I'll try...

Directional AM (medium wave) antenna arrays came about to increase the number of available frequencies to a allow more stations on the air. Two stations can use the same frequency as long as they direct the signal away from each other. A directional array can be as simple as two towers (I will now refer to towers as elements) or as complex as 12 or 13. Each of these elements has a role in creating the directional pattern. The height and the space between each element is carefully calculated to cause the elements to interact with some/ all / none of the other elements. The second pic shows a five element array.

Phasing. Depending on the number of elements and the desired patters a phasor is installed usually in the transmitter building. It is a box with adjustable coils capacitors as well as other stuff with adjustments that allow signal generated from the transmitter to be split and sent separately to each element. By varying the amount of power as well as its timing (phase) the signal may arrive at the element slightly delayed or in a different phase. This will interact with the signal being sent to the other elements at the same time and induce a pattern. Adjusting one element willl usually effect the behaviour of the others. It is a balancing act. There are measurements taken around the transmitter site to sense the direction and the signal. Also the elements usually have a sampling loop (pic 1) that sends information by wire back to the transmitter shack to measure the radiation of the element. Usually this is a one time set up, once set it is not adjusted again unless a change in the array is needed.

Depending the pattern desired these elements can be switched in or out changing the direction of the signal. Not all elements may be phased, they may simply work as a reflector; kind of like having a mirror behind a light bulb.

This is a very simplistic explanation. There is lots of information much more through than this. I hope this helps a little.

In my opinion a well engineered, constructed and maintained directional array is a work of art! You need to know what you are doing!


 

Last edited by darcyh (December 7, 2023 12:18 pm)

WindsorWatcher wrote:

Much appreciated, darcyh. 
Your explanation is  comprehensible in explaining the various features and how they interact.
The task is indeed a work of art as well as an engineering challenge. It also explains why so many AM sites are no longer up to snuff due to  shortchanging maintenance and upgrades due to  cost. 
So theoretically, a 10,000 watt station could split the power of 2,500 watts each to 2 towers and 5,000 to another via the phasor if that would achieve the directional properties sought?

Yes. Kind of. It depends. Don't you love these answers . There is no requirement for the split to be balance or  linear.
 
So, theoretically to establish a day pattern and assume 4 towers: you may apply power /  phasing as 80 % tower one, 5 % tower two, 11 % to tower three and 4 % to tower four. At sundown this equation could change to any other combination. The pattern can be calculated by computer to some extent. Back in the old days not so. Even today I find the models off sometimes, but they are accepted by the FCC and Industry Canada.

Perhaps a simpler answer would be WJR operates on clear channel 760 with a non directional pattern. I can pick them up in my car during the day with a listenable [to a dxer] signal. CKLW on the other hand has a null to the east to protect CJBQ Belleville. I rarely hear CKLW during the day in Toronto.

I believe CJBQ Belleville is 10 kilowatts. From radio locator although they have been known to be wrong.

https://radio-locator.com/cgi-bin/finder?call=CJBQ&x=8&y=4&sr=Y&s=C

Last edited by darcyh (December 8, 2023 1:02 am)

CKLW Windsor is 50 KW, DA-2
CJBQ Belleville is 10 KW, DA-2
CJAD Montreal is 50 KW days / 10 KW nights, also DA-2

One night I stayed in Ingersoll (between London and Woodstock) and all I heard on AM 800 was something in Spanish which was likely coming from Mexico. In Northern Ontario I could hear all three of those Canadian stations trying to come in over top of each other. 

I believe its call letters are PJB.

Here is another story about behemoth AM stations. Cincinnati's WLW was on the air with 500,000 watts in the 1930s. 

https://mwcircle.org/most-powerful-mw-am-stations/