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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rickety
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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