Antenna Primer Part III:
Select an Antenna, and Build a Groundplane
Copyright by Clem Small KR6A
To select an antenna to fit your needs, you should consider the frequency, or frequencies, on which you want it to perform, whether it should be directional or relatively nondirectional, and if you wish to utilize it primarily for DX or for close-in communication paths. You should also consider the potential sites you have available for the antenna, and the antenna’s cost.
Except for the cost factor, we’ll discuss these considerations below. For cost, just keep in mind that building your own antenna can often save you the major portion of the expense of an antenna as compared to buying a ready-made, commercially constructed antenna.
Some Easy, General-Purpose Solutions
Many Monitoring Times readers want one of their antennas to support listening from the LF band on up to the HF band, and maybe even cover a bit of the lower end of the VHF band. Such an antenna would serve to monitor LF signals, beacons, the AM broadcast band, tropical bands, HF utilities, ham radio HF bands, and more.
For decent performance, convenience, modest cost, and ease of construction it is hard to beat a random-length wire antenna for the application just described. If long enough, it will have nulls (directions of reduced response), but they are not deep for antennas at practical heights. This antenna’s response is modestly non-directional. Building your own random-length antenna was covered in this column two months back.
For general local listening on the VHF-UHF bands, a groundplane antenna gives good response toward the horizon in all compass directions. It is perhaps the best solution for catching the action in your area. We’ll discuss making these useful antennas below.
Some Other Antenna Solutions
Perhaps you’d like a random-length antenna, but don’t have the space. Then an active antenna is often a good alternative. Active antennas are small enough to sit on your operating table, simple to operate, and will usually give reception equal to a 50 ft or longer random-length wire antenna. On the downside, they will cost more than a random-length wire, and can be subject to intermodulation distortion and overloading from strong stations.
If you want to listen primarily to one frequency, or to one band, a good solution for general monitoring is often a halfwave dipole cut to the frequency utilized. For reception on MF or HF, it should function reasonably well over an entire band. Building your own dipole was covered in this column last month.
Mounting your horizontal antennas approximately a quarter wavelength (246/freq. in MHz) above ground tends to favor close-in communication paths, whereas mounting them a half wavelength (492/freq. in MHz) above ground gives better DX performance.
Keep in mind that on MF and most of the HF band received noise is high enough that, except for directional antennas, most antennas tend to yield similar quality of reception. As long as the antenna is large enough, or efficient enough, to capture sufficient signal for decent reception, then more signal strength is of little value. This means that on MF and the lower portions of the HF band, except for directional antennas, expensive or complex receiving antennas are generally no more effective than a random-length wire. If received noise is man-made, then a horizontally-polarized antenna may be less noisy than a vertically-polarized one.
It is true that directional antennas increase signal strength in their favored direction. But more importantly for HF and lower frequency reception, they reduce interfering signals and noise in other directions. This makes an important difference in reception if you are bothered by antenna-received interference. At the higher HF frequencies and above, where received noise is generally lower, a directional antenna’s greater signal strength output is quite important in bringing weak signals above the noise level. HF beams, such as the Yagi-Uda or quad, are excellent choices from about 15 MHz through UHF, and can be rotated to direct your antenna’s response in various directions. Phased beams are sometimes useful, but generally cannot be rotated.
If You Transmit
For transmitting, the concept of antenna reciprocity tells us that an antenna’s gain, feedpoint impedance, radiation pattern, and most other of its variables are the same, whether the antenna is transmitting or receiving. But, it is important to realize that a transmitting antenna and its feedline must be capable of handling the transmitter power fed to them.
Active antennas contain a small RF receiving amplifier, thus they cannot be used for transmitting. Desk-top loops, and Beverage antennas are not very useful for transmitting.
Groundplane antennas are practical and useful from microwave frequencies on down into the HF band. Their low vertical radiation-reception gives good all-around local coverage for HF and higher frequencies, and also great DX performance on HF. Fig. 1 gives the essentials for building one. For lower frequencies where the elements are several feet long, one method of construction is to make the antenna from wires terminated in antenna insulators for connecting to mounting ropes. The antenna is then tied in position with the top held high by a post or tree limb, and the radials tied to shorter posts, or other points, with ropes.
For antennas mounted outside, don’t forget lightning-induced damage protection. The minimum is to never use them in weather likely to produce lightning, and disconnect and ground them when they are not in use.
Of Course There’s More Than This
I may have mentioned some antennas with which you are not familiar. I wish there were space enough to define them. On the other hand, there are many good antenna designs which I haven’t mentioned at all. If you find that the above discussion whets your appetite, I’d like to urge you to look at some books on antennas. Then select the one that most appeals to you, and cover it in some detail.
W6SAI’s HF Antenna Handbook is a good introduction, especially for hams, with directions for building a variety of antennas. Joe Carr’s Practical Antenna Handbook (4th edition) is excellent, and covers a very-wide range of topics, plus it includes lots of information on building and testing your own antennas. The ARRL Antenna Book is both technically excellent and filled with many practical plans for building amateur-radio antennas.
Many more good antenna books are available. Some libraries carry one or more books about antennas. To buy books, check both new and used book stores, internet bookstores, Ebay, and radio and electronic supply house catalogs.
This article first appeared in Monitoring Times, April 2002 "Antenna Topics"