What is the difference between active and passive LTE filtering?

It’s getting to be more of a problem all the time. As cell service gets better and better, it threatens over-the-air antenna service. LTE service operates on frequencies that were once used for broadcast television and a lot of antennas are still capable of receiving it. These signals are much stronger than distant antenna signals and can cause problems with TV reception.

Even though you can’t tune those channels anymore, there’s a funny thing about all radio transmissions and it’s called a “harmonic.” When you broadcast at 700MHz, there’s also an effect at 350MHz, 175MHz, 89.5 MHz, etc. There’s even some effect at 1400MHz. The point is that every frequency has an effect on the frequencies at half, 1/4, 1/8, etc. It’s just the way this sort of thing works, and it’s not just radio frequencies either… the same effect happens if you pluck a string, which is why the term “harmonic” is used. There are even “fractional” harmonics which makes it very hard for anyone but a math genius to predict the frequencies that are going to be affected by any other particular frequency.

This sort of interaction happens all the time and it’s not a huge deal because usually you’re dealing with transmissions that are about the same strength. Digital TV is designed to avoid these problems and it rarely comes up… except now with cell towers everywhere you potentially have a much stronger broadcast source that could be interfering.

That’s why you need an antenna with built-in LTE filtering. But there are two kinds, and here’s what you need to know.


Passive LTE filtering
An antenna like this Televes DAT970 Mix is designed to passively filter LTE signals out just through the way it’s made. The unique way that the three directors at front aim and focus the signals helps reject LTE signals from coming into the antenna at all. That’s the first step in avoiding interference problems.

Active LTE filtering
This very same antenna actually has circuits that make sure that any remaining LTE frequencies are filtered out. First of all, a band stop filter circuit is used to reject all LTE frequencies, and then the amplifier is tuned so it does not amplify those frequencies. So, with an antenna like this you’re actually getting three different levels of protection against LTE signals.

LTE is here to stay and if you’re an antenna enthusiast you’ll need to be aware of it as it evolves. Start by choosing an antenna designed to help filter out those frequencies and you’re already one step ahead.

About the Author

Stuart Sweet
Stuart Sweet is the editor-in-chief of The Solid Signal Blog and a "master plumber" at Signal Group, LLC. He is the author of over 8,000 articles and longform tutorials including many posted here. Reach him by clicking on "Contact the Editor" at the bottom of this page.