Duty Cycle Calculation
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Duty Cycle Calculation
Hello group. I need assistance in explaining to an amateur radio group the need to run their primary VHF(2 Meter) repeater at reduced power output as not to exceed the rated duty cycle of the transceiver. Can anyone assist in helping me word a simple explanation that will convey this point. The station is in fact intermittent duty, however experiences continuous periods of use at different times of the day. When I suggested lowering the repeater to 50% duty cycle at 25 watts, I was told "the repeater is not transmitting 24 hours a day, therefore is not at 100% duty cycle. I need to convey that duty cycle is not based upon a 24 hour day or clock. Thanks all. Jhansen27
Re: Duty Cycle Calculation
If memory serves, the industry standard metric for duty cycle is the maximum amount of transmit time in any five-minute period. If the TXer is keyed up for 5 minutes straight at any point in the day, it's a continuous duty system. ANd, if it's a hammy repeater, it's a continuous duty cycle system.
The primary concern is heat dissipation. In very simplistic terms - let's say the transmitter produces X BTUs of heat per some unit of time. In a continuous duty system, the cooling system has the ability to remove Y BTUs of heat (Y > X) over the same time period. So, the cooling system can always keep ahead of the heating system (TXer) and nothing burns up. For an intermittent duty system, the TXer produces the same X BTUs of heat, but this time the cooling system's Y constant is less than X. Thus, the transmitter temp will continue to rise until either something burns up or the TXer turns to plasma (the first is more likely). The cooling system relies on the transmitter to turn off at some point and let it catch up. Obviously, this time period is much shorter than 24 hours.
In the real world, a continuous duty system will reach some equilibrium temperature and remain there assuming conditions remain the same. Intermittent duty will see much more fluctuation.
By the powers you've given, I'm going to guess Kenwood TKR-750? If so, 25 watts is the absolute maximum it should ever run in hammy service. Mine is turned down to 10 watts driving a 150-watt external PA (with a huge heat-sink on the back and a fan that runs continuously).
The primary concern is heat dissipation. In very simplistic terms - let's say the transmitter produces X BTUs of heat per some unit of time. In a continuous duty system, the cooling system has the ability to remove Y BTUs of heat (Y > X) over the same time period. So, the cooling system can always keep ahead of the heating system (TXer) and nothing burns up. For an intermittent duty system, the TXer produces the same X BTUs of heat, but this time the cooling system's Y constant is less than X. Thus, the transmitter temp will continue to rise until either something burns up or the TXer turns to plasma (the first is more likely). The cooling system relies on the transmitter to turn off at some point and let it catch up. Obviously, this time period is much shorter than 24 hours.
In the real world, a continuous duty system will reach some equilibrium temperature and remain there assuming conditions remain the same. Intermittent duty will see much more fluctuation.
By the powers you've given, I'm going to guess Kenwood TKR-750? If so, 25 watts is the absolute maximum it should ever run in hammy service. Mine is turned down to 10 watts driving a 150-watt external PA (with a huge heat-sink on the back and a fan that runs continuously).
Re: Duty Cycle Calculation
Thank you, and 25 watts is where I placed it today. Good guess, and yes it is a TKR-750. I was hoping to find something posted on the industry standard understanding completely that it is a variable based upon heat etc. as you explained. My goal is to put it into simple and "unarguable" terms. If they want more TX power, its time to pony up for continuous duty amplifier. That should cover their 24 hour key-down goal!
Jeremy
Jeremy