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[Articles Home]  [Add Article]  

Your First HF Dipole

from Phil C. Sr. K4DPK on July 20, 2014
View comments about this article!

"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 6/16/2010





Your First HF Dipole

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 youíll find helpful. I encourage you to read those in the ďTower TalkĒ forum, under the heading ďPutting up HF DipolesĒ.

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 youíve 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.

Hereís 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 itís 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 ďa little longerĒ, 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 doesnít 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 itís 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), donít 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, itíll sure simplify getting your dipole up right and do it in a hurry.

Now for some more stuff.

Insulators:

Obviously insulators need to be non-conductive; otherwise they wouldnít 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. Iíve made a lot of them from ľ inch Plexiglas. Years ago when glass Coke bottles were common, you could pop the rings off the top of íem. Iíve used those glass rings for low-power antenna insulators. The point is, you just have to be resourceful. Consider the power youíll be running and the mechanical stress the insulator will need to withstand. Anything fairly close will work.

Coaxial Cable:

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.

Baluns:

The word balun is a combination of the words BALanced and UNbalanced. As the name hints, itís 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 isnít 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 itíll pick up the noise and bring it inside to the receiver.

Thatís not to say antennas wonít work without a balun. Quite the contrary, but it generally will be true youíll 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. Youíll 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.

Wire:

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 wouldnít surprise me to hear of someoneís having used barbed wire. Iím sure Iíve 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 doesnít matter if the wire is insulated or bare, solid, stranded or a mix of them allÖ.. If you follow the steps given earlier about finding a new constant replacement for 468, itíll all turn out well. Just make sure your connections are OK.

Measuring:

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.

Donít 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 ďmoveĒ 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, itís 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
Slingshot
Bow and arrow
Potato gun

There are lots of others that I canít 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, thoughÖ.Donít be tempted to use your socket wrenches for weights. Sometimes, the line becomes tangled in the tree, and you donít get your weight back. Thereís 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.

Iíve learned that a 2-3 oz lead pyramid weight painted fluorescent orange works really well. It comes down through the limbs nicely and itís 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.

Donít EVER throw wires or anything else over power lines, and donít ever haul antennas up over the top of power drop wires. Thatís as good a way as I know to make a complete ash of yourself.

Rope:

Twisted or braided polyester in either 3/16Ē or ľĒ diameter size is probably the best general-purpose rope for putting up antennas around here, but up ďNawthĒ folks may need something a little more substantial.

Polyester has good UV resistance and doesnít 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 DuPontís 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, itís a good idea to melt the cut end together, so it wonít fray. If the end of the rope catches on fire, donít try to snuff it out with your fingers. Youíll see why if you try it.

Pulleys:

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 ďJohnny ballĒ 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, youíll 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 itís 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 itís 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. Donít 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 ďwiggle roomĒ.

Just be safe, stay away from power lines and think about what youíre doing so you donít unwittingly set a trap for someone.

Coax connectors:

It is very likely that most of the problems experienced by new hams (if they find their newly-erected first antenna wonít 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. (Donít 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, youíre 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 donít 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.

Weatherproofing:

A lot of folks use a putty-type coax sealant on their connectors. I donít because itís very difficult to remove and separate the connection after itís 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.

Conclusion:

Remember, your license is a license to learn. Study the handbooks and antenna manuals, and youíll understand how what youíve 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. Donít 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 weíve done here is scratch the surface with of the simplest of all.

Good luck.

Phil C. Sr.
k4dpk

Member Comments:
This article has expired. No more comments may be added.
 
Your First HF Dipole  
by W2EM on July 20, 2014 Mail this to a friend!
Good basics. Nice write-up
 
RE: Your First HF Dipole  
by W4KYR on July 20, 2014 Mail this to a friend!
This should be a 'sticky' on the antennas forum. Well done article and should be required reading for every new ham.
 
Your First HF Dipole  
by G3VGR on July 21, 2014 Mail this to a friend!
"Whatever actual finished length you have, WRITE IT DOWN"
Such good advice
 
Your First HF Dipole  
by NU4B on July 21, 2014 Mail this to a friend!
Its amazing my first dipole worked at all. It was a really sad looking dipole with even sadder components - but it did work. It was a 40/15 meter antenna for the novice bands.

BTW, I have found that a Hyperdog ball launcher used as a slingshot works really well. Much more powerful than those rinky dink slingshots Wall-Mart sells (or used to sell). I use a 2 ball launcher, an egg sinker with a roll of fishing line, then once the proper limb is "hooked" I trade up to stronger rope in a couple stages until I get the final rope up there.

With some practice it becomes really easy after you learn how to aim properly. (As always make sure there are no people, cars, or other things that can get damaged in the way)
 
Your First HF Dipole  
by K0DCH on July 21, 2014 Mail this to a friend!
Another nice feature of dipoles for beginners is that they avoid the problems associated with stray RF from improperly grounded verticals or random wires. I started out with a random long wire on a tuner and struggled for quite a while with the stray RF monster.

Very good article.
 
RE: Your First HF Dipole  
by K4PIH on July 22, 2014 Mail this to a friend!
Very important to seal the raw braid end of the coax if it's connected directly to the elements. I learned the hard way as the coax feed on my 40 meter dipole turned into a direct water hose into my shack during the rainy season in Korea. Another thing to remember about dipole antennas ... they don't cost much to build and every dollar you save can go toward a better rig.
 
Your First HF Dipole  
by N0AH on July 22, 2014 Mail this to a friend!
I have read this article and pick up more new information each time I read it- excellent write up-
 
RE: Your First HF Dipole  
by K9MHZ on July 22, 2014 Mail this to a friend!
>>>>Use a good hot iron. 250-300 watts is ideal.<<<<

Mileages may vary, but that seems like way, way too much heat. I get what he's driving at in his later text, but that's very easily done with a good quality 100-watt iron like a Weller 100P3.

Also didn't see any mention of strain-relieving the PL-259, if it's hanging down from the feed point. Even if there is no supporting structure at the center, it's still very important to strain relieve the connection. DX Engineering has some products, or you can make something easily.

Overall good points in this article.

 
Your First HF Dipole  
by JOHNZ on July 22, 2014 Mail this to a friend!
What makes this tech article superior is the talent of the author to clearly and concisely in easy to understand language give instructions which anyone with a modicum of intelligence can understand, then proceed to construct said antenna.

If the author is not a professional tech writer, he missed his calling in life. Of course, we know hams come from many walks of life.

For those who have taken tech writing courses in college, you know how emphasis is given to making the complex easy to understand, and the author has certainly accomplished that.

On a lighter note, the author is one of our southern home boys, y'all. No wonder he did a great job. He ain't from up Nawth.
 
Your First HF Dipole  
by KB2DHG on July 24, 2014 Mail this to a friend!
GREAT ARTICLE AND A WEALTH OF INFORMATION! BRAVO!
I have used home brew dipoles ever since first licensed back in 1987! To this day I am using a home brew G5RV with great results.
When it comes to simple cheap and effective antennas, you cannot beat a dipole!
 
Your First HF Dipole  
by AI2IA on July 24, 2014 Mail this to a friend!
After 52 years, I am still amazed at what can be achieved with a simple inverted vee antenna.
 
Your First HF Dipole  
by K2YWE on July 30, 2014 Mail this to a friend!
Nice write-up with good advice. I would suggest trying the coax-fed dipole without a balun first. It's been my experience it works fine that way provided the coax runs at right angles to the antenna, like straight down.
 
RE: Your First HF Dipole  
by KB6HRT on July 30, 2014 Mail this to a friend!
A W9!NN 78' 10-80m was my first dipole, needed an antenna like it to fit my very small lot, had always used verticals before the W9INN, by the time I got the antenna working as well as it should, had learned a great deal about tuning an HF antenna and lots of the tricks of the trade on how antennas work , them ordered the 87' big brother, did the same with it. Still use the tuning tricks today, Bill tought in his instructions today on coiled antennas...............KB6HRT
 
RE: Your First HF Dipole  
by KA4KOE on August 10, 2014 Mail this to a friend!
Incomplete....needs some verbiage about balanced feedline and its advantages for multibanding a dipole.
 
RE: Your First HF Dipole  
by KA4KOE on August 12, 2014 Mail this to a friend!
Retract my previous comment vis a vis balanced line. The author's intent was a simple, coaxial fed dipole. My bad.
 
RE: Your First HF Dipole  
by W8IFI on August 30, 2014 Mail this to a friend!
A good review and refresher for this old timer!
Thanks!
 
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