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Introduction

A Center-Fed Half-Wave Dipole is probably the simplest of antennas to construct and use. It is usually suspended between two supports, from it's end insulators, and has the feedline hanging from the center. The drawing below shows the esential parts of a dipole. A good wire to use is a #14 or #16 stranded copper wire, for flexability and to minimize weight. Needless to say, the end-insulators, center-insulator, and the wire need to be fairly strong, especially when you are dealing with low frequency antennas due to their length.

The recommended height for a dipole is 1/2 wavelength above ground. Finding two supports at the recomended height may be fairly easy for higher frequency antennas (15 and 10 Meters), but may present a problem at lower frequencies (80 and 40 Meters). At low frequencies, like 40 Meters, you would need two vertical supports that are 65 Feet (20 Meter) high to meet the recomended height. But, don't let this bother you too much. Lower heights will reduce the feed impedance and change the radiation angle, diminishing the overall effectiveness of the antenna, but the antenna will still be very usable.

Feed Point

At the antenna feed point there is usually a device called a Balun. The name Balun is short for Balanced to Unbalanced". The antenna wires, which contain equal an opposite voltages, are balanced while the Coaxial Cable is unbalanced. You could simply connect the coax shield to one side of the antenna and the coax center conductor to the other. However, without a Balun, the feed line may radiate, be difficult to adjust, and cause issues with RF in the shack. Using a Balun at the feed point help you get around those possible issues.

On the left is a picture of a Choke Balun. It consists of 8-10 tightly wrapped turns of Coaxial Cable and is installed at the antenna feed point. The Choke Balun doesn't actually do a balanced to unbalanced transformation, but it does form a RF-Choke which keeps the RF off of the feed line. The Choke Balun can be made with whatever coax you are using as a feed line. The 8-10 inch diameter specified in the drawing is about as tight a coil as you are going to get, if you are using RG-8 coax. If your using something like RG-8x, you can probably make a smaller coil.

The turns are bound together with electrical tape or UV stabilized tie wraps. When it's made from RG-8, the Choke Balun will be quite heavy. So you might want to provide some means of taking the strain off of the dipole feed point.

There are also commercially available Baluns. These are usually very well made and provide for strain relief of the radiator elements. Some provide an attachment at the top that can be used for hanging. I use one that I found in the trash over 30 years ago. A little cleanup and it worked fine. Internally, these Baluns contain several turns of wire wrapped around a ferrite core (rod or doughnut). The usable bandwidth is very large (3-30 MHz) making it useful on multi-band antennas. The drawing below is an example of one type of commercial balun. The drawing shows the Balun schematically and physically. You can usually find these type of Baluns at a ham fest for about $8 but they come in a variety of shapes and sizes.

Depending on many factors (height above ground, close objects, element lengths, etc.), you may not be able to obtain a 1:1 SWR, however, properly adjusted it should be some where between 1:1 and 1.5:1. Don't worry about a couple of tenths in your SWR, just get it as low as possible, and use it. The SWR will change anyway as you move across the band. Just get the dipole up as high as you can and minimize close metal objects.

Antenna Length Approximations

Approximate antenna lengths can be determined for 1/2 wavelength antennas by using the formula to the right. These simplified equations are based on the standard wavelength formula of λ(m) = 299,792,458/F(Hz) and accounts for a Velocity Factor of 0.95. The lengths listed are for a full 1/2 wavelength, to the nearest 1/4 inch or centimeter, and are for the low frequency ends of the band. The antenna may need to be shortened a bit for the high end.

Amateur Bands Shortwave Broadcast Bands
Band Frequency Length Band Frequency Length
160 M1.800 - 2.000 MHz 120 M2.300 - 2.495 MHz
80 M3.500 - 3.600 MHz 90 M3.200 3.400 MHz
75 M3.600 - 4.000 MHz 75 M3.900 4.000 MHz
60 M5.330 - 5.406 MHz 49 M5.900 6.200 MHz
40 M7.000 - 7.300 MHz 41 M7.200 7.450 MHz
30 M10.100 - 10.150 MHz 31 M9.400 9.900 MHz
20 M14.000 - 14.350 MHz 25 M11.600 12.100 MHz
17 M18.068 - 18.168 MHz 22 M13.570 13.870 MHz
15 M21.000 - 21.450 MHz 19 M15.100 15.800 MHz
12 M24.890 - 24.990 MHz 16 M17.480 17.900 MHz
10 M28.000 - 29.700 MHz 15 M18.900 19.020 MHz
6 M50.000 - 54.000 MHz 13 M21.450 21.850 MHz
2 M144.00 - 148.00 MHz 11 M25.600 26.100 MHz

Antenna Length Calculation

The dipole antenna is really only good for use on one frequency band. Sometimes you can use it on it's third harmonic, in a pinch. For example, a 40 Meter Dipole is usable on 15 Meters, but the SWR may not be as good. For multi-band operation see the sections on Trap Dipoles or Fan Dipoles.

The formulas on the right can be used as a starting point for a center-fed half-wave dipole. It's a good idea to cut your wire a few inches longer than the calculated values to allow for securing to the insulators and final adjustments. The length can then be changed incrementally for the best match. Enter your required center frequency in the box on the right.

Enter the required Center Frequency, or the Frequency of Minimum SWR.... MHz.

Output: A Half-Wave Dipole for this frequency should be x long, which would make each leg of the dipole x in length.

Antenna Length Scaling

Current Center Freq. (MHz):
Required Center Freq. (MHz):
Current Dimension
(One Leg Only):

Output Data

The current frequency, of minimum SWR, is x and the required frequency, of minimum SWR, is x. The dimension of one leg, as specified above, is x. The specified dimension would need to be changed to x, which is a x of x.

So, you say you cut your antenna to the dimensions specified, but when you put it up the frequency of minimum SWR is higher, or lower, then you intended. The equations assume that your erecting your antenna at the recomended height of 1/2 wavelength and there is nothing close by to interfere with it, like house wiring or trees or just about anything. That's usually not the case and there is the remotest possibility that you made a slight error in measuring. But none of that really matters. The real point is that you should always be ready to make some final adjustments.

However, if you planned well, you included some extra wire for pruning purposes. If, at each end, you had a 1 foot loop of wire you would probably be able to easily adjust the antenna for any frequency offset. When you finally get the attenna to the length you need you can cut off any excess. But a better idea is to just leave it there. It won't bother the performance and will give you room to adjust the antenna later, if necessary.

In the text boxes provided, enter the frequency where the SWR is currently at a minimum. Then enter the frequency you would like the SWR to be a minimum. Finally enter the current dipole dimension for one side of the dipole. Note that it is assumed that both sides are equal and pruning will apply to both sides.

A synopsys of the scaling data is then presented below. As we are really just dealing with a ratio, the dimension (Ft or M) is not really needed but I included it for clarity.