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Thread: 70s AM powerdown

  1. #1

    70s AM powerdown

    I am curious about how many different ways there were to power down AM stations at night in the US in the 1970s.
    Was it always as simple as pushing a button or were there more technical or complicated ways of reducing power, dependent on the age of the equipment?

    Thank you for your input (and output)!

  2. #2
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    Quote Originally Posted by Cheesewater View Post
    I am curious about how many different ways there were to power down AM stations at night in the US in the 1970s.
    Was it always as simple as pushing a button or were there more technical or complicated ways of reducing power, dependent on the age of the equipment?

    Thank you for your input (and output)!
    This is a very simplified explanation of a rather more complicated subject that changed over time as technology developed.

    There are really two ways of powering down. The first is by selecting the proper power on one transmitter and having it reduce power. The second is by having a different transmitter for each mode of operation.

    Older transmitters, particularly tube models, had to be ordered with the proper cutback levels.

    Because there were were 6 Class IV channels where for some time 1 kw days and 250 watts at night was the standard, manufacturers had a model for cutback to 250 watts. With solid state transmitters, the ability to have programmed power presets at almost any level became possible. And many regional channel stations ran with 5 kw day and 1 kw nights, and manufacturers could offer single units that ran well at each level.

    Problems came when the FCC let stations have nearly any power level.

    Prior to that, the FCC had only a few power levels, 100, 250, 500 watts and 1 kw, 5 kw, 10 kw, 25 kw and 50 kw. You could not have, for example, 780 watts or 13,450 watts.

    When the FCC changed to allow stations that could increase power to pick the exact and highest possible level, manufacturers had to enable the ability to run at preset but almost random power levels. The advent of solid state transmitters with computerized controls made this relatively easy.

    With stations with huge day and night power differences, it was generally more economical to have a low power night transmitter, as running the big transmitter at very low power was not efficient. This would be the case when Robin Mathis build a number of 50 kw day, 250 watt night stations in Mississippi.

    A seldom used old practice was to use a resistive network to burn off some power when standard power options from makers of transmitters were not available.
    Last edited by DavidEduardo; 05-15-2019 at 08:52 PM.
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  3. #3
    Really old equipment could be a serious pain to power down. Partly because power-down also came with a directional pattern change. So the antenna impedance would change -- maybe substantially -- which could cause the transmitter to fold back. However, in such cases it would generally be advisable to fix the antenna tuning unit to properly match the night array. That was certainly possible in the 70s, but the theory was not well understood until (I want to say...) the 40s.
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    Quote Originally Posted by PTBoardOp94 View Post
    Really old equipment could be a serious pain to power down. Partly because power-down also came with a directional pattern change. So the antenna impedance would change -- maybe substantially -- which could cause the transmitter to fold back. However, in such cases it would generally be advisable to fix the antenna tuning unit to properly match the night array. That was certainly possible in the 70s, but the theory was not well understood until (I want to say...) the 40s.
    Antenna impedance, including resistance and reactance, were well understood in the 30's. The problem was more in the construction and design of components and the difficulty in doing measurements of the system.

    For directional systems, there is a common point where the transmitter feeds the antenna system, whether one tower or twelve. That point is tuned to match the output of the transmitter. Now, as back in the 30's, the common point for day and night operations were designed to present essentially the same impedance to the transmitter.

    Today systems are designed by computer. Back in the 40's, Carl Smith developed a "mechanical computer" to design thousands of directional patterns and the spacing and tuning elements of each. Transmitters don't like working into a mis-matched load and some are very intolerant of them.
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  5. #5
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    Quote Originally Posted by DavidEduardo View Post


    This is a very simplified explanation of a rather more complicated subject that changed over time as technology developed.

    There are really two ways of powering down. The first is by selecting the proper power on one transmitter and having it reduce power. The second is by having a different transmitter for each mode of operation.

    Older transmitters, particularly tube models, had to be ordered with the proper cutback levels.

    Because there were were 6 Class IV channels where for some time 1 kw days and 250 watts at night was the standard, manufacturers had a model for cutback to 250 watts. With solid state transmitters, the ability to have programmed power presets at almost any level became possible. And many regional channel stations ran with 5 kw day and 1 kw nights, and manufacturers could offer single units that ran well at each level.

    Problems came when the FCC let stations have nearly any power level.

    Prior to that, the FCC had only a few power levels, 100, 250, 500 watts and 1 kw, 5 kw, 10 kw, 25 kw and 50 kw. You could not have, for example, 780 watts or 13,450 watts.

    When the FCC changed to allow stations that could increase power to pick the exact and highest possible level, manufacturers had to enable the ability to run at preset but almost random power levels. The advent of solid state transmitters with computerized controls made this relatively easy.

    With stations with huge day and night power differences, it was generally more economical to have a low power night transmitter, as running the big transmitter at very low power was not efficient. This would be the case when Robin Mathis build a number of 50 kw day, 250 watt night stations in Mississippi.

    A seldom used old practice was to use a resistive network to burn off some power when standard power options from makers of transmitters were not available.
    Wow... I knew the late Robin Mathis personally. Haven't heard his name mentioned in a while. The 50 kW day / 250 W night stations that he and his brother Marvin put together were something to see. Both 50 kW stations are still in operation in Mississippi, but have been updated with solid state transmitters over the years by new owners.
    RFB

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