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Good info on duty cycle for repeater builders



 the MaxTrac, M-series, Radius LRA series and GM300 are MOBILE radios, made with minimal heat sinks, and while they can be used quite readily as a low-to-medium performance repeater receiver, or as a link receiver, you can NOT use it as a repeater transmitter, as a link transmitter or as an IRLP node radio without due consideration to the normal mobile radio limitations on RF power and duty cycle.

Remember that the beauty of a repeater is that it is a device that allows one-to-many communications rather than a cellphone, which is limited to one-to-one. As such, while any individual user may only transmit for 10 to 30 or even 60 seconds the repeater is transmitting for the duration of the transmissions of all of the users continuously one after the other. And if it's an IRLP-linked repeater then the user count - and transmit time - just increased to add all the users on the other end of the link or reflector.

As a 10% to 15% duty cycle radio the MaxTrac, Radius and GM300 (and the GR series repeaters that were made from them) are designed to transmit for no more than 10 to 15 seconds out of each 100 seconds. This is not to say that you can't transmit for three minutes, but the transmitter is going to get hot, and the longer it talks the hotter the mobile radio gets. These radios were designed for a 10% to 15% transmit duty cycle, and the internal power control circuitry is the only thing preventing the radio from burning itself up.

The MaxTrac and Radius radios have power control circuitry that turns the transmitter power down if you talk too long (all it does is measure PTT time), where the GM300 series radios actually measure the PA deck temperature with a thermistor (hence they "know" if there is a fan present or not). Due to the timer design the MaxTrac and Radius radios assume that there is no fan.

The MaxTrac and Radius power control timer can create a situation where the transmitter goes into power shutdown on its own, which can happen in the middle of a conversation. This WILL happen in a long transmit situation. The radio might not be hot (due to a strategically placed fan), but the microprocessor "knows" it has to be, and steps the transmitter power down to zero. It'll recover for a few minutes after it's unkeyed, then do it all over again until it has adequate time to rest. You really don't want your repeater fading out during something important like a search and rescue operation, or even a 911 autopatch call. The article on Manual Power Control (in this section) shows how to address this problem by bypassing the shutdown control. Just make sure you have a suitable external temperature control system in place (fans, etc). And it would still be a good idea to put a thermal snap-action switch on the heat sink and use it to trigger a backup fan - and maybe a repeater controller announcement.

Despite these comments, if you ARE going to use one then pick a 33 and 34 series radios (the lower power ones) as it is a better choice than a 43 or 44 series as they use the same heat sink casting.

In short, with very, very few exceptions, any mobile radio (Motorola or otherwise) used as a link or repeater transmitter is a recipe for failure, especially if the system includes EchoLink or IRLP... as I said above, the repeater is transmitting for the entire conversation of all the users, one after the other, however with a linked system the user pool now includes everybody on the local system plus everybody on the far end node, or if connected to a reflector then every person on every node that is connected to the reflector.

I've seen a photo of a 110w Spectra trunk mount mobile where the power control failed and let the transmiter run at full blast for as long as PTT was held down. Then one day the driver sat on the microphone for an hour during a road trip. The transmitter PA burned itself up and the radio was not economically repairable (which is why you program the idiot timer into any mobile radio that has one).

The "Executive Summary": If you are going to put up a repeater (or a point-to-point link that will have a similar duty cycle), don't use a low-end low duty cycle mobile radio (from any manufacturer) as the transmitter. It's one thing to use a mobile radio based repeater like a GR300 (or similar) in a shopping mall environment to talk to the rent‑a‑cops or to tell housekeeping to clean up little Johnny's spilled ice cream cone, but you want something with a higher duty cycle as your primary area repeater.

These radios were designed for a range of RF power - for example, the D44 (or M44) series is a 40 watt radio, and is designed to run from 25 to 40 watts, which means not more than 40, but also not less than 25. If you run outside that range, it will be inefficient and run hot.   Yes, strange as it may sound, a D44 run at 15 watts runs hotter than when run at 30 watts.

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I won't disagree with most of that, however being a M employee in the hay day of those mobiles, you may be surprised at their hardiness. Yes, while they are certainly not a MSF5000, they make decent little repeaters. For HAM use? No, HAMs like to TALK! You need a 100% duty cycle repeater for that, but for GMRS, or even light duty commercial, they can and do work. 


I ran a 40 watt M120 as my GMRS repeater TX for 16 years and never had to touch it. When I did remove it and replace the cabinet with a MTR2000, that M120 was on freq, had a good RX even though that wasn't really being used, and keyed to 41 watts, dropping to about 37 after 2 minutes. Now, I am not saying it would do that continuously, but for the sporadic conversations we had on the repeater, it was fine. Never had an issue. I had TOT set for 3 minutes with a re-key time, so people knew to give it a rest, and I mounted it with the heat sink up at the top of the cabinet with 2 - 4 inch fans right above it and controlled air flow in the front and over the radio. The RX was a M208, and it still works great too, as a receiver on my secondary repeater.

The later M120/GM300/Maxtrac series had power cut back, so unlike the older M100/M2xx/Maxtracs, you didn't have to set the power back on a 40 watt to say 25, or a 25 to 10 to keep from frying it easily. The Maxtrac series PA was also used in Desktrac bases and repeaters. 


So yes, these are not meant to be heavy duty repeaters, but for some GMRS guys, they make good ones, and can be the difference in spending a few hundred dollars and a few thousand with no better results. I wouldn't scare anyone off from using them, especially in a first time project.   

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I have a post some where on here with a graph of how duty cycle (stated from the service manuals) relates to power output. I used both Motorola Radius (M1225, SM50/120 have the exact same stated specs in their service manuals) and GE Custom MVP. I figured is add in using active cooling later when I felt like calculating the general thermodynamics.


Might be worth merging the two threads.


Shows the three common power splits for the motos, 1-10, 10-25, 25-40/45. Note, the 1-10W radios can actually handle a 100% duty below 4W. They are the most inefficient of the three but my math was actually backed up by the service manual.

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