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Many AM stations are required to switch powers and patterns for daytime/nighttime operation. How is that done?
Do they need to have more than one transmitter? Several sets of towers?
There’s a station near here with three powers and patterns, daytime, critical-hours and nighttime!
Is the change done manually or can it be done automatically?
The station site has the answer to the pattern thing, several towers which I assume are transmitting towers. Certain ones are on or off depending on the directional pattern they have to have.
I guess power adjustment is like us doing it. The main transmitter RF controls can be adjusted for a certain wattage out in a control room where an engineer would look after this.
A station near Toronto which I liked quite a while back when I was talking to them said when I asked why you have to turn the power down at night told me of the reason why they do this and they said they forget sometimes. So someone manually did it. They knew they were loosing evening listeners but it was mandated in their license agreement. CHML a Hamilton station near here has old radio dramas every evening but it’s harder to get them in all the clutter at night as they turn the power down too.
I wonder how part 15 AM stations deal with being blocked out at night as even commercial stations which are perfect in the day are not listenable at night. Like CHML from Hamilton, west of Toronto.
One of the stations I engineered in the past was a 1 KW day and 500 watts at night. To achieve the night-time pattern, another tower located just to the north of the main tower was powered at 15 watts via a phasor. This was just enough to skew the pattern seen here:
The signal at night never gets much past the local prediction.
Questioning the Answer
How did you get from 1 kW to 500 W at night? Were there two transmitters for that?
And adding the extra tower at 15 Watts, was there a 15 Watt transmitter, or was the power derived through the phasor by taking it from the larger transmitter and using resistance to lower it?
Is my question confusing or do you think I am confused?
Hello Carl:
There is a switch on the transmitter that is pre-programmed to a specific power lever. In this case, 500 watts. And yes via the phasor the 15 watts is derived from the single transmitter.
A local AM station I worked at in 1990 had two transmitters for different power levels: a full-bore Nautel for daytime power, switching to a classic LPB low-power transmitter for ten watts at sunset. Otherwise, many modern transmitters are modular, and their power ratings are based on how many modules are contained in the chassis. Changing wattage simply involves configuring the transmitter to turn off and bypass a bunch of the modules, and just modulate one or two of them.
And, as AMRL pointed out, phasing a signal through two or more towers spaced a certain distance apart will cause cancellations and reinforcements of the waves to occur in a chosen direction. This is something worth looking into for Part 15 operators: conceivably, we can place a parasitic reflector (an antenna connected to nothing) a certain distance behind our whip, which will act as a reflector and “steer” our signal in the direction of the most coverage. Whether or not this is legal will determine if it is proper for us to do this, but if you Google “Yagi”, you’ll see that ham operators do this all the time so we know the technology is valid.
Steering Towers
BillyBurg’s excellent idea: “for Part 15 operators: conceivably, we can place a parasitic reflector (an antenna connected to nothing) a certain distance behind our whip, which will act as a reflector and “steer” our signal in the direction of the most coverage. Whether or not this is legal will determine if it is proper for us to do this.”
I believe we can safely assume the practice to be legal so long as it’s not expressly disallowed, and there’s nothing in the rules against doing it.
What We Take from This
We have learned from your comments that AM stations have several ways of switching powers & patterns.
A local 5 kW station shuts carrier for about 1-second of dead air at sundown then the nighttime pattern comes on with a poof.
The station, KTRS 550 kHz, keeps the same power at night but changes patterns.
Carl: Many AM transmitters need to power down momentarily to allow the power supply to stabilize before powering back up.
Billy burg Station8 has been experimenting with (and has enjoyed great success) with reflectors for part 15 transmitters.
You should had attended the discussions at the ALPB meetings when he tried to explain how his antenna design worked because by doing this…can you say DB Gain?
Let Us Know
Many of us are very curious about Station8’s findings with reflectors but cannot go back in time to be attend a meeting we never knew about.
We know that JohnyC RAG-FM records ALPB meetings, so arrange to get the presentation linked here so we can learn about it.
Yeah Carl. Can you say DB(sic) Gain?
I Can Say More Than That
Not only can I say “…DB(sic) Gain“,
I can also say DB Gain (sig) as in significant.
The Hy-Gain ham antenna people included a fairly basic but thorough lesson in phasing right inside the instruction manual of their 12AVQ vertical antenna. And its free!
http://www.hy-gain.com/support.php?productid=AV-12AVQ
Download the product manual and check out the last few pages to get a few ideas moving around in your heads. Amateur or Part 15, RF is RF and the principles are the same.
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