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Directional AM Towers

T

trusty

From the "The Only Dumb Question is the One Not Asked" Department:

Do the "other" towers in the array reflect - or amplify - the signal off the main tower while others "absorb" some of the signal to create a null? Or both? Or something else?

(Reason: I've seen towers set in a N-S direction whose pattern was E-W. I've also seen towers set in a N-S direction with a N-S pattern. Are the other towers (OK, antennas) sending a signal to amplify or null the main antenna's signal?)
 
Much like connecting up multiple speakers, they can be in-phase (additive), out-of-phase (subtractive) creating nulls, or somewhere in between. I don't know (off hand) of any arrays that use passive towers...they are usually all fed with signals, at different levels and phases.

When you get in to some higher frequencies, like shortwave, then the (usually wire) elements may be active or passive....that's when some are fed with signal, and some reflect or guide that signal along.
 
Like kenglish said, it's mostly all about phasing (timing). You can vary the effects of cancellation or addition by increasing or decreasing the power to each tower. You can change the phase electrically, or physically, by moving the towers closer or farther and by changing the length of the transmission line.

There are some old arrays that have passive towers. These are not popular because they're harder to control than a "driven" tower.
 
This condition is common in phased arrays and referred to as “towers which are negative in impedance”. Mostly found on multiple in-line arrays where the lobe is off the end of the line of towers. Usually the last tower or towers towards the lobe are driven into negative impedance. In these cases, the power from the negative tower is sent back to the phasor common point and added in-phase so that the loss is minimized. In other cases where the power is very low, it is simply dissipated in a resistor. Most designers today can see this phenomenon develop and stay away from it because it makes the system very difficult to setup and a problem to maintain. Broadcast antenna systems are required to meet minimum efficiency standards for the class of operation, so that is why the power has to be conserved.
w/
 
I've seen a couple of arrays where one tower has a very slightly negative impedance. This condition makes the pattern unstable. In these cases, a resistor was added so that the negative tower was further from zero ohms impedance. Using a resistor prevents the tower from swinging from a negative impedance to a positive impedance.
It stablizes the array a bit.
 
I built an array where one of the towers was driven down to 7 ohms. Its stable, but I sure wouldn't want one that went below 5. To much risk of the tower doing what you said. Changing sex on you.

I've actually seen only one array with a "non-driven" tower. It was the "front" tower in a three tower array. The passive tower was tuned via a stepper arrangement which could be remotely adjusted to switch various resistance into the tuning network.
 
stacker said:
I built an array where one of the towers was driven down to 7 ohms. Its stable, but I sure wouldn't want one that went below 5. To much risk of the tower doing what you said. Changing sex on you.

I've actually seen only one array with a "non-driven" tower. It was the "front" tower in a three tower array. The passive tower was tuned via a stepper arrangement which could be remotely adjusted to switch various resistance into the tuning network.
Click to expand...

In my case, the resistor was actually driven by the Phasor. This arrangement gave me infinite control of the phase and ratio for that tower. The series resistor merely acted as an attenuator as it increased the tower's apparent impedance.
 
I think the worst I ever saw was 4 tower parallelogram spaced 60° around the corners and 54° across, 59° tall + 20° top load on 560 kHz. RSS/RMS = 4.5 and 2 of the 4 were negative about 5 and 5 ohm respectively. Mercifully, site was sold and is currently being converted to housing tract. Had a few 4, 5, or 6 inline that might come close, but tossup.
 
One of mine was a four tower Parallelogram. On the day pattern, all towers had a good positive impedance. On the night pattern, one of the towers was about -1 ohm.
Adding a 10 ohm resistor brought the tower to 11 ohms and made the pattern much more stable.
 
think the worst I ever saw was 4 tower parallelogram spaced 60° around the corners and 54° across, 59° tall + 20° top load on 560 kHz. RSS/RMS = 4.5 and 2 of the 4 were negative about 5 and 5 ohm respectively. Mercifully, site was sold and is currently being converted to housing tract. Had a few 4, 5, or 6 inline that might come close, but tossup.
Click to expand...

RSS/RMS ratio 4.5!! Amazing. Whoever convinced someone to build that needs to be selling airtime!
 
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