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Antenna Primer Part II:

Antenna Terms, and Build a Dipole Antenna

 Copyright by Clem Small KR6A


In this installment, we continue with our Antenna Primer series by defining and discussing some terms which are useful in dealing with antennas. We also build another antenna.


Antenna Gain and Response Patterns


Antennas differ in their sensitivity or response to signals which they receive. A more sensitive antenna is said to have more “gain” because it responds to signals which it intercepts by producing a greater signal output for the receiver than will an antenna with lower gain.


Nondirectional antennas have equal gain to signals coming to them from all directions. Directional antennas are more responsive to signals coming from certain directions than from other directions: thus their gain is different in different directions.


A figure showing an antenna’s gain or responsiveness to signals from different directions or vertical angles can be called its “ reception pattern” (Figs. 1A and 1B).  The performance of an antenna in transmitting the power which it receives from a transmitter and sending it in different directions gives a “radiation pattern” identical in shape to its reception pattern. Because reception and radiation patterns are identical, either one may be referred to as the “radiation pattern.” However, to avoid confusion they can be referred to individually by separate terms, or collectively as the “radiation and reception” (R&R) pattern.


The portion of the pattern showing directions of maximum response (or gain) are called “lobes,” (1A & 1B), and those showing minimum response are called “nulls” (fig. 1A & 1B). An antenna’s gain is usually specified as the gain of its most responsive lobe.


Although a minimum amount of gain is necessary for satisfactory reception or transmission, it is not necessarily true that more gain is always better. For example, a directive pattern may allow us to reduce received noise from certain directions and hear weak signals from other directions better than with an antenna of higher gain and a different pattern. Appropriate patterns can also help avoid radiating interference to locations not involved in our communications link.


Horizontal vs. Vertical R&R Patterns


An antenna’s R&R pattern in horizontal directions (fig. 1A) shows the antenna’s relative gain in the various compass directions. The vertical R&R pattern shows gain at different elevation angles.


Antennas with considerable functioning at low-vertical angles (fig. 1B) send and receive well toward the horizon. This gives maximum coverage out toward the horizon. On the HF and MF bands this low-angle radiation sends signals to refract from the ionosphere such that they produce very long distance (DX) communication.


Antennas with patterns giving very high angles of vertical radiation are useful on HF for relatively short-distance HF paths, from valley to valley in mountainous areas, and for communication with aircraft, spacecraft, and satellites in the HF, VHF and higher bands.




Impedance is one measure of opposition to RF current flow. In connecting a transmitter (source) to an antenna’s feedline (load), the impedance of transmitter’s output circuit and of the feedline must match, or power from the transmitter will not be transferred to the feedline efficiently. Similarly, when any connection must be made between antenna, feedline, transmitter or receiver, the impedance of the source of the signal and the impedance of the load receiving the signal must match reasonably well for efficient signal transfer.


Where mismatches occur, there are circuits which we can use to make the match better. In some applications matching is more important than in others. We will discuss this in a future column.


Standing Wave Ratio


As mentioned, there is efficient transfer of power between a source and load when the two are impedance matched. If they are not matched then there is some reflection of power from the load back toward the source. On a feedline this returning power interacts with the power coming forward, and causes stationary points of high and low current and voltage along the line.


The distribution of these currents and voltages are known as “standing waves.” A high standing wave ratio (SWR) is indicative of a poor impedance match between source and load. Although fairly high SWR can be tolerated fairly well in some situations, in others it leads to unacceptable power loss, or destruction of components. We’ll discuss this in a future column.


Physical Length vs. Electrical Length


We generally define wavelength, or electrical length, as the distance that a radio wave travels in space in one cycle of its operation. The wave’s length would be about the same in air as in space. As an example, in space or air a 30 MHz signal will travel very close to 10 meters during one cycle of operation. So for 30 MHz one wavelength is said to be 10 meters long.


Radio waves traveling in, or on, a medium other than space or air have a lower speed than that they have in space. And so waves traveling on a wire antenna are somewhat shorter than their commonly designated wavelength. For instance, a halfwave wire antenna at 30 MHz (10 meters) is not 5 meters long, but somewhat less. We have a formula which takes this shortening, as well as something called “end effect,” into account. The formula is: 468/frequency (MHz) = length (feet), or 143/frequency (MHz) = length (meters). Thus, a halfwave antenna on 30 MHz is: 143/30 = 4.77 meters, not the 5 meters one might otherwise expect.


Let’s Make an Antenna:


The halfwave dipole antenna (fig. 1C) is found useful from the upper portion of the MF band on into the microwave region. It is most common on HF where it is more responsive to distant stations when strung a half wavelength above the ground, and to closer-in stations when strung a quarter wavelength high. Never mount it near power lines.


Cut your elements by the formula given above, and solder them in place on the three insulators as shown in Fig. 1C. An acceptable antenna-to-feedline match for HF or lower frequency reception will usually be obtained using any good coaxial cable for the feedline. We’ll discuss why this is so in more detail another time.


Solder the feedline to the antenna as shown, and insulate the exposed end of the coax with coax sealant. Then run the feedline to your receiver. We won’t worry about using a balun for now, we’ll talk about their function another time. But don’t forget lightning-induced damage protection: the minimum is to disconnect and ground the antenna when it is not in use, and never use it when weather is likely to produce lightning.



For more on dipoles check out And here is a short tutorial of antenna technology:



This article first appeared in Monitoring Times, March 2002 "Antenna Topics"