Why don’t all TV antennas pick up VHF?

This is a UHF antenna. It even says so in the title. It doesn’t look so terribly different from this antenna, which says it’s a UHF/VHF antenna. Sure, the careful observer will realize that there are a few more chunks of metal here and there, but to most people they look pretty much the same.

So how can one be UHF-only, and one be UHF-VHF? Can an antenna really be that smart? If so, that would mean it was able to selectively listen to some things and not others. I have relatives who are not that smart.

One way an antenna can pick up some frequencies and not others is through the size of the antenna itself. An antenna picks up frequencies best when it’s an even fraction of the wavelength, and the bigger the fraction, the better it does. Let’s look at that more closely.

The wave for channel 7 is about 67 inches long. Really. Picking it up effectively means using an antenna that’s 1/4 that length, about 14″. Smaller than that and it won’t do a good job. On the other hand, the wavelength for channel 51 is about 16 inches long, meaning that an antenna 4 inches long will do a reasonable job. On the other hand, that same antenna won’t do a good job with channel 7 because it’s less than 1/8 the size of the wavelength and it’s not close to the actual fractional wavelength. This is why you can have a small antenna that picks up UHF and it won’t do VHF very well.

See, that’s the key to this… the words very well. You’re right if you’re thinking that an antenna is just a stupid piece of metal and it can’t “decide” what frequencies to pick up. Even an antenna that’s designed for satellite TV frequencies will pick up VHF a little, it just won’t do it very well at all. When we say something is a UHF-only antenna we usually mean it just do such a bad job at VHF that there’s no point to it.

The other way an antenna can be UHF-only is through the use of a notch filter. A notch filter is actually smart; it uses electronics to almost completely block a set of frequencies. Some more expensive UHF antennas use notch filters so that they can be combined with standalone VHF antennas for better performance. Without a notch filter it’s possible that the weak VHF signals from the “UHF” antenna could interfere with those from the VHF-only antenna. Notch filters are also used in some combiners so you don’t have to worry whether or not you have one on the antenna. (Having two of the same notch filters doesn’t hurt anyone.)