A good first antenna, and maybe your last; It’s that good!
This article will focus on the venerable dipole; the good old half-wave flattop and a couple of it’s cousins. It’s not a very glamorous antenna, doesn’t cost much, won’t rotate (usually) and doesn’t attract a lot of attention, but it’s the “gold standard” of antennas. A properly constructed and installed dipole will have unity gain, and that’s what every other antenna is compared to in technical discussions and advertisements. The two most commonly seen references are dBi and dBd. dBi refers to an “isotropic radiator”, one suspended in free space, away from any earthly influence including that pesky feedline. An impossible antenna, of course, but it makes a handy reference. A more realistic comparison is dBd or referenced to an ordinary half-wave dipole, tuned to resonance and suspended at least a half-wave above the ground and any nearby objects. You won’t see many antennas compared to a dipole if it exhibits less gain, only more. No manufacturer wants to claim credit for a 90 pound weakling. You will, however see many, many advertisements for just that, only the manufacturer will make all sorts or outlandish claims as to it’s performance. Just remember, if it’s less than full size it just ain’t going to perform like one that is. Here’s the basic configuration:
Nearly every ham, it seems, is hampered by inadequate space for a decent antenna. I know I am. Living in a somewhat larger than average subdivision lot, within the city limits, room to stretch a full size dipole for 80 meters just about maxxes out my lot. For a while, I did what many hams do with limited space; I put up a vertical antenna. The trouble with a vertical is, if it’s a quarter wave radiator, you need to give it something to “work against”. They work great on a sail boat because it’s working against salt water but in a typical city lot, the earth beneath the antenna might as well be glass. You overcome this problem by installing radials; copper wires about a quarter wave length long and lots of ’em. Now we’re back to the space problem…if you have room for 100 or so quarter wave radials, you have room for a dipole. There are verticals that don’t require radials, but they are usually 5/8 wavelength or half wave radiators. (wait! that’s a dipole standing on end!!!) Now you have a different kind of space problem; can you install your antenna sticking up in the air 130 feet for 80 meters? No? Neither can I.
A dipole, installed a half wave up in the air will be directional. It will radiate, and receive signals with maximum strength in a direction at right angles to the antenna. In other words, if your wire runs north-south, the maximum radiation will be east-west. Your location might determine the orientation, since it is the lucky ham that has room to run a dipole in any direction. For field day operation in northern California, I like to orient the antenna east-west, that way I can have the best signal on a 3,000 mile path to the east coast. If I were to orient it north-south, I would do well along the west coast but I might have a null off the end of the wire, in effect blacking out 80% of the country. If your lot is big enough to give you a choice, this is food for thought; if not, then you may be stuck with what ever you have.
If your antenna is not installed a half wave up in the air, let’s say only up about 30 feet on 40 meters, then the directional characteristic will be less noticeable, and orientation will be less important. Another thing that affects directivity is whether or not the antenna is mounted on a level plane. If your dipole is suspended by the center, and the ends are quite close to the ground, it may still work very well, but directivity may be quite low. Not necessarily a bad thing. Antennas installed like that are commonly referred to as “inverted Vee’s” although a true inverted Vee is normally a wave length or more in length.
Angle of Radiation
Sometimes referred to as “take off angle”. This can be pretty important if your desired location to contact is specific. Remember, sky waves are refracted by the ionosphere and the angle formed by the wave approaching the refractive layer determines how far your signal will go on each hop. Keep in mind that some signal strength is lost each time your signal hops so you want to keep the number of hops low. You will get the longest hop if your antenna is installed a half wave or more above the ground. If it is installed lower than that, the angle will be steeper and the hops shorter. Some pretty effective communication can be had from an antenna installed only ten feet off the ground. The take off angle will be quite steep but it might give you a “killer” signal if a short hop is what you want.
Holding it up
Some old timers will tell you that “If your antenna doesn’t come crashing down each winter, it either isn’t high enough or big enough.” Balderdash, I say. If you put it up right it will not only work well but it will stay up until you want to take it down. The dipole has 3 critical points that must be considered when designing it: The ends, and the center. Some dipoles are installed by stretching the wire between two supports, allowing the feedline, which is usually attached to the center, to just hang from the wire. That can work, if you are able to pull the wire tight enough so that the center does not droop too much. A long, heavy feedline such as RG-8 coaxial cable can put a pretty good strain on the wire if you go high enough. If this is the way you plan to do it, fine but you will need a stronger wire. A 12 or 14 gauge steel wire, plated with copper will do the trick but it may be more expensive than pure copper wire.
If you support your dipole from the center, that will take all the strain off the wire and a soft copper wire will work fine; even one of a lighter gauge. If you plan to support your wire from the center, an inverted Vee is quite simple although if you have the room, and supports in the right places, getting the ends up the same height as the center will give you better performance.
Buy it or build it
Not long ago, a ham with even a rudimentary workshop would not even consider buying a ready made dipole. But times have changed. Locating materials for a good antenna can be a challenge. There are specialty vendors such as Halted Supply Company that stock most of what you need but you may need to travel some distance to find one. HSC in my area is located in Rohnert Park, about an hour’s drive. Materials can also be found on the internet, but you may find that the cost to build one this way is equal to or greater than just buying a ready made dipole. Check the ham catalogs to help you make the decision.
Links on the web for antennas or the material to make them: http://thewireman.com/products.html One stop shopping for parts or entire antennas. Wireman has copper clad steel wire at a reasonable cost. They also stock a 7 strand copperweld antenna wire which is much easier to handle. Many commercial wire antennas are made with this stuff.
MFJ equipment, in my humble opinion, runs the gamut from terrible to terrific. A large selection of antennas available here including the G5RV. http://www.mfjenterprises.com/
The center support.
Figure 1 shows the most basic center support. This is simply a method of insulating the two quarter wave halves of the dipole from one another while allowing the attachment of the feedline. It’s nothing more than an insulator. These used to be commonly available at hardware stores when there were still farms using electric fences, but now days these insulators are not easy to find. The copper wires forming the two quarter wave legs of the dipole are passed through the eye on each end of the insulator, twisted a few times and soldered. The lanyard, if one is used is tied tightly around the insulator in one of the grooves. Finally, the coaxial cable (if used) is looped over the insulator and taped in place, the ends of the coax are stripped, passed up through the eye and soldered to the dipole legs. MFJ stocks these insulators in 4-packs.
This page shows a simple way to make your own center insulator that will work with either a 2 or 3 support configuration. It’s made from schedule-40 PVC and is available at many hardware stores. If your local store won’t cut you a foot of 1-1/2 pipe, just buy a slip coupling. It will do the same thing.
The first few photos show the simplest possible way to build the support. The last photo shows how to use a balun to build a dipole that will last for years.
Ok, that’s one band. What about the others?
Good question. If you built your flattop dipole about 64 feet from one end to the other, you’ve got an antenna that will perform well on the 7.2 mHz band, referred to as 40 meters. If you put an antenna tuner in line between the transceiver and the feedline you may be able to get it to perform on other bands as well but a simpler solution is to convert your dipole to a “multi-band” dipole. This is pretty simple to do. It just requires more wire and some fabricated insulators. Figure 2 shows a basic dipole modified to work on 14.3 mHz as well as 7.2 mHz. You can continue to add band elements and your only constraints will be the difficulty in handling the beast when you get finished. A single band dipole is pretty easy to handle but one built for 3 or more bands gets pretty difficult; you may need help to keep it from getting tangled up. Once in the air though, it will stay forever if you do it right.
Turn a single band flat top dipole into a multiband antenna by simply adding a wire cut to the appropriate length. There will be some interaction, that is, adding a new wire will usually throw the existing wire(s) slightly out of resonance. It can be helpful to have an antenna analyzer handy to retune as you go.
Multi Band wire antennas
The difficulty described above is what gave birth to the G5RV multiband antenna. These are available in nearly every ham catalog and it works well if you can get it high enough. The basic G5RV will work without a tuner on some bands but your transmitter will probably tune up easier if you put a tuner in the line. The G5RV is 102 feet from end to end and is fed with a combination of 450 Ohm “window line” and 50 Ohm coaxial cable. The window line, similar to twin lead is 32 feet long and should be configured so it drops straight down from the center of the dipole. For this to work right your center support should be 40 feet up or better. The G5RV is available from AES for $40, and while it’s not built to last forever, with maintenance it should last for many years. MFJ also stocks the full size G5RV as well as a shorter one that does not cover 80 meter. MFJ also sells an extensive line of tuners, some good, some not so good. You’re better off paying a little more up front and get one that will last.
Another type of multiband antenna uses traps to accommodate additional bands. Traps are coils that present a high impedance to some frequencies and low impedance to others. By inserting tuned traps in strategic points along the wire, multiple bands can be tuned. Trapped dipoles work well enough but can be difficult to tune up. They tend to be not as broad banded* as un-trapped antennas. A 5 band trapped antenna (Unadilla) is available from AES for $86 but definitely requires a tuner in the feedline. Overall length is 120 feet.
*Broad banded antennas can operate over a wide range of frequencies. A typical single band dipole for example, might tune up with minimum SWR at 7.2 mHz, and work all the way up to 7.3 mHz with just a small increase in SWR. Tuning down, you may also go as far as 7.1 mHz without a significant SWR. This is highly desirable since it allows you to move around the band without constantly fiddling with your tuner. Antennas with “features” in the elements such as loading coils or traps, or antennas made from very fine gauge wire tend to be narrow banded.
A word about SWR
Some hams fixate on SWR. If the reflective power is even slightly noticeable they feel it must be fixed. The problem is, SWR appears at the radio’s antenna terminal when there is a mismatch in impedance somewhere in the line. Most modern radios have a feed point impedance of 50 Ohms and if you use a 50 Ohm impedance feedline such as RG8 or LMR-400, then the only possible place for a mismatch will be where the coax cable connects to the antenna. A typical dipole, installed at a half wave in the air will not be 50 Ohms, it may be 60 or 70 Ohms even when it’s properly trimmed. As a result you will show a slight SWR due to the mismatch, but if your feedline has a low loss, this mismatch should be of no concern. In the above example, dividing 70 by 50 gives you 1.4. An SWR of 1.4:1 should load up just fine and should be of no concern. Often times however, the antenna is tested (and adjusted) while only a few feet off the ground. At this point, the antenna impedance may be slightly less than 50 Ohms. You carefully adjust the length for minimum SWR, you may even get it to 1.0:1 and then when you raise it up…bingo! 1.4:1. What’s going on? Test your resonance by taking readings at various frequencies within the band. If your lowest SWR is 1.4:1 and your highest is 2.0:1, you’ve got a great antenna. Use it and don’t worry about it.
Let’s get specific
There’s no point in re-inventing the wheel, or in this case the dipole. A very complete article on the dipole can be found on Wikipedia . This article has all the information you need to design the dipole you need for your station. if you are put off by formulas and math, don’t be discouraged. There is sufficient information on this page to help you build the antenna you need. Pay particular attention to the section near the bottom of the page, which deals with baluns. (Bal-un, not Ballums. Stands for Balanced/Unbalanced). A balun can be a great help in keeping stray RF off your feedline and out of your station.
http://www.arrl.org/tis/info/dipole-h.html Offers a good selection of articles on simple antennas, most of which you can build with simple tools.