Wonderful World of Antennas

One of the most important pieces of equipment a Ham can have is a good antenna. Sometimes it is difficult to decide on a particular antenna, because you are not really sure how to determine how effective the antenna would be in your situation. Most of us have a particular brand we favor for whatever reason, whether it be a previous experience, the recommendation of a another Ham, or even the recommendation of someone on the internet or a sales representative. A basic understanding of antennas might help you in your decision, or at least get you thinking about why one antenna may be better than another in your case. First things first. Here are some antenna-related terms to be familiar with before you decide. The terms are not in any particular order.

Gain: The gain of an antenna is how well it focuses radio frequency (RF) energy in a specific direction. Normally gain is compared to an ideal such as an isotropic radiator, which is theoretical. Higher gain means a more focused or narrower beam. Lower gain means a broader and less powerful signal. Gain is measured in decibels (dB). See dBi and dBd below. Remember that gain does not add power; it reshapes the radiation pattern, making it longer and narrower in the focused direction. See Figure 1.

Figure 1 Gain & Pattern

Isotropic Radiator: An isotropic radiator is a theoretical antenna which radiates equally in all directions from a point, i.e., a sphere. An isotropic radiator has 0 dBi gain.

dBi: dBi is the measurement of an antenna's power gain compared to a theoretical isotropic antenna. High dBi (8+ dBi) is good for long-range, point-topoint communications. Low dBi (2-5 dBi) is good for wide-area coverage. dBi = dBd + 2.15

dBd: dBd is the measurement of an antenna's power gain compared to a standard half-wave dipole antenna. As with dBi higher dBd values are good for longer range and lower dBd values are good for wider area coverage.

Omnidirectional: An omnidirectional antenna radiates and receives radio signals nearly equally in 360 degree horizontal directions. This produces a doughnut-shaped pattern. The pattern may actually be focused in one or more directions depending upon how the antenna is installed, but in general the pattern will be mostly 360 degrees horizontally, with reduced strength straight up. Omnidirectional antennas (whips, vertical dipoles, etc.) are good for mobile systems, but also work fine for fixed systems. See Figure 2 for a 2- meter ground plane antenna radiation pattern on the vertical and horizontal planes.

Figure 2, Omni Pattern & Gain

Directional: Directional antenna focus RF energy into a beam, and this enhances signal strength, range, and efficiency in one direction and reducing interference from other sources. See Figure 5.

Figure 5, Main & Back Lobes

Front-to-Back Ratio: The F/B ratio of an antenna measures the directivity, comparing the signal strength from the intended forward direction to the signal strength from the opposite direction (180 degrees). In other words, it is the ratio of power radiated forward (main lobe) and the power radiated in the opposite direction (back lobe). The F/B ratio is expressed in decibels (dB). A higher ratio indicates a more directional antenna. See Figure 4.

Figure 4, Gain & Front to Back

Impedance: Antenna impedance is the complex ratio of voltage to current at the feed point. Just for clarity it is expressed as Z = R + jX. where Z is the Impedance, R is the Resistance, j is an imaginary unit (beyond the scope of this article), and X is the Reactance. It is measured in Ohms and usually with a vector network analyzer (VNA). Impedance consists of resistance (radiation and loss) and reactance (capacitive & inductive). When reactance is zero in this case, the antenna is considered to be resonant. See Figure 3.

Figure 3, Resistance & Impedance

Efficiency: Antenna efficiency measures how effectively an antenna converts RF energy at its input into radiated energy. Efficiency is expressed as a ratio or percentage. Example: If 100 watts is at the input to the antenna and 95 watts is radiated, the efficiency is 95%, i.e., 95 / 100. Conductor losses, dielectric losses, and impedance mismatch all affect an antenna's efficiency.

Wavelength: A wavelength is the distance a radio wave travels in one complete cycle and that determines the size necessary for resonance. Wavelength is calculated by dividing the speed of light (c) which is around 300 million meters per second, by the frequency (f) in Hertz. Example: the wavelength of 146 MHz is (300 x 106) / 146 x 106 = 2.05 meters, the 2-meter band.

Feed Line: An antenna feed line is the cable that channels the RF energy between a transmitter/receiver to an antenna. Common types are coaxial cable, twin-lead, and ladder line. The impedance of the feed line should match the transmitter and the antenna. Impedance of 50 ohms is popular in the Ham world. Other impedances are also possible, but will need matching systems to ensure the best power transfer between the transmitter and the antenna. An impedance mismatch can cause signal loss and reflected power (high Standing Wave Ratio [SWR])

SWR: The SWR of an antenna is a measure of how efficiently RF power is transmitted from a radio, through a feed line, and into the antenna. SWR is expressed as a ratio, e.g., 1.5:1. An SWR as close as possible to 1:1 is desired, although up to 2:1 is acceptable. Performance is reduced at SWRs higher than 2:1 and greater than 3:1 can lead to equipment damage. A high SWR at lower frequencies means the antenna is too long. A higher SWR at higher frequencies means the antenna is too short. Damaged cables and loose connectors can cause a higher SWR. See Figure 6.

Reflection Coefficient: The reflection coefficient is a measure of the amplitude of a reflected wave to an incident wave at an interface point or impedance mismatch point. The calculation of the reflection coefficient is based on incident impedance and load impedance. Reflection Coefficient = 0 is a perfect match, no reflection. Reflection Coefficient = 1 means a total reflection (open circuit). Reflection Coefficient = -1 means a total reflection with phase inversion, i.e., short circuit.

Figure 6, SWR & Reflection Coefficient

Radiation Pattern: An antenna radiation pattern is a 2-D or 3-D representation of the field strength or power radiated (or received) with respect to direction. Omnidirectional patterns are doughnut shaped radiating equally in the horizontal plan but varying in the vertical plane. Directional patterns show energy concentrated in a specific direction. Yagi, Cubical Quads, and Dish antennas are examples of directional antennas. See Figure 1 shows an example for a 10-meter dipole antenna. The horizontal pattern is red and the vertical pattern is blue. Note the maximum gain is about 7 dBi.

Resonance: Antenna resonance is the frequency where the antenna's electrical length perfectly matches the operating frequency. The impedance at this point is equal to the resistance, i.e., the reactance is zero because the capacitive and inductive reactances cancel each other out.

Near Field: An antenna near field is the area right near the antenna, usually within a few wavelengths. The field acts like a bubble storing energy. Objects in the near field actually become part of the antenna system, causing impedance changes, detuning, and unpredictable radiation patterns.

Far Field: An antenna far field is the area far from the antenna where electromagnetic waves are more organized as planar waves. This allows more accurate measurement of the radiation pattern. Most wireless communications occur in the far field.

Now, that seems like a rather large number of terms. But remember, you don't need to go beyond a basic understanding of them. Go into the details just far enough so you can make a decision on the right antenna for you. No requirement for complex math or engineering concepts unless you really want to delve into that level. Antenna theory is quite complex and involves a lot of difficult mathematics. However, you don't have to get involved with the math. Evaluate your needs by asking yourself some basic questions:

What frequency or frequencies do I want to user the antenna for?

Where am I going to install the antenna?

Is it for a fixed or mobile site?

How much power am I going to be using?

How much gain do I need/want?

Will I need a matching system?

Should I buy or build the antenna?

What is my budget?

There are other considerations, but these a some of the more important. Hope this helps.//Bill