How is antenna performance measured?

TV reception is a funny thing. While we do our best with the wide variety of tools we have, there’s no way to guarantee that a particular antenna will work in a particular location, short of actually putting it up there. However, we do have a lot of tools that help us predict what an antenna will do and feel pretty confident about it. One of those tools is the antenna’s measured performance.

Antenna performance can be measured several different ways, depending on how precise you feel you need to be. Let’s take a look at how we measure an antenna’s performance and look at how it’s possible to make a pretty good guess about how an antenna will do in your home.

How performance is measured.
We say that an antenna has a “gain” number and that’s the number that we use as a general guide. Sometimes we’ll split that number out into VHF and UHF, since different parts of an antenna are used for different frequencies. Gain is measured in decibels, or dB. Decibels are a relative measurement that help you understand how much more of something you have. When you see an antenna has a 14dB gain, there are three possibilities:

  • The gain is measured against something called a “reference dipole” which is a very basic antenna that everyone agrees to use for measuring stuff. When a reference dipole is used, generally gain numbers are quoted as “dBd.”
  • The gain is measured against a theoretical antenna called an “isotropic radiator.” This means it’s not a real-world test but the number is better at predicting what you’ll get in an ideal condition. When this is the case, gain numbers are quoted as “dBi.”
  • If any other method is used, the gain numbers are quoted as “dB” meaning that the antenna is better than whatever the lab used to test. Often times a bare copper wire is used or a signal meter is left disconnected to get a basic measurement.

Getting more detailed
While we’ll often say an antenna has a gain like “15dB in the UHF band” that’s still pretty broad. If you’re interested in how an antenna performs over the entire frequency range you need to look at a plot. This is a typical antenna range plot.

You’ll see that the performance of an antenna changes depending on the frequency that it’s trying to pick up. Two antennas both rated with the same overall gain number might perform differently because of the actual channels you’re trying to pick up, and that’s important because the channel allocations are different in every city.

Getting even more technical
If you really want to understand an antenna’s performance, you need to realize that most antennas are directional. That is, they perform differently depending on how you aim them. So if you’re really going to delve into antenna performance, you need to look at a polar plot like this one:

What you’re seeing here is how an antenna performs as it’s pointed. The blue line is gain, and where it gets further from the center, it’s showing more gain. So this particular antenna works great for things directly in front or in back, but not so well on the sides. There’s a lot more to this sort of chart but this is that part that’s important.

So really when you look at a gain chart and polar plots you’ll get an idea how that particular antenna is going to perform, at least in theory. When you get the antenna into place, it can be affected by everything from the trees and hills in front of you to the weather on a particular day. Of course, starting with the best information will help, but in the end the only true measure of how an antenna will perform for you… is how it does perform.