Extra Class question of the day: antenna gain

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 antenna gain? According to question E9A08, antenna gain is the ratio relating 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.” An  isotropic antenna is a theoretical antenna used as a reference for antenna gain. (E9A01) An isotropic antenna is an antenna that has no gain in any direction. (E9A03) That is to say it radiates the power input to it equally well in all directions.

Let’s take a look at a practical example. I often say that the 1/2-wavelength dipole antenna is the most basic amateur radio antenna. Well, 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 have compared to an isotropic antenna is 2.15 dB. (E9A02) Sometimes, you’ll see this value as 2.15 dBi, where dBi denotes that  and 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. An antenna will have a gain 3.85 dB compared to a 1/2-wavelength dipole when it has 6 dB gain over an isotropic antenna. (E9A13) 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.

Similarly, an antenna has a gain of 9.85 dB compared to a 1/2-wavelength dipole when it has 12 dB gain over an isotropic antenna. (E9A14):

Gain over  a dipole = gain over an isotropic antenna – 2.15 dB =
12 dBi – 2.15 dBi  = 9.85 dBd

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