Odd Directional Pattern

[wpiv926]

While most of my engineering has involved FMs, I've done some AM in the past. I have a question about the pattern on KCMC-740 AM in Texarkana. The station runs 1KW, day and night from a 3 tower, in-line, evenly spaced array that is oriented due north to south. Funny thing is, the pattern goes just the opposite direction, east to west. All directional arrays that I've seen of that type, 3 in-line towers evenly spaced, always have a pattern that goes in the direction they (the towers) are themselves pointed, but not this station. Note: KAAY -1090 AM in Little Rock has 3 towers, in-line, evenly spaced and of the same height, and the nighttime signal goes exactly the way the array is situated, NNW to SSE. Any ideas?

 

[ncradioeng]

Depending on the phasing and power ratios of the towers, the pattern may very well be the way you described.

 

[Doctor_Technical]

Like NC said, one can design lobes and nulls in any direction, depending on the phasing/spacing/relative power levels of the towers. The limitation of an in-line array is that the minima and maxima on one side of an inline array will be the same as on the other side. In other words: an AM pattern is symmetrical around the line of an in-line tower away.

Thus the reason for dog-leg, square, parallelogram or my favorite "towers placed in seemingly random places where the computer said to place them" arrays: desired-maxima and mandated-minima don't always land at convenient angles to where you can practically place a tower array.

Look up the KTNQ/KTLK site on http://www.radio-locator.com. Note the new feature where you can click on the coords, then select hybrid or satellite view for an aerial view of the site. Also, here's a great book on DA design: http://www.broadcastbooks.com/

Paul E. Burt
Jackson, MS

 

[wpiv926]

One thing that I didn't consider but just realized was the real estate that was available at the time of the construction of the site. It might have had to be built that way in the past. Also, if you check out KCMC at radio-locator, note the slight "hump" to the north and south.

 

[PTBoardOp94]

The basic idea for such a site would be middle tower does all the work, and N & S towers just negate signal, right? I'm sure there are other ways, but that seems most obvious to me.

 

[mr_elbows]

it's a variation of the classic 'spaced 180 degrees / fed in phase' set-up, with three towers used (probably) to get a decrease in skywave in the N-S direction

note that the spacing is 192 degrees, on the south tower the + 20.5 phase angle almost compensates for the increased spacing; on the north tower the phase angle is -5.5 degrees, that accounts for the small 'bump' due north, the differences in field ratios between N and S towers (0.49 and 0.56) are what causes the major lobes to be skewed slightly south of E and W

 

[radiosaur]

. Note: KAAY -1090 AM in Little Rock has 3 towers, in-line, evenly spaced and of the same height, and the nighttime signal goes exactly the way the array is situated, NNW to SSE. Any ideas?

Actually, that's incorrect. The KAAY towers are on a SW to NE line, with the main lobes NW to SE. Interesting site, BTW. No phasors, as the towers are all in phase.