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Switching day/night power without cutting carrier?

K

kenglish

Guest
Is there a way for a station to switch their power from, say 10 KW to 250 watts, with no interruption of power or signal at all?
Wondering if there is a way to make the "switch" seamlessly, without that annoying "loss of signal" for a second or three.
 
don't know the answer, but ours is a 2 second delay while it switches. One thing to think about is the age of many directional systems - very few are less than 30 years old, ours is 65 years old. Not likely that there is much you can do with equipment that old, and incurring cost if nothing is broken makes little sense. So there may be a way to do it, but it most likely won't make sense to proceed.
 
Only way i know of would be to have two separate sites. As I assume your 10kw has to go off and your switching to your 250w, and that shouldn't be done "hot".
 
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Well, if it is a non-DA, then a modern transmitter could probably handle a cutback that far without a burp.
Day to night pattern? "modern" phasor (anything built in the last 30 years) should be able to do that with just a minor burp.
Depending, of course, of the age of the transmitter (s) driving the system.
 
In our case the 5kw switches just under a second. The 50 kw around a second. Relays much bigger in the 50 kw phasor.

The other concideration is this. No matter how you get rid of the RF it does not turn of immediatly. Tuned circuits on the transmitter 's output act like an RF flywheel that has to wind down. Thus there will be a decaying RF signal when the transmitter is muted. So it's important to give it a little time before giving telling the relays to change after the transmitter is muted. Somewhere in the millisecond range depending on the transmitter.

As soon as the relays get into the right position then to transmitter can be un-muted.
 
If it is a DA the answer is no. The problem there is not the transmitter. If you switch patterns with the transmitter on you will draw arcs across the RF contactor contacts in the phasor and the ATUs, causing them to pit and eventually to fail. In a properly designed phasing system the phasor master nterlock closure should drive the failsafe interlocks on all transmitters. Another reason for connecting the interlocks is that, should a contactor freeze in mid-excursion, the transmitter will not come up, preventing possible damage to the phasing system or the transmitter.

In 30+ years of working with DAs I have often come into an old station with a DA problem and found that the problem was due to a phasing system that was not properly interlocked with the transmitter.

If you are talking about a non-D the answer is still no. The transmitters should be interlocked with a system that prevents them from being connected to the antenna system at the same time, which could damage one or both.

How long a "burp" you have to put up with when switching depends entirely upon how long it takes each transmitter to go from the RF off (or as I still tend to call it, "plate off") command to zero RF output.
 
I've seen several contactors toasted by someone trying to switch hot. And, a second or two should never be enough to make someone tune away. If it does, you've got other problems.
 
Hot? No. Not at that power level. If you really need to, you can probably cut the down time to 1/2 second or so, depending on your RF switching. Leave enough time for the carrier to drop. let your contactor(s) seat in their new positons, then set switch logic to put the transmitter back up.

Most new rigs have RF kill circuits that are pretty fast. You can ground a terminal and cut carrier without shutting the whole transmitter down. Lift the ground & it comes right back up.

Really though... Do you think the short switching time has any effect on listeners? I have one tube-type AM left that has a 1-second power change... nobody really notices.
 
Remember also that the final PA tank is a big RF flywheel with lots of stored energy that takes several milliseconds to dissipate.

The old GE 4BT50A1 had a great VSWR circuit that you could also use to mute the RF for pattern change. It was biased beautifully and would sit there with no RF forever while muted. It could mute and change pattern in a 200 milliseconds. Any shorter and you would see an tiny arc on the contacts so it was set to 700 ms for safety.

The original pattern changer on that GE was a motorized wood cam with microswitches that would turn off the plates, wait a second, change the relays, wait a couple seconds, change the power level, wait a second, then turn the plates back on. They made a big deal about the pattern change on the air. Later the power level stayed at 50 kw day and night and a solid state controller was built that used the RF mute instead.
 
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