Antenna

Directional or Omnidirectional Antenna?

Do you need a directional antenna or an omnidirectional antenna? That question is basic for amateur radio operators, shortwave listeners and scanner operators. The answer is simple: It depends. I would like to give you a simple rule for all situations, but that is not possible. With radio antennas, the "global solution" is rarely the correct solution for all users. In this paper you will find a discussion of the issues involved so that you can make an informed decision on the antenna type that meets most of your needs. But first, let's take a look at what we mean by "directional" and "omnidirectional."

Antenna Patterns

Radio antennas produce a three dimensional radiation pattern, but for purposes of
this discussion we will consider only the azimuthal pattern. This pattern is as seen from a "bird's eye" view above the antenna. In the discussions below we will assume four different signals (A, B, C, D) arriving from different directions. In actual situations, of course, the signals will arrive from any direction, but we need to keep our discussion simplified.

Omnidirectional Antennas. The omnidirectional antenna radiates or receives
equally well in all directions. It is also called the "non-directional" antenna because it does not favor any particular direction. Figure 1 shows the pattern for an omnidirectional antenna, with the four cardinal signals. This type of pattern is commonly associated with verticals, ground planes and other antenna types in which the radiator element is vertical with respect to the Earth's surface.

The key factor to note is that for receivers all four signals (or signals from any
direction, for that matter) are received equally well. For transmitters, the radiated signal has the same strength in all directions. This pattern is useful for broadcasting a signal to all points of the compass (as when calling "CQ"), or when listening for signals from all points.

Directional Antennas.

Gain and directivity are intimately related in antennas. The directivity of an antenna is a statement of how the RF energy is focussed in one or two directions. Because the amount of RF energy remains the same, but is distributed over less
area, the apparent signal strength is higher. This apparent increase in signal strength is the antenna gain. The gain is measured in decibels over either a dipole (dBd) or a theoretical construct called an isotropic radiator (dBi). The isotropic radiator is a spherical signal source that radiates equally well in all directions. One way to view the omnidirectional pattern is that it is a slice taken horizontally through the three dimensional sphere.

Figure 2 shows a bidirectional or Figure-8 antenna pattern. This pattern is associated with half wavelength dipoles, quad loops, and a number of other antennas. There are two preferred directions (maxima) and two null directions (minima). In the half wavelength dipole the minima and maxima are positioned as shown. For receivers, signals arriving from the direction of the minima (Signal "A" and Signal "C") are suppressed because the antenna is not sensitive in that direction. The suppression is not complete, but it can be tremendous (e.g. 60 dB). The signals arriving from the direction of the maxima (Signal "B" and Signal "D") are received the loudest. For transmitters, the radiated signal is the lowest in the direction of the minima and greatest in the direction of the maxima. Again, the signal level radiated off the ends of the antenna, i.e. in the direction of the minima, is not zero, but is very low.

Local installation factors can affect the radiation pattern. In "free space," i.e. the antenna is installed at great distance from the surface of the Earth, trees, houses, wiring and so forth, the pattern will be nearly perfect. But in practical situations, the two lobes might not be equal, or the minima might be less distinct.

It’s obvious that the antenna system is an important part of an RF communication system — without it the ystem wouldn’t work. Equally obvious is the fact that the antenna system is common to both the transmitter and the receiver; any change made in the antenna system affects both transmission and reception. This brings us quickly to consider the economics of a radio system. We can help the talk-out range — base station to mobile — by doubling the transmitter power, but this doesn’t help the talk-back range — mobile to base — in the least. On the other hand, if we can change the antenna to effectively double the transmitter power (and we’ll see how this is done a bit later in our discussion), then the power of the mobile transmitters will also be effectively doubled. By changing the antenna system we help both base-tomobile and mobile-to-base ranges. And, generally, it is less expensive to change the antenna system than it is to change the transmitter power of the base and mobile units.

What is an antenna?
The antenna is the portion of the radio system found at the top of the tower. It could be a simple one-element antenna, or it could be a complex multi-element array. The antenna takes radio energy from the transmission line and radiates it into space; it also receives radio energy from space and feeds the received energy down the transmission line to the receiver. To oversimplify, an antenna is designed to radiate radio energy into space and collect radio energy from space. We already know that an antenna changes radio energy from the transmission line into radiated energy and vice versa. What is remarkable, though, is how efficiently this occurs. A common household light bulb is only about 20 percent efficient in changing electrical energy into light (another form of radiation), whereas a two-way antenna is nearly 100 percent efficient. Of course, we don’t quite get all of the energy out that we put in. Factors affecting this include a coaxial line that doesn’t perfectly “match” the input to the antenna and power lost due to such things as “skin effect,” insulator dielectric, eddy currents, etc. But, since we can typically claim that an antenna radiates better than 95 percent of the watts it receives from the coaxial line — provided it “matches” the line — an antenna is a pretty efficient device when compared to most other energy-emitting things we know.

Antenna Gain

Antenna gain and pattern shape are interrelated; if you change one, you generally change the other. Just as we need a starting or reference point when we survey land, we must have a reference point to start from when we talk about gain. In addition, just as inches, feet, and miles, (or centimeters, meters, and kilometers), etc., are used as a unit of measure in surveying land, we need a unit of measure when we talk about gain. The reference we use in two-way base station antennas is the half-wave dipole and the unit of measure is the decibel. As the point of reference, we use the half-wave dipole and say it
has a gain of one (or unity), or sta ted in decibels, it has a value of zero decibels.

How Do We Get Antenna Gain?

There are only two approaches to antenna gain:
1. We could increase, or multiply, the power or current density in the antenna so the antenna radiates a given pattern shape with greater intensity. Unfortunately, however, we can’t increase the power, so this can’t be done.
2. The other option is to change the shape of the pattern so it radiates more of the antenna’s signal in a particular direction. This is something we can do. Since we don’t need radiation in all directions, we can increase the signal’s intensity by changing the shape of the antenna pattern. We do this by designing the antenna so that it radiates the same amount of total power, but we change the shape of the pattern so that it directs the radiation where we want it. To get a better idea of how this works, let’s compare it to a lawn sprinkler. The sprinkler head, which would represent the antenna, is attached to a water hose; the water hose would represent the transmission line. If we adjust the sprinkler head so that it covers a full circle and turn the water on full force — that is, we set it at full power — we distribute the water in a circular pattern around the sprinkler head. Now, if we readjust the head so that it only covers one corner of the lawn and turn the water on full force again, we output the same amount of water as before, but this time, because we have limited the pattern to one direction, the water — or our antenna signal — goes further and outputs more
into a given direction.