Antenna gain is one of the most misunderstood topics in amateur radio. There are several reasons for this, including:
- Antennas don’t really have gain in the same way that an amplifier has gain. When you use a linear amplifier, you get more power out than you put in. Since transmitting antennas are passive devices, there’s no way to get more power out than you put in.
- It’s not easy to measure antenna gain. There is no antenna gain meter that you can simply hook up to an antenna to measure its gain.
So, what is meant by antenna gain? Antenna gain is the ratio of the radiated signal strength of an antenna in the direction of maximum radiation to that of a reference antenna. What this means is that when you talk about antenna gain, you have to know what kind of antenna you’re comparing it to.
When talking about antenna gain, antenna engineers often refer to the “isotropic antenna.” In practice, an isotropic antenna is a theoretical antenna that has no gain in any direction. That is to say it radiates the power input to it equally in all directions.
QUESTION: What is an isotropic antenna? (E9A01)
ANSWER: A theoretical, omnidirectional antenna used as a reference for antenna gain
Let’s take a look at a practical example. The 1/2-wavelength dipole antenna is the most basic amateur radio antenna. The dipole actually has some gain over isotropic antenna. The reason for this is that it is directional. The signal strength transmitted broadside to the antenna will be greater than the signal strength transmitted off the ends of the antenna.
The gain of a 1/2-wavelength dipole in free space compared to an isotropic antenna is 2.15 dB. Sometimes, you’ll see this value as 2.15 dBi, where dBi denotes that an isotropic antenna is being used for this comparison. Since the isotropic antenna is a theoretical antenna, some think it’s better to compare an antenna to a dipole antenna. Let’s look at an example:
QUESTION: How much gain does an antenna have compared to a 1/2-wavelength dipole when it has 6 dB gain over an isotropic antenna? (E9A12)
ANSWER: 3.85 dB
You obtain this value by simply subtracting 2.15 dB from the 6 dB figure:
Gain over a dipole = gain over an isotropic antenna – 2.15 dB = 6 dBi – 2.15 dBi = 3.85 dBd
Sometimes, the gain over a dipole is denoted as dBd.
Effective radiated power
When you use an antenna that has gain, you are increasing the effectiveness of the power input to it in the direction the antenna is pointing. We call this the effective radiated power, but it is not just the transmitter’s output power times the gain of the antenna. You also have to take into account losses in other parts of the antenna system.
This is especially true for VHF and UHF repeater systems, where losses in the feedline, duplexer, and circulator can be significant. The power that reaches the antenna may be substantially lower than the power output of the transmitter.
Let’s look at an example. Say that your repeater station had a transmitter output power of 150 watts, a feed line loss of 2 dB, 2.2 dB duplexer loss, and 7 dBd antenna gain. To calculate the effective radiated power, you have to first subtract the losses from the gain, as expressed in dB to get the total gain of the system:
total system gain = 7 dB – 2 dB – 2.2 dB = 2.8 dB.
Now, recall that 3 dB corresponds to a power ration of 2:1, as shown in the table below. 2.8 dB would then be slight less than that. In fact, 2.8dB corresponds to a power ratio of approximately 1.905, so the effective radiated power is the transmitter output power times the total system gain:
effective radiated power = 150 W x 1.905 = 268 W.
QUESTION: What term describes station output, taking into account all gains and losses? (E9A13)
ANSWER: Effective radiated power
QUESTION: What is the effective radiated power relative to a dipole of a repeater station with 150 watts transmitter power output, 2 dB feed line loss, 2.2 dB duplexer loss, and 7 dBd antenna gain? (E9A02)
ANSWER: 286 watts
Let’s look at another example. In this example, your repeater station has 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain. The total gain of the system is, therefore, 10 dB – 4 dB – 3.2 dB – 0.8 dB, or 2.0 dB. 2.0 dB corresponds to a power ratio of approximately 1.585, making the effective radiated power 200 W × 1.585 = 317 W. Note that in this system, the high feedline and duplexer losses almost completely negate the benefit of using a high gain antenna.
QUESTION: What is the effective radiated power relative to a dipole of a repeater station with 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain? (E9A06)
ANSWER: 317 watts
Here’s a third example. Notice that in this example we comparing the effective radiated power to an isotropic antenna, not a dipole. In this example, the repeater station has 200 watts transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, and 7 dBi antenna gain. The total gain of the system is 7 dB – 2 dB – 2.8 dB – 1.2 dB, or 1.0 dB. 1.0 dB corresponds to a power ratio of approximately 1.26, and the effective radiated power equals 200 W × 1.26 = 252 W.
QUESTION: What is the effective isotropic radiated power of a repeater station with 200 watts transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, and 7 dBi antenna gain? (E9A07)
ANSWER: 252 watts
Feedpoint impedance
Other antenna parameters are also important, of course. One of the most basic antenna parameters is the feedpoint impedance. The reason that the feedpoint impedance is important is that you want the feedpoint impedance to match the impedance of the feedline that you use and the output impedance of the transmitter. When these are all equal, we say that the system is “matched,” and it will radiate the maximum amount of energy.
Many factors may affect the feed point impedance of an antenna, including antenna height, conductor length/diameter ratio and location of nearby conductive objects. For example, we say that the feedpoint impedance of a half-wavelength, dipole antenna is 72 Ω, but that’s only really true if the antenna is in free space. When it’s closer to the ground than a quarter wavelength, then the impedance will be different. That’s why you have to tune the antenna when you install it.
QUESTION: Which of the following factors affect the feed point impedance of an antenna? (E9A04)
ANSWER: Antenna height
Radiation resistance
Another antenna parameter that’s frequently bandied about is radiation resistance. The radiation resistance of an antenna is the value of a resistance that would dissipate the same amount of power as that radiated from an antenna. In the case of an antenna, however, that power isn’t being turned into heat, but rather turned into radio waves. The total resistance of an antenna system is the sum of the radiation resistance and ohmic resistance. The ohmic resistance is the combination of all the physical resistances in the system, including the resistance of the antenna wire, feed line, and connections.
QUESTION: What is the radiation resistance of an antenna? (E9A03)
ANSWER: The value of a resistance that would dissipate the same amount of power as that radiated from an antenna
QUESTION: What is included in the total resistance of an antenna system? (E9A05)
ANSWER: Radiation resistance plus loss resistance
If you know the radiation resistance and the ohmic resistance of an antenna, you can calculate its efficiency. Antenna efficiency equals the radiation resistance divided by the total resistance.
QUESTION: What is antenna efficiency? (E9A09)
ANSWER: Radiation resistance divided by total resistance
Vertical antennas, bandwidth
Vertical antennas are sometimes criticized as being inefficient antennas. The main reason for this is the lack of a good radial system. Installing a good radial system will improve a quarter-wave vertical’s efficiency. Another for poor vertical performance is poor soil conductivity. If soil conductivity is poor, ohmic resistance will be high, and the antenna’s efficiency will be low.
QUESTION: Which of the following improves the efficiency of a ground-mounted quarter-wave vertical antenna? (E9A10)
ANSWER: Installing a radial system
QUESTION: Which of the following factors determines ground losses for a ground-mounted vertical antenna operating in the 3 MHz to 30 MHz range? (E9A11)
ANSWER: Soil conductivity
Antenna bandwidth is the frequency range over which an antenna satisfies a performance requirement. Normally, the performance requirement is an SWR of 2:1 or less. In fact, you’ll sometimes hear this parameter referred to as the 2:1 SWR bandwidth.
QUESTION: What is antenna bandwidth? (E9A08)
ANSWER: The frequency range over which an antenna satisfies a performance requirement
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