[time-nuts] What are these towers?
Lamar Owen
lowen at pari.edu
Fri May 20 22:59:53 UTC 2011
On May 20, 2011, at 6:34 PM, David Martindale wrote:
>
> Apparently, about 2/3 of the transmit power is fed to just one of the
> eight towers, and most of the remainder goes to a couple more. So the
> majority of the towers are there for nothing more than creating and
> aiming the null in the pattern.
>
Phased array AM's are rather interesting beasts; for eleven years
('92-03) I served as Chief Engineer for some AM stations, two of which
are directionals. The specific field ratio and angle phasing
(relative to the reference tower) can be very low, very high, and even
negative, based upon the self and mutal impedance of the towers.
Negative resistance towers (that 'eat' power) make things
interesting. One of the stations I CE'd for is a four tower inline,
each tower 90 degrees tall, and spaced 90 degrees. Four towers with a
pattern as tight as a twenty element yagi. The two center towers had
approximately the same base current, and the two end towers had
approximately the same base current, but the phases set things.
Specific engineering data (first column: tower number, second column
relative field (relative to tower 1), third column phase, fourth
column spacing from tower 1, fifth column bearing from tower 1):
> Tower information:
> 1 1.000 54.00 0.00 0.00
> 2 1.963 -101.90 90.00 122.00
> 3 1.963 101.90 180.00 122.00
> 4 1.000 -54.00 270.00 122.00
>
The active phasing cabinet (the 'phasor') has a pair of controls to
set common point operation, a pair of controls for current and phase
for each tower, and an input tank tuning to set the common point
impedance. Each tower then has an antenna tuning unit to match the
line impedance of 50 ohms to the base impedance of each tower; all in
all a very fun system to tune.
That specific array is fed with 1.1kW input power, produces a 1kW RMS
field, and has a forward power approximately equal to 6kW and a
reverse power of 0.18W. Behind the array there are places you can
physically see the array, but not receive the station.
Anyway, phased arrays are fun.... in theory. In practice, when the
array is in a mixed cow/horse pasture and you need to fix a contactor
at a tower (for pattern change, RF contactors) at 2AM.... well, not so
fun. Which is why I'm now in IT. Much less stress.
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