HF Antenna Types Antennas started out as long wires, sometimes held up by kites. Next step was the classic Marconi setup, an antenna and a grounded counterpoise, mounted vertically or horizontally. You couldn't get too much metal into the air. You've seen the old pictures of several horizontal wires, the more and longer, the better. The antenna was regarded as the aerial half of a "condenser," the ground making up the other plate. They were sort of right. The modern vertical antenna, the Beverage, and many of the other designs we see, are still pretty much all grounded and/or terminated Marconi antennas. Most of these can be mounted very low to the ground, or even on it. In fact, if very conductive soil or salt water is available, this is the best place for achieving acceptable radiation angles. Hence the tendency for AM broadcast stations to favor flat, swampy areas. Other designs, however, have their own counterpoise. Typical is the wire or metal dipole, which works best when suspended completely free of the ground. Most rotatable "beam" antennas are arrays of dipoles with different phasing arrangements, so these, too, work better high off the ground. Finally there's the ground-plane vertical and all its variants, where a metal screen or set of radials stands in for the missing ground. These antennas are common on CB. But all this is about transmitting. What about listening? Well, the "reciprocity" principle means that the effects described in antenna textbooks for transmitting will similarly affect receiving. For example, an antenna designed to increase signal in a given direction will also hear better this way, but will have to be turned or phased for maximum signal from all other directions. All but short, active antennas are supposed to use the resonant tuning so well investigated by people like Tesla, where the antenna length bears at least some relation to the wavelength of the desired signals. This means that HF antennas are rather large. It could be worse. Some military VLF antennas have to be several miles/km long, and towed by aircraft. On ELF, antenna farms can be the size of small countries. Resonance, of course, brings up the first buzzword people use on beginners so they'll keep thinking antenna design is a black art. This is the dreaded "Impedance." Well, impedance is a special sort of resistance that takes into account the behavior of alternating currents, but that's not on the quiz. For us, it's just a measure of how efficiently radio waves are transferred from the antenna to the radio. If the numbers match, this is good. If they don't, this is bad, but not as bad as it would be for transmitting, when things smoke, burn up, and smell bad. Of course, the typical hobby listening antenna is usually too low, too short, or just too random to make much use of resonance. This is not a problem, but it can be an annoyance. As you go across HF, about a 10 to 1 frequency ratio, both the receiver and the antenna make huge peaks and dips in their impedance (impedance for us being basically a measure of how efficiently the signals are transferred into the radio.). Thus you'll have some frequency bands where the stations leap out of your speaker, and other places (usually your favorite band) where they vanish into black holes. Enter such matching devices as baluns, coaxial sections, or antenna tuners. The idea here is to tune out the impedance mismatch with various resonant circuits until the radio and antenna understand each other and can proceed to have a good dialog. Kind of like people. So here's where the laws of physics kick in. There is no free lunch, ever, with antennas. There's no black magic. The tuner can't increase the performance of the antenna. All it can do is make the lousy frequencies sound more like the good ones. Now the balun, which stands for BALanced-UNbalanced, is not only a matching transformer but a converter from unbalanced (Marconi) to balanced (counterpoised) antenna designs. For example, the dipole is a balanced antenna, or at least it's supposed to be, but most decent radios use unbalanced coaxial cable with a grounded shield side to cut noise. While no law (including those of physics) says you can't just connect the coax shield to one side of the antenna, this means that it's not a dipole any more, but a kind of a funny bent Marconi, with the coaxial shield very much part of the antenna, and so you get back some of the noise. An aside: how can the shield be grounded to the radio, and then to a water pipe or whatever, and still be part of the antenna? Skin effect. The common-mode noise on the outside of the shield goes to ground, or at least it's supposed to, and the RF on the inside doesn't. Neat, huh? Baluns, though, have also become a bit overrated. Some wire antennas, like the T2FD or the Beverage, need special ones with a 9:1 matching ratio. Since these are pretty darn good antennas, they hear like champs, and their baluns get too much of the credit. Like the tuner, it's not a magic snake oil. Also, there are good baluns and lousy baluns. Lousy ones, usually those with poorly-designed ferrite cores, don't do anything except enrich the company selling them. The only other antenna mumbo-jumbo is the decibel (dB). This is a ratio. That's all it is. One Bel, named for Alexander Graham Bell, a very bright man who started the National Geographic Society, endowed several schools for the deaf, and by the way invented the telephone, is a 10:1 ratio of anything. Your taco could be 10 dB, one Bel, hotter than the mild one, in Scovill heat units or however they measure these things. You would have a ten dB taco ratio. Your taco could be advertised on the banner out front as a 10 dB taco. (This could be a tacoBel, but I am not going there, nor did I make this one up, so don't flame me!) You can see why spec writers are in love with dB. It creates the impression that they know something. Often, that's about where it ends. For hobby antennas, the published gain spec means that the advertising department has asked engineering how much Jalapeno its taco has, and they've sent back a number based on calculation from a theoretical isotropic source. An isotropic source, now, is an antenna with absolutely no direction. It's so wretched that it only exists in theory. You can't build one, because even the structure holding it would radiate/hear better. The closest electrical analogue is a light bulb. Light bulbs don't hear much, though actually it is possible to transmit a radio signal on one. This is the only "antenna" that will ever show a LOSS relative to the theoretical isotrope, since quite a bit of the RF goes to light the bulb up! That's right. You'll always see your lousy antenna in the dark. Airplanes won't hit it. If the gain spec shows the number in dBi (Taco Bells to an isotrope), this is what's up. Sometimes you see dBd (tacos to a real dipole in "free space," whatever that is), but that's usually for transmitting. At least one radio magazine, QST, makes the advertisers cover up the gain specs (sometimes with a magic marker). I would regard any dB figures for antennas as a logarithmic quantity indicating how badly a company wants you to buy their antenna, unless it's verifiable from a real-world test range. My own personal bias is toward wire antennas for listening. If you have the space, they get the most bang for the buck. They get a lot of RF voltage going, and they can be extremely simple and inexpensive. The only drawbacks are size, and sometimes noise. Here are descriptions of several of the hundreds of antenna types. Antennas are the most fun part of the radio hobby. Good luck, and watch the power lines! ================================================================== Active antenna- Any of several types of short spikes, coils or loops that use sensitive RF preamps to make up for the tiny amount of signal, weak voltage fluctuation really, that gets picked up. These can vary from horrible to pretty good, and can achieve a huge saving of space. The laws of physics continue to assure that nothing beats a nice wire, but the gap is becoming ever smaller. These are receive-only, of course, with transmitting having expensive consequences. Adcock array- A four-antenna setup used for direction finding by comparing signals at the different points. Unlike a loop, it need not be rotated. Antennas can be anything from verticals to huge rhombics. Beverage- You'll think I'm drinking same when I describe this antenna, but it's a real design, named for its inventor. It's a terminated, Marconi longwire for the lower frequencies. The good news is that it can be made of just about anything that conducts, and works best near or on the ground. The bad news is that the best ones are at least a mile (1.6 kM) long, and preferably way more than that. People in rural areas, where these antennas can be run atop long fences, and so on, have heard standard, commercial, AM broadcasting stations from halfway around the world. A recent QST described a smaller Beverage for HF. It was an interesting article, and the antenna could actually fit a very large back yard. DDRR- Stands for directional diversity ring radiator, a special case of the horizontal loop that makes a very efficient, low-profile antenna perfect for car roofs on VHF. A bit large on HF. Delta loop- A three-sided wire antenna, usually mounted vertically, often in pairs to achieve gain. They are popular on the lower end of HF, as they outperform dipoles without needing huge spaces. Dipole- Simple wire antenna composed of two tuned segments, usually connected to the radio in the middle, often through a balun, an balanced/unbalanced transformer, since a dipole is a balanced antenna and coaxial feed systems are not. It's possible to think of the dipole as two tuned monopoles in doublet configuration without ground. It's possible, but confusing. It does, though, explain why it's also called a "doublet." Dipoles get a bad press because they are hardly ever strung high enough to get optimum performance either listening or transmitting on HF. A dipole in what engineers call "free space" is actually a pretty good antenna, with a low pickup angle and a figure-8 pattern slightly favoring signals arriving perpendicular to the wire. A sharp null appears at either end, which is sometimes useful for getting rid of nearby interference. Some very good DX is heard on simple dipoles. Unfortunately, free space on HF requires very tall mounting supports. The whole antenna has to be a certain fraction of the lowest wavelength away from anything conducting electricity - other antennas, buildings, even the ground. This works out to something like 300 feet/100m at the low end of HF - ouch! And so the dipole's performance degrades, with more of the pattern going straight up (good for space men) and less efficient a match. At some point, loops or vertical antennas become better uses of the same labor and money. Discone- A wideband antenna consisting of a metal disk atop a cone, which achieves a 10:1 frequency bandwidth, say from 3 to 30 MHz. This is a real interesting antenna. The cone resonates the disk, using more and more of its surface until finally the whole cone is involved. Above or below the cone's bandwidth, the performance degrades. While the discone has been described as an antenna that performs equally mediocre on all bands, this is in many cases better than one that works real well on one band and not at all on the others. Discones are real hot scanner antennas on VHF/UHF, they are also popular with the military, and we even see wire ones on HF, often folded to reduce the otherwise great height. Dish- HF dishes, in which a parabolic reflector concentrates great signal levels onto a dipole or voltage probe mounted at the focus center, are possible, but prohibitively large. Typically, HF radiotelescopes use phased arrays. Fan dipole- Several dipoles cut for different frequencies, fanned out from a common center feed point, thus saving space and complexity. Folded dipole- This is basically a dipole that has been folded back upon itself, using parallel wires with insulated spacers. Not only does the antenna become shorter, but its impedance changes, often for the better depending on application. Most people know these antennas from the little twinlead dipoles provided with consumer FM tuners. On HF, the folded dipoles are very popular with the military, due to their ruggedness, portability, and adaptability. They can be hung from flagpoles as an inverted V, stretched out straight, or sloped, depending on the tactical need of the situation. The classic military dipole is by Barker & Williamson, and you can buy one too, for a price. Fractal antenna- I don't know much about these except that they exist. A fractal is a self- similar curve. By folding back on itself in a manner that replicates the original curve in ever smaller scale, it concentrates more surface area in less space than was originally thought possible in this universe. In plain English, fractals grow to incredible lengths, without necessarily getting any bigger. This means you can get a hell of a lot of wire onto a small circuit board, a "smart card,' or whatever your restricted-space application is. The wire interacts with itself, of course, and so you don't really get a fractal dimension on RF. This is not a Star Trek subspace radio, but it works, up to a point. Helical antenna- There's suddenly a lot of interest in this design for HF. The more common use we see every day is on VHF radios, where a coil inside a flexible casing makes the popular "rubber ducky" antenna. However, several designs for HF antennas wound on broomsticks or inside concentric, plastic pipes have been shown to receive extremely well, with a better signal/noise than wires. Both active and passive designs exist. You can spend a fortune, or roll your own from plans on such Internet sites as dxing.com. Inverted L- Simple wire antenna popular for shortwave listening in a limited space. The several variations all share the basic, and well named, shape of a wire running vertically up a wall (or whatever) and then horizontally to a remote support. Main advantage is the large signal pickup area (aperture), and extremely low cost. Inverted V- Descriptive name for an upside-down vee, really a drooping dipole where the ends are lower than the middle. If done right, the inverted V saves space while improving on the low dipole's angle of radiation, wasting less signal. The pattern begins to resemble that of a vertical, but without needing to bury radials. It also lowers the feedpoint impedance of the antenna to something closer to the 50 ohms desired for most equipment. However, it's tricky to get all this right, and most people don't. Inverted Vees are popular for multi-antenna installations where a rotatable yagi or log periodic is mounted on a tall tower for the high end of HF, and a long Vee is hung with this tower in the center. I used to have something like this for 80 and 40 meter amateur, and it worked out very nicely. J-pole- Descriptively named vertical dipole with a tuned stub at the bottom, more popular at VHF than HF, where these get rather large. Log periodic- Antenna typically using many phased elements of a gradually decreasing size to achieve gain over a wide frequency range. It's hard to explain, but the clever phasing arrangement makes only a few elements actually contribute at any one frequency, but the many different lengths mean more frequencies with real gain. The standard TV antenna is a VHF/UHF log periodic. HF log periodics are sometimes rotated on large booms, or fashioned from wires between poles. The TCI company makes some real killers for HF broadcasting stations. No more going outside in the snow to throw antenna switches. Long wire- Technically, a wire antenna so long as to lose all phase cancellation effects on a desired frequency, becoming something of a traveling-wave antenna and getting awesome signal/noise ratio via this feature as well as through its enormous signal pickup aperture. At HF, though, this is very long indeed, sometimes thousands of feet/ Hundreds of meters. Shorter wire antennas of random length, while Usually called longwires, are technically random wires, because they are still showing resonant peaking/cancellation over the HF spectrum. Often, an antenna tuner is used to improve HF randomwire performance by filling in the antiresonant dead spots. It does nothing for the peaks. Loop- Basically a loop of conductive material, resonant at or near the desired frequency. They can be made of wire or, at higher frequencies, metal hoops. Vertical loops have a sharp null (lack of signal) at their short sides, making them popular for radio direction finding. The same null makes the loop a good choice for DXing crowded AM broadcast bands, as it can get rid of unwanted stations from other directions. (Nothing gets rid of stations in the same direction except an anti-radiation missile.) Old AM loops were air core, with many turns of wire on the large wooden form seen in radio museums. Today, they are wound on ferrite rods or toroids (donuts), and remain very directional. Loops don't have to be round. More practical for HF listening is the horizontal loop, which is a full-sized loop antenna all the way around a backyard, or wherever. this gets a lot of wire into the air, with very little directionality, making a good listening antenna. Noise antenna- These were real popular on VHF mobiles in the tube era, when car engines were much noisier than now. The idea is that a second antenna is optimized for noise pickup, as opposed to signal pickup (voltage probes are perfect), and this noise signal is combined out of phase in the receiver to create a filtering effect. This has been tried on HF, and it works like a charm, but it remains a design road not taken. Phased array- Historically, these were multiple verticals, spaced and phased by different-length feeders, to achieve either broadside or end-fire gain. AM broadcasting stations still use these, due to the enormous antenna sizes required. A newer type of phased array uses many small antennas mounted in the same plane, and fed by precise, often computer-controlled, networks of delay lines. On HF, these are popular mostly for research. Finally, it's perfectly possible to phase vees, loops, yagis and log periodics on HF. Smaller arrays can still be rotated, and they look cool. Quad- A four-sided loop antenna, with a balanced feed at the bottom. Quads can be arranged in parasitic arrays on the same boom, like yagis, in which case they perform better, though at a considerable cost in size, and general maintenance hassle, especially in icy climates. The quad is better transmitting antenna than the yagi, especially on the higher end of HF, where it's a practical size. 3- and 4-element quads are popular with CBers and freebanders who want to get serious about radio. If you see a vertically polarized quad (diamond shape) on HF it's probably one of theirs. Horizontal (box shape) is used by hams. Quagi- A quad with a square reflector and several directors, combining characteristics of a quad and a yagi. Popular on UHF, but a bit large for HF. Random wire- The typical shortwave hobby antenna, just a wire stretched out until there's no more room, thus a random length. Wire antennas get lots of signal. Sometimes a simple tuner/preamp helps these, other times it just increases noise. Rhombic- Large wire antenna, shaped vaguely like a diamond, and hundreds of feet/meters long on lower frequencies. Typically suspended from telephone poles, and capable of awesome signal gain on HF, especially when terminated. Old rhombics were matched through ominous coils, preferably with a fire extinguisher nearby, as these coils could really heat up at higher transmitter output powers. Full, global, multi-frequency coverage requires many acres/hectares of rhombics, but if one has the real estate the antenna still delivers surprising bang for the buck, easily outperforming yagis or just about anything else short of Sterba curtains. The rhombic is also a good listening antenna, sometimes allowing a layer of stations that you never knew were there. Four rhombics in an Adcock array made up a very nice, if very large, direction finding setup, though Wullenwebers are used for this now. Rubber ducky- A coil, usually wound on a vertical pipe, whose name comes from the flexible antennas popular on VHF walkie-talkies. Yes, you can make an HF rubber ducky. I saw one wound on a 2-liter drink bottle once. With a lot of attention to matching, and stern words to all bystanders about getting too close to the thing, you can even transmit. But, with some really cool loaded verticals available, why would you want to? Sense antenna- A sense antenna is one, usually an omnidirectional vertical, that is combined with the signal from a sharp loop to resolve the otherwise ambiguous double null that is created in direction finding. In other words, it means that your bearing is the right one, and doesn't have a 50/50 chance of being 180 degrees off. Sloper- A dipole arranged with one end near the ground, and the other one higher than it would otherwise be. This lowers the dipole's otherwise high angle of radiation at any practical mounting height, and and can also sometimes change the impedance match to something closer. Four slopers around a pole or typical amateur tower give a nice, directional setup when the desired one is connected. Such arrays are common, though they need a lot of open space to work well. Sterba curtain- Enormous, complex, stacked, phased array, often made unidirectional with a mesh reflector, and usually hung from two or more very large towers. Antenna gain on HF rivals that of rhombics, in less horizontal space (vertical space is easier to come by). Curtain antennas are very popular with super-power HF broadcasters. T2FD- A wire antenna vaguely resembling a sloped, folded dipole, but fed through a special balun transformer. This is a good listening antenna, because it tends to reduce some of the noise relative to simpler wires. A Dutch company makes a very nice, and very expensive, listening setup which is an active T2FD on steroids. Trailing wire- Aircraft have used these on HF in the past. The classic has to be the seven-mile version that a Navy TACAMO (Take Charge And Move Out) command post airplane can unreel to allow VLF transmissions to submarines - while the subs are under water! (Water, especially salt water, shorts out radio waves really well, except at ridiculously long wavelengths.) They don't use this long version over land, for obvious reasons, though in a nuclear war they would. They figure that not too many people would care about falling wires at that point. Vee- Basically a half-rhombic, with two wires arranged in a V shape and either aimed in the desired direction or mounted vertically for omnidirectional coverage. Vertical- Any of a class of several antennas that use a tuned wire or pipe oriented vertically to the ground. The simplest is a grounded Marconi, a half- or 5/8-wave monopole with a counterpoise of radial wires buried in the ground. On the low end of HF, the antenna becomes more efficient, and easier to tune and match, as the counterpoise grows, often into acres of buried, expensive copper. At the high end of HF, "ground plane" verticals become small enough to use metal radials and take their counterpoise up the tower with them. The vertical has the seemingly impossible ability to achieve gain in all compass directions at once, explaining the ads for "gain verticals" in ham radio magazines. It's not black magic, just the result of the antenna's vertical orientation, allowing the beam width to concentrate nearer the ground (where the people are) and less in space (where they don't use HF much anyway). When everything's perfect, the vertical can achieve a very low beam, giving nice DX reception all directions at once. Better still, the thing sure takes up less space than a field of rhombics. Unfortunately, unless you live on a gently sloping hill of salt water, it'll only be absolutely perfect on a small boat in the ocean. Worse, it'll only be just right on a narrow band of frequencies. This is not a problem for the local AM station, which never changes frequency, or at least better not. It's a problem for us, though. Various traps, coils, and top-hats are often added to verticals, but it's still not as broad an antenna as some of the alternatives. Worse, HF verticals see an effect from soil conductivity, for several miles around the antenna in some cases. The counterpoise won't change this much. In sandy soils, the vertical deserves its reputation in this hobby as not a very good antenna. Many people will also tell you that the vertical gets more powerline noise. I haven't found that to be the case here, but I can't speak for everybody. Voltage probe- Simply a metal spike stuck into the RF field, the way you'd stick a wet finger into the wind. The feeble voltage fluctuations thus picked up are typically sent to hot FET preamps/ matching circuits, or used to this advantage by the hot front ends of modern radios. The whip antenna on the typical consumer "World Band" receiver is most definitely a voltage probe, as are small, active antennas marketed under such names as "World Probe." Decent voltage probes can be an alternative to hanging 15 feet/ 5 meters of wire from hotel windows. :-) Windom- Off-center fed, broadband, wire antenna named for inventor Laurent G. Windom, who described it in a 1929 QST. The original Windom used a single-wire feed and a ground counterpoise, developing an impedance that remained near 600 ohms on both odd and even harmonics. In other words, it was cut for the lowest ham band you wanted to use, and all the other harmonically-related bands fell into place. Modern "Windoms" are usually 2-wire coaxial feed through a balun, and without a ground counterpoise. This makes them more like Zepps. While the "Windom" name is wrong, it often sticks. Wullenweber- A circularly disposed antenna array, used for direction finding on HF, that was invented in World War II. From a distance, one sees this huge, round collection of poles and wires, the low-frequency end of the array. Closer up, the rows of smaller wires and spikes come into view. From the air, one sees something that looks very much like Stonehenge, and in fact Wullenwebers are among the easiest man-made objects to spot from space. The antenna is sometimes called an "elephant cage," though the scale more resembles a Godzilla cage. These are very popular with the government and military for High Priority Intercept, real spook stuff, and the classic one is at Imperial Beach, CA, where the US Navy trains crypto types. It replaced a rhombic farm, four of them in Adcock config, but actually it's not all that much smaller. It's visible 10 miles away. Directional bearings are obtained by comparing signals as they enter the different segments of the circle. Yagi- Directional antenna, really a Yagi-Uda, as it is named for its two inventors. It uses the clever principle of parasitic excitation, where one electrically hot dipole is phased with a passive reflector and any number of passive directors. All these are dipoles of just the right electrical size and spacing to achieve gain by narrowing the pickup and radiation beam and concentrating on one direction in all 3 dimensions. Yagis are thus often referred to as "beam antennas." Like log periodics, they can be mounted on a rotating boom or strung with wires. Yagis are popular because of their size, with a chief disadvantage being the very narrow bandwidth. Like dipoles, yagis have to be away from conductive objects to work as well as they did when measured on the antenna range. The 50-60 foot (20 meter) metal tower with its multiband yagi on top has become a symbol of ham radio. For shortwave listeners, the yagi makes sense mostly as a second antenna for one band (say 12 MHz marine boating) if and when you need to hear absolutely everything out there. Otherwise, it's probably too narrow. Zepp- Off-center, wire antenna with good frequency coverage, named from its early use on Zeppelins. Various types and phasings exist, all the way up to the awesome "extended double Zepp," a very large antenna array pair with bandwidth approaching that of the discone and gain approaching that of the rhombic. # # #