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Duty Cycle Explained


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#1 Corey

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Posted 14 March 2019 - 06:10 PM

What does “duty cycle” mean?

 

I bring up duty cycle every time I hear somebody talking about making a repeater out of cheap Chinese mobiles and worse any type of handhelds. Duty cycle is the maximum time an amplifier may transmit within a five minute interval, expressed as a percentage, to avoid overheating. Suppose a mobile amplifier is rated at 30% duty cycle. This means that it may transmit for no longer than 1.5 minutes and must remain off for not less than 3.5 minutes. Some people forget that a repeater is transmitting for 2 or more people, duty cycle will be reached quickly if you get into conversation. More people in the conversation just amplifies the issue.

 

Once a radio reaches it's thermal design limits it will no longer be able to adequately cool the output transistors. Even if a radio is not hot to the touch the transistors are, in part because of the inefficient transfer of heat to the units housing or internal heat sink. The longer you exceed the duty cycle the more heat builds on the transistors, surrounding electronics and heat sink effecting it's ability to remain on frequency without spurious emissions. Exceed duty cycle long enough and you will need a new transmitter or radio.

 

I have tested a few Baofang and TYT radios on my service monitor without great results. All of the radios started deviating outside of the allotted channel bandwidth after simulated conversation at 50% duty cycle, the longer I allowed this the worse if got. Testing was done using an Aeroflex 2975 IFR recently back from the calibration lab. 

 

GMRS is a tiny sliver of spectrum surrounded by the commercial land mobile part 90 service. It is important that any repeaters that are built or re-purposed are held to the highest standards and operated as to not cause any interference inside or outside of our allocated spectrum. I wont get into the part 90/95 debate but i do stand firm that non certified import equipment has no place on GMRS. 


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Just My $.02

 

Corey

 

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#2 WRAK968

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Posted 15 March 2019 - 07:15 AM

Good explanation Corey. Its also important to note that even repeaters have some form of duty cycle and that the duty cycle can change depending on output power. A great example is the motorola 1225 series repeater. At full power, 45 watts, the repeater is limited to a 50% duty cycle. In low power, 25 watts, they are 100% cycle.

Luckily, good repeaters have protection in the form of a Time Out Timer (TOT) and usually a ReSet Timer (RST) When set up correctly, the TOT should cut off the TX BEFORE the duty cycle is reached, and the RST should prevent transmissions until the appropriate rest period has passed. This helps extend the life of the repeater even when your not there to monitor it.

Almost ALL radios now-a-days have a TOT as well. Typically I set mine for 2 minutes which is more than I need to transmit for, but is less than the duty cycle permits thus protecting the transistors.  I've had people tell me its dumb to do as no idiot would want to talk non-stop unless they are in a net or special operation. That's when I give them this story:

 

A while back when I was on the first aid squad, I was getting ready to go to sleep. Turned out the lights, laid my head on the pillow, then came the sound of a soft moan from a woman. Now, this would have been great, if there had been a woman in bed with me. The moaning continued and a bit confused I began looking for the source. Didn't take long to find my squad radio was receiving, and after turning it up a little, yep you guessed it. Turns out, the LT's portable radio had fallen between the bed and the night stand and as they and their lover were going at it, it was all transmitted quite clearly to the repeater, then retransmitted for the ENTIRE COUNTY to listen to. To make matters worse, the radios all had ID numbers so we knew whos radio was transmitting, and even worse, the LT had shut their phone off, so calling them to tell them their private session was public was not doable. Finally, the radios did not have TOT's at the time and so for a good while (I just shut my radio off but a friend at the county said they heard every minute of the 15+ minute incident. When the portable finally cut out, either from overheating or a dead battery or from falling loose of the PTT button, there wee quite a few catcalls to let them know everything was public including a few repeats of nick-names used during the session.

So folks, learn two things from this story. A: TOT's work, especially when a mistake is made or you don't realize the transmit button is pushed, it will work and save your radio from damage and possibly you from some embarrassment. B: Someone is always listening, so keep in mind what you say or do on the radio, especially if you work for a public safety organization because if you say the wrong things you could find yourself in quite a bit of trouble.


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#3 Corey

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Posted 15 March 2019 - 07:45 AM

Great follow up! I was hoping this topic would generate some conversation. I personally don't own any repeaters that are less then full duty cycle and I dont use time out timers on the repeaters. We link several machines together using VOIP and at times the conversations can get long winded with many users engaging I have seen them go for 45 min or longer. In the beginning we set the TOT for 2 min and had different machines resetting at different times causing lots of missed conversations etc. After a year of that we went to a system wide TOT of 5 min and that's been working out great as all the machines will reset at once. As far as the repeaters, we only use Motorola MTR2000's of the 100 watt configuration. At full power these machines are designed for 100% duty cycle, so at our allowed 50 watts they are hardly breaking a sweat.


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Just My $.02

 

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#4 JohnE

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Posted 15 March 2019 - 07:51 AM

same here. 100W machines turned down 3min TOT.  MTR's will run 24/7 @ 100W w/no problem.


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