much more securely to the hooks on the ends of the bungee cords than this photo appears to show. This allows the cord to stretch when the wind blows the tree limbs about, yet keeps tension on the antenna wire. Frankly, I have no idea how long the bungees will hold up to the elements. I will find out the hard way!
So far, so good. We have had some pretty good storms in the nine months since I hung this antenna and it is still up there and still working fine. Here in Alabama, I don't usually have to deal with ice, thankfully, as that could create a whole new set of concerns.
Of course, if you are using trees to hang the antenna, the hardest part is getting those support ropes across the right limbs. As already confessed, I had the tree guys hang some of the ones I needed, such as the one at the feed point near the house. For the others, I finally invested in the method I have heard other hams describe. I must say, with a little practice, I got pretty good at my "Zebco Antenna Launcher!"
The poor man's antenna launcher, designed to get a line over tree limbs, a fishing reel, a slingshot, and, in my case, a small socket from a socket set...nothing I would mind losing, though.. There are commercially made versions of this type apparatus but the whole thing cost me less than $15 at WalMart. Be careful! Even a cheap slingshot can propel a projectile a long way and cause it to hit very, very hard.
I found a Zebco fishing reel on sale at WalMart for $4. The slingshot was one aisle over and cost about $10 with tax. I used a fishing weight for a while, tied to the end of the line, and shot from the slingshot. Trouble was, it was difficult to find the thing in the trees, bushes, and leaves. I finally went with an extra socket from my socket wrench set as shown. It is bright stainless steel and it shows up much better in the foliage. Still, resolve yourself to losing whatever you shoot, and to having to cut lots of tangled line and starting over. Some recommend mounting the reel on a piece of PVC pipe or using an actual fishing rod. I found that if I take care and set the reel on the ground away from obstructions, it spools off pretty well. You can get a nasty bruise if the line snags and sends the projectile back at your noggin. Don't ask me how I know!
And that reminds me. Be careful! Even a cheap slingshot like this one can launch an object very far and very hard. Windshields, picture windows, and neighbor kids cost lots more to fix than you planned to spend on this antenna!
I was lucky with my measuring and ended up, once I pulled all the wire up from each support point, with almost exactly what I expected. There is not much room for error in my system since the supports are all pulled up almost snug to the tree limbs. There are a couple of points where I'm running through tree limbs and they keep the wire from being pulled as high as it could be. That could be rubbing off all my precious insulation, too. Too bad! I can't get up there to do anything about it. I'll make do with what I have.
Now, with the antenna hung and secure, the next problem is how to get the feedline into the shack. As mentioned, ladder line should not be run close to anything metallic, like gutters, towers, water pipes, or coax cables. By "close," I mean within at least six inches, and a foot or more to be safe. You may be lucky enough to be able to go right beneath a closed wooden window or have some kind of non-metallic feed-through scheme at your disposal. Maybe not, though. There are a number of ways of using feed-throughs, including at least one commercially made one of which I am aware, from MFJ. If you have a direct, unencumbered run from the antenna feedpoint to your ham shack, then bless you. I'm not so lucky.
My shack is in the basement, in a former den area that I have converted to my office, and the only practical way in is through a hole in the brick wall where the air conditioner condenser line goes. I have at least five runs of coax coming through that way already, too. And once inside, the feedlines have to go past air conditioning ducts, a chest-type freezer, a filing cabinet, and then through a hole in the sheetrock, alongside those same other coax cables and the telephone line.
My "temporary" method of splicing the ladder line leads to a pair of parallel coax cables to get the feedline inside the shack. The brass objects are called "butt splices." It also allows quick disconnecting when lightning threatens. There are better ways of doing this. As I say, mine is temporary...for almost a year now. The ladder line is NOT as near the ground as this photo seems to show. It is at least a foot away from the dirt.
Luckily I stumbled upon a method used by several hams. I took two pieces of RG-8 coax about twenty feet long each and taped them together, parallel to each other. Then I tied the two sides of the balanced feedline from the antenna to the two center conductors of the coax using the method pictured above (more on that later). Those two taped-together pieces of coax then came in the hole in the brick and into the house, past all that metal, but the shield of the coax protects it from coupling with the other stuff. The coax shields were not connected together at the ladder line end. They are, however, connected at the shack end and then attached to the station ground. This last point is crucial. Both coax shields are tied together and connected to a good station ground. You do have a good ground, right?
Purists will give you grief about this method. "Oh, you are taking 450-ohm line and attaching it to a 100-ohm line!" "That coax will be lossy! You'll lose many, many DB!" Not really. The impedances presented by a multi-band antenna such as this will vary all over the place, and this method will not really amount to much in the grand scheme of things. Yes, you should absolutely keep the run of parallel coax as short as possible since coax cable--even good quality, lower-loss types--will still introduce enough resistance to sap reflected power. But this is not a bad compromise at all. Mine works fine, so far as I can tell. Would a straight shot of low-loss, 600-ohm, old-fashioned ladder line be better? Sure! But all antennas are a compromise to one degree or another. And while I could be sitting around, wringing my hands, worrying about whatever could be, I'd rather be on the air, ragchewing, chasing DX, and talking antennas.
In the photo above, you see my current method of attaching the ladder line to the twin coax run. The brass gadgets are called "butt splices" by electricians. The shrink wrap keep the two splices from shorting. I can unscrew the screws on the ladder line end and drop the feedline to the ground for lightning protection. (Did I mention that having hundreds of feet of copper wire in the air can create something of a lightning risk? Take precautions! A strike can be miles away and generate enough of a charge on the antenna to do damage!)
I don't delude myself. This is not the best method of splicing the feedlines together. I will eventually change to a knife switch of some kind. I have already had the experience of sloshing around in rainwater, wearing only a pair of sweatpants and slippers, in the middle of a sudden, unexpected 3 AM thunderstorm, holding a flashlight in my mouth, trying to unscrew those little screws to disconnect the antenna from the shack. All the while, I was thinking about the fact that I was sloshing around in water, holding onto 475 feet of copper wire in the middle of a lightning storm!
Matching the Antenna to Your Transmitter
Okay, you have the feedline inside the shack. Now what? How in heck do you attach those two pieces of wire to the coax connector on the back of your radio? And whatever impedance the antenna system presents to the output of the rig, it probably ain't gonna be the 50 ohms the rig craves.
Well, first, it is a leadpipe cinch that with any all-band antenna, you will encounter a wide variety of impedances, as well as potentially unmanageable capacitive and/or inductive reactance. There will be standing waves present at almost any frequency you want to use, and your modern, transisitorized radio will balk at the mere thought of trying to transfer radio frequency energy to such a mismatch. A matching device is required, and typically something much more robust than the auto tuner found inside most of today's rigs.
The best choice is a true "balanced" antenna system matching device (usually referred to as a "tuner"), one designed to match a balanced antenna system such as the one you just built. Such tuners are available from several manufacturers or, if you are technically adept and have access to the right size capacitors and inductors, you can build one for yourself.
However, once again, you can compromise and still have a very effective antenna system. Several commercially available antenna tuners accept balanced feedlines and, using an internal balun ("balanced/unbalanced" transformer), can match to them quite effectively. Make sure it is a current-type balun, not voltage-type. You may also be able to purchase a commercially made balun and place it between the balanced feedline and the unbalanced (coax connector) output of your tuner. Be careful in selecting the tuner or balun, though, because it will need to be robust enough to handle very large mismatches and high standing waves. Loss is dissipated in the form of heat, and it is not good form to melt down a balun there in the ham shack! I would suggest that if you intend to run more than 100 watts, invest in either a balanced tuner or one that has an internal balun that is rated to handle the power you intend to use...plus some!
Some will advocate hanging a balun outside, right there at the feedpoint of the antenna in an effort to "step down" the impedance to whatever it is to a nice manageable 50 ohms, and then use coax all the way to the tuner. Not a good idea. First, the impedance of your antenna will be all over the place, not necessarily a nice 6-to-1 or 9-to-1 that the balun expects. And there will still be standing waves coursing up and down the coax feedline, making it nice and toasty instead of being radiated into space as we desire. See the aforementioned Mr. Maxwell's writings on this subject. A good quality, 1:1 current balun is a good choice, and it can simply be mounted next to your antenna matching device in the shack, then have a very short of run coax to the coax, unbalanced input on your "tuner."
There is another option and it is not a bad one. You can mount a matching device ("tuner") at the feed point of the antenna or close by and tune it remotely. Then coax from the matching device to the rig is not a bad choice. But you have to have a device that is protected from the weather. It also has to have power to it in order to operate whatever mechanical mechanism changes the inductor and capacitor values to arrive at a match. And if it is heavy, it will have to either be on the ground somewhere or supported up there at the antenna. There are some commercially made "tuners" that are designed to be used near the antenna feedpoint and may be worth investigating if you are so moved.
I use the MFJ-993B Intellituner, an automatic memory tuner, which features a balanced output and is rated at 300 watts. I have had no trouble with the internal balun becoming hot, and it works quite well over most of the band (see "Performance" below). Truth is, I have gotten quite lazy, and the advantages of having a literal "one-click" matching device has over-ridden any sacrifices that such a solution presents. Once I have allowed the 993B to find a match, it is only one click the next time I'm in the vicinity of that frequency and I'm matched to the loop, ready to make a call.
My antenna matching device, the MFJ 993B Intellituner. Though not the best choice for matching to a balanced antenna system, it seems to do a good job for me and my applications. They are actually a good piece of gear to have in the shack for anyone who enjoys experimenting with antennas.
Rating antenna performance is a dicey thing. We see advertisements for commercially made ham antennas that promise huge DB gain, but even if there is fine print, we really have to dig to see that gain is compared to the performance of a ten-penny nail or a piece of limp linguini noodle. On the air and in the review forums on the amateur web sites, the performance ratings are usually anecdotal: "I worked New Zealand on my first call and he gave me a 59 plus! It's the greatest antenna ever invented!" That may mean the antenna is working fine, or it may have just been good propagation, and that strand of limp linguini might have done almost as well.
Few of us have sophisticated test gear, field strength instruments, or an antenna test range, so we have to rely on our own experience, or compare it to existing antennas in our little antenna farms out back.
All that being said, here's my take on the skywire loop. I have to compare it to my two other current antennas--a G5RV (I will try to do an article later on this very popular but very misunderstood antenna) and a telescoping loop that can be deployed for any frequency/band I want. Both are pretty good antennas--and several dipoles cut to a specific band that are no longer in the sky. The G5RV does quite well on 40 and 20, compared to those previous dipoles (that's what I do...put up new antenna "B" and if it is dramatically better than antenna "A," then "A" comes down to make room for something else). The vertical has an 85-radial groundplane beneath it (see photos below) and seems to do a good job (it's a great QSO-starter...it's a World War II-era telescoping vertical...see photos below).
My World War II-era telescoping vertical antenna. That's "Maggie" in the left photo, helping me check the condition of the radial field--85 radials cut to be at least a quarter wavelength long when the vertical is telescoped out to its maximum height for 40 meters. That is a DX Engineering radial plate, an extravagance that I would highly recommend for anyone using a vertical antenna at ground level. It is not only effective but makes the job much, much easier than it could be.
The loop wins about 90% of the time. On 40, the G5RV and the loop are typically neck-and-neck, depending on where the other station is and what the propagation happens to be. They are relatively close most of the time on 20 but the loop is usually the better performer. On 17, the loop is stellar. Occasionally a DX station might fall into one of the nulls (as frequency increases, a loop will have nice gain lobes but also corresponding sharp nulls, too...see W4RNL's pattern graphs) and either the vertical or the G5RV will be better. The G5RV won't work on most of 75/80 or on 30 or 15. And certainly not on 160!
I don't care. The loop is spectacular on each of those bands.
There are a couple of problem areas. I can't manage a match with the 993B on the lower 500 kHz of 160 or a stretch of 10 meters. I have not taken a look at the reactances in these spots. I might be able to play with the feedline--either ladder line or twin-coax feed or both--and work it out, but I'm also afraid I will mess up some of the other areas where I operate much more.
Since the antenna is only up from 15 feet to about 35 feet above the ground, it is not a DX antenna on 160 or 75/80. I can, though, reliably work out about 500 to 600 miles on 160. I played some in the recent 160-meter contest and did fine at that distance. If I tried to reach out farther, I had a rough time making them hear me with my 100 watts and the contest QRM. I have worked Europe several times on 80 and 75 but it is not easy. I do get South and Central America often during the winter months but I suspect they are struggling with me. On the other hand, I do fine checking into the Alabama net on 3.965, and have had stations doubt my claim of 100 watts. And unlike a dipole cut to one end or the other of 75/80, the loop does just fine from 3.5 to 4.0 mHz.
You may have heard that a horizontal loop antenna is quieter than other antennas. That is absolutely true. Sometimes a quiet antenna simply means the thing can't hear, but in the case of the loop, it is simply the nature of the critter that it ignores a lot of both manmade and natural noise while still reaching out and grabbing RF signals quite effectively.
Bottom line: I'm on all HF ham bands with a single antenna (though I don't have 60-meter transmit capability) that seems to work very, very well on most of those bands. And except for a roaring case of poison ivy I got putting the thing up, it didn't cost me much at all.
The most expensive part of my loop was the wire. I bought it originally to be part of that radial field for the vertical, but decided I would save it for other antennas and "settle" for "only" 85 quarter-wave-for-40-meters radials. Truth is, more radials would have had practically no effect on the performance of the vertical! The loop is a much better use for that precious copper. The spool of 12-gauge insulated copper wire cost me about $60. That was over a year ago. It would be more today, of course, but you can watch for sales or alternate sources of wire. Counting rope, bungee cords, feedline, wire, and all, I have less than $100 in the antenna.
It took me most of a Saturday--working by myself--to build and hang the antenna. The feedline-into-the-house took a few more hours.
As you can see, I have become quite evangelical about the skywire loop. No, it is not a 4-element monoband beam, but I don't have to have a tower and rotor to use it, and I don't cringe with every wind gust ahead of a thunderstorm. I would encourage anyone to undertake a similar project, or to at least take a look at other information on the subject that is available on the Internet...much of it linked within this article...or in the various antenna books. Maybe start small with an antenna for 40 or 30 and up. But I promise you that you will be impressed with what that big circle or square of wire will do.
Oh, and feel free to let me know how you come out with your loop. I'm always glad to set a schedule and talk about the subject while we listen to each other's aerials. Look for me on the air, or email me at email@example.com.
(NOTE: The WWII-era vertical has been replaced with a Hustler 4-BTV, so no more having to climb a stepladder to change bands. The loop, however, remains up after three years, and is still a go-to for 160 meters. I also often use it for listening on other bands since it is so quiet. My high dipole and homebrew hexbeam are usually better for transmitting, but the loop still surprises me sometimes.)
Return to N4KC home page
Go to Don Keith's personal web site
[ close window ]