What you need to know to make your first antenna work.
A few weeks ago I posted a little recipe on the Elmers forum, showing how a simple single-band, coax-fed HF dipole antenna could be constructed, with the least possible investment in time and study. My thinking was and remains that the new ham wants to get on the air as quickly and effectively as possible, and then, if he stays with it, learn more about how antennas actually work.
There were a great many comments and much discussion about resonance, matching and about other antenna types that I know you'll find helpful. I encourage you to read those in the "Tower Talk" forum, under the heading "Putting up HF Dipoles".
It was at some point suggested this should have been an article, so I will now attempt to correct that mistake and to add some other simple, non-technical suggestions. I hope new hams are able to benefit from this writing, and I hope the more experienced of you will add helpful suggestions you've picked up along the way.
The original recipe (more or less):
Putting Up HF Dipoles
This is going to be a mono-band antenna one-half wavelength long, fed in the center with 50 ohm coaxial cable. This is probably the most common antenna used on the ham bands today, and it is useful for both local and DX work.
Here's how to do it:
1. Decide what frequency you want to have in the middle of your planned operating range.
2. Divide the number 468 by that frequency in MegaHertz. The result will be the overall length (in feet) the antenna should be, between the ends of the loops on the extreme ends of the antenna, where the wire passes through the insulators.
3. Take notice that the overall length is just a little longer than the sum of the two wires from inside loop to outside loop. That's because the part of the feedline that attaches to the middle insulator (where it's fed) is antenna, too. The length of the antenna is the total length from where the wire passes through the insulator on one end, to where the wire passes through the insulator on the far end. If you want to be really precise on the higher bands, add an inch or so for the loop going through the insulator. Each side of center will be one-half the total length of wire.
4. Cut the wire for each side a little longer than the formula says. Adjustment will be required, and it's easier to remove wire than it is to add. By "a little longer", I mean ten inches or so on 20 meters, proportionally more on the lower bands, less on the higher. Whatever actual finished length you have, WRITE IT DOWN.
5. There are several good reasons to use a 1:1 balun or a ferrite choke balun at the feed point. Without going into detail, suffice to say using one will help in terms of telephone and other RF interference, and will probably help keep RF off your microphone.
6. Put the thing in the air as high as you can. Then find the frequency where the SWR is lowest. This might be at the bottom of the CW band or at the top of the phone portion. It doesn't matter. RECORD that frequency.
7. Then take the actual length of the antenna (you wrote it down, remember?) and multiply it by the frequency (in MHz) of the lowest SWR. That number will be your new constant, to replace 468.
8. Divide the new constant by the frequency you want to have in the middle of your preferred range. This is the length the antenna should be. Now you need to adjust the one you have in the air to this length. You might find it's easier to simply add or take away equal lengths on either side near the center insulator rather than on either end.
9. After doing this haul the antenna back up into position. It should now give you the lowest SWR at the desired frequency.
If for some reason you later want to trim an HF wire antenna (say, you decide to move to a different band segment), don't waste your time cutting a half-inch at a whack. You can estimate how much to cut or add based on the band and how far you have to move it.
For example, compare 468/ 14.0 = 33.42 ft with 468 / 14.35 = 32.61 ft , so only about 10 inches to move the width of the entire band on 20 meters.
On 75/80 meters, the difference between the band extreme edges is better than sixteen feet.
So you see, if you think about it and plan ahead, it'll sure simplify getting your dipole up right and do it in a hurry.
Now for some more stuff.
Obviously insulators need to be non-conductive; otherwise they wouldn't be called insulators. You can buy porcelain or plastic insulators at most hardware stores, including Tractor Supply, Home Depot and Ace Hardware. Basically, they just need to be a piece of plastic or porcelain about 2-4 inches long with a hole in one end for the wire and a hole in the other end for a rope. I've made a lot of them from inch Plexiglas. Years ago when glass Coke bottles were common, you could pop the rings off the top of 'em. I've used those glass rings for low-power antenna insulators. The point is, you just have to be resourceful. Consider the power you'll be running and the mechanical stress the insulator will need to withstand. Anything fairly close will work.
Coaxial cables are grouped in classifications of an electrical property called impedance. It is this number which gives us an indication as to where along the length of a piece of wire the transmission line should be attached to obtain optimum transfer of power. Impedance varies along the length of the antenna wire, and has to do with the length of the wire and the ratio of voltage to current at particular points along that wire. There is a wealth of information available on this subject when you are ready.
The most common coaxial lines used in feeding ham antennas are probably the 50-ohm variety. Over the years, this family has been called both 50 and 52 ohm, but we are talking about the same thing.
Some of the common 50-ohm lines are RG-8U, RG-8X, RG-213, 9913 and a couple of small ones for low power, RG-174 and RG-58. Usually, the smaller ones are less expensive but they also have greater loss. That is, not as much of the transmitted energy reaches the antenna to be radiated, as compared to their larger kin. There are charts for comparison in the ARRL Handbook and on the Internet.
It is worthy of note that most manufacturers have adopted 50 ohms for the design output impedance of modern transceivers.
The word balun is a combination of the words BALanced and UNbalanced. As the name hints, it's a device that connects a balanced system to an unbalanced one.
Baluns are often used to connect a balanced antenna (like the half-wave dipole) to an un-balanced feedline (like the coax). The balun, among other things, helps prevent current flowing on the outside of the coax. Otherwise, when this current (called common-mode current) appears on the outside of the shield, the feedline behaves as if it were an antenna. There are several reasons why this isn't what we want to have happen. If the feedline is behaving like an antenna, and it passes near a phone line on the way inside, you will probably interfere with the telephone. Since antennas work both ways, if the feedline comes close to a noisy power line, chances are it'll pick up the noise and bring it inside to the receiver.
That's not to say antennas won't work without a balun. Quite the contrary, but it generally will be true you'll have fewer problems with noise and interference if you take steps to avoid current on the outside of the feedline.
Some baluns have the ability to transform to a higher or lower impedance. This has to do with the ratio of turns contained in the windings of the balun. You'll see baluns called 1:1 or 4:1, etc. This is the ratio of impedances the balun is intended to connect. For instance, if one wanted to connect a 450-ohm balanced feedline to a 50-ohm unbalanced line, he would select a 9:1 balun. You will learn later this is a convenient way (for instance) to bring a balanced open wire line from a multi-band antenna into the house, using a short length of coax.
In our case however, since the center of a half-wave dipole is closer to 50 ohms impedance than it is to most other standard feedlines, and since we are using 50 ohm coax, we need to have a 1:1 balun.
Antenna wire can be almost anything at all, so long as it will support the weight of the antenna, conduct electricity, withstand expected wind and ice load, and lend itself to a good long-term low-resistance electrical connection. Usually this means a copper wire of sufficient size, although it wouldn't surprise me to hear of someone's having used barbed wire. I'm sure I've come pretty close in my early years.
Antenna wire for HF can be new wire, or it can be pieces of dissimilar wire properly soldered together. What matters is the length, and as mentioned earlier, its mechanical strength. It doesn't matter if the wire is insulated or bare, solid, stranded or a mix of them all..... If you follow the steps given earlier about finding a new constant replacement for 468, it'll all turn out well. Just make sure your connections are OK.
Measure the antenna length from one outside end to the other, counting the center insulator as part of that length. If you have loops that go through the end insulators, and the ends are wrapped back around the wire, then measure from the outside ends of the loops.
Don't get carried away with precision. There is no need for great pains to be taken with the measurements on HF antennas. On two meters, a half wave antenna is around 38 inches, and obviously an inch is an appreciable percentage. On eighty meters, though, an inch will only "move" the antenna about three kHz.
As mentioned elsewhere, it takes around fifteen feet to change from one end of the 75-80m band to the other. Obviously, the higher in frequency, the greater the measurement precision required and vice-versa.
Putting it in the Air:
There are as many ways to hang a wire as there are situations, but in general, to pull the antenna up you first must have a rope already over a tree limb, yardarm, post, pole or some other elevated stationary point.
The simplest way is to just throw a rope over a limb. Remember the old Gene Autry movies? Well that would work, but we want to be a little higher off the ground than was customary in dealing with horse thieves.
Usually, it's easier to put a pilot line of, say, 15-20 lb. monofilament fishing line over the limb, pull the rope up with that, and then pull the antenna with the rope.
You can put the pilot line up in a number of ways:
Throw it by hand with a weight on the end
Use a fishing/spinning/casting rod
Bow and arrow
There are lots of others that I can't think of right now, and these can all be used in combination or with great modification and still work, but you get the idea. One caution, though....Don't be tempted to use your socket wrenches for weights. Sometimes, the line becomes tangled in the tree, and you don't get your weight back. There's a nice house over in Rome GA with a complete socket set dangling high up in the pines. I wonder if the new owners ever noticed.
I've learned that a 2-3 oz lead pyramid weight painted fluorescent orange works really well. It comes down through the limbs nicely and it's easy to spot in the brush.
Anyway, once you get the line over the tree, remove the weight, tie on the rope and pull it up through the tree. Then tie on the antenna and haul it up.
Don't EVER throw wires or anything else over power lines, and don't ever haul antennas up over the top of power drop wires. That's as good a way as I know to make a complete ash of yourself.
Twisted or braided polyester in either 3/16" or " diameter size is probably the best general-purpose rope for putting up antennas around here, but up "Nawth" folks may need something a little more substantial.
Polyester has good UV resistance and doesn't rot or degrade over time. It also has better abrasion resistance than many other ropes, and has normal moisture content of only 0.4%, compared to nylon 6,6 having 4.4%. Dacron is DuPont's registered name for polyester. Most of these ropes are available in colors, and OD or black is best. Your wife will explain this to you if you hang a new white rope across the front lawn.
Nylon is second choice. Polypropylene or olefins are awful and should be avoided.
Whenever you cut any synthetic (thermoplastic) rope, it's a good idea to melt the cut end together, so it won't fray. If the end of the rope catches on fire, don't try to snuff it out with your fingers. You'll see why if you try it.
Sometimes, situations present the need for a pulley, but pulleys are not a good idea when putting up wire antennas. Use ceramic egg insulators, or "Johnny ball" insulators instead.
Pulleys can rust if they are the wrong material, or the rope can jump off the wheel and jam itself between the wheel and the housing. Whenever this happens, you'll tear the whole thing down and start over, at least on that end.
Ceramic insulators are very slick, very tough and have no moving parts. I have some up that have been in use as pulleys for over twenty years with no problems.
Shape of the antenna:
Dipole antennas usually are installed in either flat top or inverted V configurations. On the HF bands, though, antenna dimensions sometimes exceed the accommodation typical lot sizes offer, so it's occasionally necessary to stray from the ideal.
OK, now there are a few things you need to know about the electrical properties of the half-wave antenna. You should read up on antennas and understand why these are true.
1.The electrical current is highest at the center and lowest at the ends.
2. The voltage is lowest at the center, and highest at the ends.
3.It is the high-current portion of the antenna that radiates the most.
Reasons no. 1 and 2 show why we can feed the dipole in the center with a low-impedance line. The impedance is lowest there on the band it's cut for, and at practical heights it is very near the impedance of the 52 ohm coax we are going to use for this single-band dipole. (Later, you can learn how to build multi-band antennas fed with open wire line.)
You can see from this information how important it is to have as much of the center portion at the highest possible point (Reason #3), but it is also permissible to allow the ends to droop or even hang straight down if need be. Don't have the ends so low the neighborhood kids or anyone else can touch them. There are very dangerous voltages present (Reason #2).
Some liberty can be taken with whether the wire follows a straight path. I suspect there are a lot of 160m inverted Ws and Zs out there, and they still, for the most part, work.
The point here is to get the antenna as high as you can, as straight as you can, but if you need to, there is some "wiggle room".
Just be safe, stay away from power lines and think about what you're doing so you don't unwittingly set a trap for someone.
It is very likely that most of the problems experienced by new hams (if they find their newly-erected first antenna won't work), are due to improperly assembled coax connectors.
Always use good high-quality connectors. If you must use the cheap nickel- plated stuff, use a small file and remove the plating around the holes in the inner barrel. Tin these well with solder before inserting the coax.
Examine the braid closely after removing the insulation. Make sure no stray shield filaments are left that might touch the inner conductor. I usually tin the braid very lightly before pushing it into the barrel. (Don't forget to put the outer body on the coax before installing the inner barrel).
Use a good hot iron. 250-300 watts is ideal. That way you can get the connection heated up in a very small area, solder it and take the iron away before the heat migrates very far into the coax insulation. If you leave the heat on the connector or coax very long, you're gonna melt something important.
A good friend reminded me of a neat trick. Keep a damp sponge or cloth handy to quickly cool the connector after soldering.
Once you have the connectors on the line, use an ohmmeter to check continuity of the shield and inner connections, and also make sure you don't have a short between the inner and outer conductor.
Do this without the balun connected. Some configurations of wound baluns show a short to a DC ohmmeter, but they are perfectly OK at radio frequencies. You can see why by reading up on baluns in any good electronics/ham radio handbook.
A lot of folks use a putty-type coax sealant on their connectors. I don't because it's very difficult to remove and separate the connection after it's been on there a while.
I use Scotch 33 black plastic tape over the connectors, stretching a couple of layers over the tightened connector and the adjacent coax, in order to prevent any possible moisture entrance. On the final wrap of tape, I leave a tab sticking out and fold the tape on itself so I can find the end later if I want to remove it.
I use RTV on coax when there is an exposed raw braid end without a connector, such as would occur when attaching coax to a dipole without use of a balun.
Remember, your license is a license to learn. Study the handbooks and antenna manuals, and you'll understand how what you've constructed works.
Get yourself an ARRL Antenna Handbook, and go to places like www.w8ji.com and start reading. Many of the Elmers on e-ham and elsewhere on the web can explain the details just as fast as you can absorb them. Don't pass up these valuable resources when you get over the excitement of the first few contacts and start your serious education.
There are hundreds of different antennas, and they satisfy uses ranging from local to DX, gain to low-noise reception, directional and mobile, simple and exotic. Antennas offer a wonderful place to begin learning and experimenting as you grow in ham radio. All we've done here is scratch the surface with of the simplest of all.
Phil C. Sr.