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Question from a non-tech radio listener: Directional signals

T

thathoustonradiogeek

How does an AM directional array works?
What do terms like phasor, parasitic and beam tilt mean?
Can an FM station be directional? How does THAT work?

In layman terms please. ;D

Thanks in advance.
 
1) Essentially, an AM directional array works by placing multiple towers in a certain pattern to produce a desired signal.

I'll use WBAA-AM, the radio station licensed to Purdue University
They utilize an offset three tower array at night to produce a signal null to the south.

This is accomplished by splitting the signal from the transmitter output and directing a different portion of it to each tower. In this array, I suspect the middle tower does most of the work. The southernmost tower would receive less power, and it would help generate this southern null. The northernmost tower would also help cancel power and generate the slight northeastern rotation the pattern has.

The more towers in an array, the more complicated the pattern, generally. A one tower array would have a theoretically circular pattern. A two tower array will typically have a carotid pattern.

2) You may know that any wire has a resistance and inductance, including the wire that stretches under the ground to provide the signal to the multiple towers of a directional array. Inductance changes the phase of a signal. Using the WBAA example again, the transmitter plant is by the middle tower. If there were no phasor, the north and south towers would be a few milliseconds out of phase with the central tower, leading to the directional array not working properly (generating a poor signal, and/or not in pattern). That is, a phasor will compensate for the imperfections of the wire in use.

3) Beam tilt can be used to focus an FM signal on one particular area. For example, the Denver area FMs probably aren't getting on the western side of the Rockies, so a couple degrees of beam tilt might be used to direct more of the signal down into the Denver metro. (I don't know if any of the Denver stations do this, but its theoretically possible)

4) An FM station can be directional. In fact, most FM stations are directional :) To create an FM directional antenna is roughly the same concept as the AM directional antenna. If you have a multi-bay FM antenna, you would alter the power going into one or more of the bays. You could also alter the alignment of the bays. I say most FM stations are directional because they are often side-mounted on towers, and the tower can affect the signal propagation.
 
AMs and FMs can both be directional, however it is done in different ways. FM directionalizes by the use of what's called parasitic elements. Basically, they are pieces of metal that are put in front or behind the radiator to deliberately distort its pattern. This is similar to how many outside TV antennas work.
The reason it's done this way is mainly simplicity. See, the FCC does not allow deep nulls in an FM's pattern, nor does it allow tight ones.

On the other hand, AM's nulls are permitted to be as deep as you can make them-and as many as you need! With directional AM, all the towers have some power in them, and the resultant pattern evolves from the number of towers, their heights, the distance between them, their geographic layout, how much power is in each one, and the timing of that power (phase). As you can see, there are many variables involved compared to FM, so computers are generally used to come up with a workable AM pattern, while with FM directionals experimentation with an actual antenna on a field range is generally done. Of course, the average person can hold an FM bay-picking up and moving around a 400 or bigger AM tower isn't really practical!

The phasor is the physical device that actually splits the power to each tower and also controls the timing of that power. The antenna monitor measures the signal generated (amount and timing) by each tower relative to one tower in the array (set up by the consultant and called the REFERENCE tower). There are actually pick-ups attached to each tower to receive these signals and they are brought back into the transmitter building by small coax cables. Since most stations have different directional patterns day and night, frequently there are different towers used in each pattern, shared towers and also different reference towers for each pattern. As you can see, things can get quite complicated!

I hope this has been helpfull to you.
 
I can't conceive of beam tilt being introduced into an AM directional.

The place I have seen beam tilt used is in the antennas for cell-phone base stations. The early cell phone installs were trying to reach as far out toward the horizon as possible, so no downward tilting of the signal.

As the customer base grew, and more towers were needed to handle all the traffic, the towers were placed closer and closer together, The carrier did NOT want to power aimed at the horizon where it would end up reaching into the area covered by the next tower using the same frequency. One of the techniques was to mechanically mount the antenna in a tilted angle. That worked reasonably well straight out in the direction the antenna is aimed. But the power that spread out horizontally 30, 45 and 60 degrees from the center did not angle down as much. The designers found ways to electrically tilt the signal with some of the same laws of physics LaGuy explained earlier. This caused the power going to the side of the main power to also tilt down. Think of taking a CD, hold it with its main surface parallel to the earth. Lean one side down a bit. That is like mechanically tilting the cell tower antenna. Take saucer or a bowl and turn it upside down. The rim or lip of your bowl droops uniformly to either side of the center of your power.

Cell phone antennas for the towers are high directional. You may have noticed that the antennas are usually mounted on a triangular "birds nest" with two or three antennas on each face of the triangular birds nest. These antennas DO NOT put out power in all directions like that saucer or bowl that I suggested you imagine. The power goes out in a narrow beam shaped like a slice of pie. The same frequency can be used to carry three different calls from the one tower but directing the power in those narrow pie shapes.
 
Null fill and beam tilt are used in FM
 
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