Electrical – How to combine two antennas that carry different frequencies

There are different types of MIMO. Those are Precoding, Spatial multiplexing, and Diversity Coding.

Precoding

The idea behind MIMO is that at the frequencies being used, the wavelength is small enough that even 30 cm apart is enough to receive the signal at different phases. As Brain said, the wavelength is about 12.5cm for 2.4 GHz. This means that regardless of how far you are from the two antennas, the delay (or phase delay) between the two antennas will always be fixed for any given angle.

You are able to take advantage of this phase difference to create beam steering. The math and actual implementation of this is complex, but the general idea is actually relatively simple. If the two signals are in phase, then you know that the source of that signal is the same distance from each antenna which means that your source has to be somewhere along the line of symmetry.

As the source begins to move around, the signal will get to one or the other antennas first and the angle from the receiver can be determined based off of the amount of delay between the two. This then allows you to setup "sectors" or beams based of off how much delay is applied to the incoming signal.

Now technically the drawing I showed is only MISO (Multi in single out), but the logic holds true when you add another antenna to create a full MIMO. Also, on the transmitting side, you can do the same thing I talked about with receiving, but instead a delay is applied to one or the other antenna to create a beam in specific direction out of the transmitter.

The accuracy of the angle in and out of each pair of antennas is determined by both the spacing of the antenna and the accuracy of electronics to produce and detect a specific phase shift.

Also things get more complex as you start to account for the fact that at some locations the signal might appear to get to the antennas at the same time but are actually 1 full cycle apart. Also there has to be a control system setup to know what direction you should be directing you beam at, especially when you have a moving device.

But to get to your question directly, it doesn't matter if your source has 2 antennas or not, it is treated the same on the receiving end. What maters is the angle that the source is from the destination. You essentially end up with a source directing its beam in the general direction of the receiver and then the receiver is steering its beam in the general direction of the transmitter.

The big advantage of using MIMO is that you are not creating a lot of extra noise for neighboring devices and so you are able to get more devices in to a small area. Also, since the signal is more directional there is less to bounce off of which results in less issues with multipath.

The electromagnetic wave and antenna don't know if the TV is analog or digital. To them, it's the same! This means that most of analog transmission experience applies to digital TV as well.

So in general, you should point the antennas correctly for best signal quality and use proper polarization as well.

You can use multiple antennas for best signal quality, but there are some bad sides to that as well. Let's say you have an antenna correctly set up for one signal source. It is designed for its frequency, has sufficient gain and is correctly pointed at the signal source. That antenna will still pick up signals from other sources as well, but they will be weaker. If you have another antenna matched to the weak signal source of the first antenna and you combine them both directly, first antenna's weak signals can interfere with second antenna's strong signals. Usual solution to that is to use a diplexer. It's a device that has a band pass filter (some implementations may just use low pass and high pass filters) and you set it up so that it passes through the frequencies which a particular antenna is receiving the best and blocks other signals. This way, at the output of diplexer you'll only have the best signals from all antennas you have and weak signal from one antenna won't interfere with a strong signal from another antenna. Of course, diplexers have their own insertion losses, so they'll decrease the signal quality. The idea is to use as few antennas as possible to get the best signal and to use diplexers sparingly in order to prevent their insertion losses from having a negative net effect on signal quality.

Some digital TV standards allow use of "auxiliary" broadcast locations so that you have one main transmitter and few smaller transmitters connected to the main transmitter. In such setups, the signals from the smaller transmitters shouldn't interfere with the signal from the more powerful transmitter. Unfortunately, I don't have any practical experience with this, so I won't go into any details.

The other important thing is the digital cliff and analog slope. In digital TV systems, as long as your signal quality isn't horrible, you'll get relatively good picture. Problems will occur once it gets borderline and then quickly after that, you'll completely lose signal. You can take advantage of that to have good picture quality even with not so good signal, but the downside is that you'll be more sensitive to bad weather than if you had good signal quality. On the other hand, in some situations setting up a proper TV antenna system can require expensive antennas, cables and expertize so exploiting the digital cliff effect can be economically justified.