Recently, we had a client question why we didn’t mount antennas higher up on a tower with an FM repeater on it. The top of the tower has an FM repeater on it so we mounted the equipment about 25 feet below that.
When you are talking about antennas and transmitters the basic thing to remember is it’s all radiation. Good antennas have predictable drop off patterns and, when paired with a good transmitter, have crisp frequency drop offs. However, there is still radiation emitting from feedline and the antenna on the tower. Many FM repeaters use a dipole design. Some are folded, others are different types. Below is an antenna pattern from a Dipole antenna.
As you can see there are a few patterns radiating from the antenna. These patterns should be taken into consideration when mounting your equipment near FM, UHF, or VHF systems. Radiation may interfere with things such as your cat-5, or your PIM. In an earlier article, I talk about low-pim cables and what affects PIM. This is very important when you are deploying LTE gear. RF radiation from high power transmitters can cause PIM issues if the wavelength happens to coincide with the wavelength of the other transmitter. This does not mean they are on the same frequency. Remember, in RF you have full wave, 3/4,1/2, and 1/4 wavelengths to deal with.
Other things to consider are near and far field patterns. If you want some heavy reading you can read about it on Wikipedia.
Our next issue and the most common issue is the radiation getting into our Ethernet cables as well as our radios on the tower. Below illustrates the propagation of signals coming out of an antenna on the top of the tower. If you notice, some of the radiation is directed underneath of the antenna. Any equipment mounted too close underneath will be bombarded with radiation.
Too much radiation can cause link negotiation issues, signal degradation, and other issues. By moving our antennas out of the patterns of other antennas we can make for a more reliable system. This is one case where higher on the tower is not always better. Just because another antenna is not mounted in front of another it does not mean they are in each other’s radiated patterns.