There is an additional problem with AM towers in that any tall metal structure within about three kilometers of an AM tower can become what is called a parasitic radiator and can interfere with AM transmission. This has meant that nobody builds other wireless towers close to an AM tower, and the areas around an AM tower are often cellular dead spots – to the detriment of folks that happen to live close to a tower. Since there are around 10,000 AM broadcast towers, this implies many thousand wireless dead zones.
But the AM towers don’t have to be a wasted asset. There are two methods that can be used to install other radios on AM towers that often get overlooked by cellular companies and WISPs. The methods both rely on isolating the new antennas from ground at the same frequency as the AM transmission.
The first technique is known as a folded unipole. This consists of a vertical metal rod, called a mast, that is connected at the base of the AM tower to a conductive surface called a ground plane. The mast is surrounded by a series of vertical wires attached at the top of the mast and extended to a metal ring near the mast base. The feed line for the mast is connected between the ring and the ground. These wires must be mounted at carefully calculated heights. If installed properly, the tower can be isolated and used for other radios. This is a common technique used to connect an FM transmitter to an existing AM tower, but it can also allow for cellular or fixed wireless radios.
The other method for isolation is to install electronics on the transmission line that carries the radio content signal to the antenna. The most common device is called an iso coupler, which allows RF signals within a certain frequency range to pass through while continuing to isolate the AM signal from ground. That might mean allowing through the signal from cellular or fixed wireless electronics to bypass the effects of the AM signals on the tower. Another device that performs roughly the same function is a coil device that can isolate the new antenna signals from the AM signals.
Both of these methods are referred to as detuning, meaning that a new radio can be isolated from the tuned AM signal that permeates the whole tower. Most engineers who are looking for towers avoid AM towers in the belief that it’s too complicated or costly to detune the tower to add other transmitters. Admittedly, getting this to work requires an experienced RF engineer who understands AM towers. But it’s a common practice used most often for adding FM transmitters. I’ve talked to some folks who say the process can be surprisingly affordable.
Anybody looking for tower space shouldn’t shy away from this option because the folks who own AM towers are likely open to negotiating an affordable connection since they don’t often get the opportunity.