Why a mobile radio is not a good repeater IMHO

While a mobile can be powered from the mains and can be connected to an external antenna, it is not optimal for use as a repeater. Here's why:

A well designed mobile radio will, at best, support a 50/x/y duty cycle. That is: 50% TX time; x% RX time and y% idle time. On a real radio system, the duty cycle may be something like 5/35/60 but this value is applicable to the users' radios only, and not the repeater. With a 5/45/50 duty cycle, the repeaters duty cycle will 50/0/50 - this is because whenever any one of the radio users transmit, the mobile which is being used as a repeater, must also transmit to relay their signal. Excessive load like this significantly reduces the MTBF on this radio.

A mobile radio is simply not suited for fixed installations where there is other radio equipment. Back in the day, I used to tell my customer to only use the GR500 on a RF site where there is nohing else. This is because a mobile radio does not have sufficient RF filtering on the receiver and transmitter. For example, the DM4400 has a adjacent channel power of -60dB*; spurious attenuation of 75dB and a 3rd order intermodulation rejection of 75dB* - by comparison, the SLR1000 has a adjacent channel power of -78dB*; spurious attenuation of 80dB and a 3rd order intermodulation rejection of 80dB*.

For a repeater, you want to ideally have the adjacent channel power as low as possible (i.e. the highest possible attenuation) as this will prevent - or reduce - interference to nearby users on adjacent channels.
A high spurious attenuation prevents your repeater from interfering with other radio system and services.
Having a high 3rd order intermodulation rejection prevents your system from being interfered with by systems on other frequencies.
On this count, a SLR repeater wins hands-down!

Of course, you could mitigate interference by installing filtering hardware, but this adds to the cost and requires additional space. Using a mobile as a repeater does not automatically mean that you will have interference, it just means that your chances of creating or being subjected to interference are comparatively greater.

You could also help the mobile cope with the load by adding a fan but that would look really stupid and there's no guarantee that the radio wont fail long term since a fan only alters the air flow over the chassis and may not help with adequate cooling.

Some of you may be asking yourself why I say this yet, for many years Motorola had the DR3000? Well, the DR3000 was based on technology that was available in the early 2000s and although the RF specifications weren't as good as the current range of MOTOTRBO repeaters, the duty cycle was still 100% at full power.

The SLR series repeaters are purpose-built and have the superior RF performance needed for fixed installations.

The above is also true for RF repeater links.

* At 12,5kHz