Shortened Dipoles for Limited Spaces
Occasionally, antenna space is too limited for a dipole, particularly for the lower bands, so some creative method is needed to get a dipole up and radiating. Also, sometimes a rotary dipole is a viable alternative to a beam when a moderately directional antenna is needed and there is no room or money for a beam. Rotating a full sized 40-meter dipole is a little much for someone to have swinging overhead and most neighbors will not fully appreciate the beauty of such things. So the shortened dipole may be the solution in most cases.
Figure 1 shows a version of the shortened dipole that has both loading coils and capacity hats to increase the bandwidth of the short antenna. There is a matching coil in the center for matching the feedline to the antenna as well. There are alternate ways to building this type of antenna, and a lot of them have been used down through the years, both with and without capacity hats.

To calculate the inductance needed in the antenna, either with end capacity hats or without, the formula in Figure F-1 is used for the required inductance. In the case of a rotary dipole, the diameter of the elements in inches is needed. If it is not going to be rotated, then the size of the wire in inches is also needed. Conversion to English measurements from metric measurements will be required for this formula.

ln = Material log
f = Frequency, MHz
A = Overall length in feet
B = Distance from Center to each leading Coil in feet
D = Diameter of Radiator, inches
A simpler version is the antenna found in Figure 2. This particular antenna was the brainchild of W0SVM. The antenna was a total of 36-37 ft. long and was tuned by tapping the center-loading coil. There are several variations on this antenna. One of them has the loading coils located just past the ends of the matching coil. Others had capacity hats on the ends of the antennas, or bowtie-shaped multiple conductor antennas. Figure 3 is to be used in conjunction with the formula in Figure F-1 and has the pertinent distances marked on it.


In Figure 4, the loading coils and matching coil are located in close proximity to each other. This may be required on a rotary 40 meter dipole to eliminate excess weight out on the elements which could cause them to bend excessively. It also will allow the adjustment of the coils to be made easily as they will be close to the tower where a climber could reach them for adjustments.

Another version of the shortened dipole consisted of two mobile antennas mounted on some nonconductor in a dipole configuration. These antennas did work but suffered from low efficiency and very narrow bandwidth, particularly at the lower frequencies. There has been some work with the helical wound dipole where the two elements consist of 1\2 wavelength of wire wrapped on two non-conducting forms, which were mounted in a manner to allow rotation of the antenna. These antennas were tuned by trimming wire off the ends of the antennas until resonance is found. Such antennas also are narrow bandwidth and some means to match them to the feedline is needed.
For the most part, shortened antennas all suffer from narrow bandwidth as well as somewhat lower efficiency, but in many cases, this is the only type of antenna that will allow operation from certain locations. Also, the ability to rotate the antenna does compensate for the other disadvantages common to these shortened antennas. There are more complicated methods of shortening the dipole, particularly the rotary dipole, but for now we will limit this discussion to the methods above.
If you build one of these dipoles, the best way to tune them up is to tap the coils equally a little at a time until the antenna reaches the frequency range desired. This means raising and lowering the antenna until it is tuned, but that is the only real way to get it right. Then adjust the VSWR to minimum by whatever methods you decide to use and then put the antenna up permanently. An alternate method is to use fixed coils and adjust the lengths of the ends of the antenna until the antenna is at resonance. Either way, you will have to raise and lower the antenna a few times. If you try to resonate the antenna on the ground, it will not be on frequency when you put it up. Everything changes when the antenna is elevated. So, like it or not, the only way to tune the antenna is to raise it and lower it until it is correctly tuned.
Using pair of mobile whips as a rotary dipole with a matching network at the center works, but this method will result in a very narrow bandwidth antenna. That is so particularly if you are going to make one of these antenna for the lower bands, 30, 40 and 75/80 meters. To tune these antennas, the whip resonators will have to be adjusted the same amount until resonance is found. Matching this type of antenna to the feedline can be done any way the builder desires.
In short (no pun intended!), the shortened dipole is a compromise.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 14th May 2024