Many readers will get this wrong!

Imagine this, if you will.

A UHF trunked radio site is located on a hilltop site. The transmitter power is set to 25W and receiver sensitivity is normal (around -124dBm for 5% BER). The site uses a 4-port cavity combiner and multicoupler with a TX insertion loss of around 4dB. The 4-stack dipole antenna is mounted at 30m and fed with 25mm heliax. The installation was done per Motorola R56. All internal RF cabling uses 12mm double shielded coaxial cable. In terms of external noise and intermodulation, spectrally, the site is clean. In the above state, the site provides service to portable radio users within an approximate radius of 25km - ignoring terrain and other influences.

Increasing the TX power to 50W will:

A) Make no difference.

B) Increase the range slightly.

C) Double the range.

D) None of the above.

Generally, in any trunked radio system, upon power on or loss of signal, the radio will begin a hunting process: It will look though a list of predefined channels or frequencies, for a Control Channel. 

The Control Channel is a constant transmission, present on one of the channels, at all radio sites (like the one above). At any time, one of the repeaters or base transceivers, at the above is hosting the control channel. In almost all cases, there is always/only one Control Channel per site.

The radio will evaluate the control channel transmission to confirm that this is the real radio system (and not a rogue system). It will then check for signal quality (RSSI and BER) and based on this, it will generally choose the site with the best signal - if the radio found more than one control channel during the hunting process.

Once the Control Channel is found, the radio will attempt to register on that site. The control channel receives this request and passes it to the site controller which completes the registration process. Once the registration process has completed, the radio will monitor the control channel for calls. When a call comes in, the radio will move to the voice (sometimes called payload) channel that is allocated by the system. When not in a call, the radio will always monitor the control channel.

But what if the system cannot receive the registration request?

A radio will only consider a RF site if the site controller acknowledges its registration request. If no acknowledge is received, it will retry a few times after which, it will continue hunting.

For the user it will appear as if there is no signal - even though the radio can receive a signal on any control channel.

This means that the coverage for a trunked radio system is determined by both the inbound (radio-to-RF-site) and outbound (RF-site-to-radio) coverage. Therefore doubling the RF power on the above RF site will probably make no difference to the range.

The above is applicable to all trunked radio systems. Don't let their salesman bullshit you! You're welcome to grill me in the comments but a MBA is not a substitute for any radio communications qualification and/or common sense.

In MOTOTRBO, the behavior of a radio in an IP Site Connect and Capacity Plus system, will be different. 

In an IP Site Connect system, the radio will remain on the current channel for as long as it can receive the repeater beacon message. The roaming process itself does not ensure that the repeater will hear the radio, should the user press the PTT. The same is true for Capacity Plus.

In any radio system, since the repeater has the advantage of location and (often) antenna gain, the outbound (repeater-to-radios) coverage will always be better than fielded radios. 

Portable radios especially, have lower power (no more than 5W); no antenna gain and are often worn on the users belt. 

An unoptimized system could have disparate coverage.

So without careful design, there could be an area where radios can hear the repeater but the repeater cannot hear the radios (blue). And, in a DMR T3 system, if the repeater (system) cannot hear the radio, the radio will ignore that site when it fails to register there.

To avoid the above scenario, the repeater (or base transceiver) TX power should be set to a level such that the the inbound (radio-to-RF-site) and outbound (RF-site-to-radio) coverage are more or less equal. The TX power can be increased slightly (if regulations permit) so as to compensate for fade margin etc.

There are a few ways of doing this but the one that has worked for me is as follows:

Firstly, carry out a coverage prediction using only the inbound (uplink) path. 

Once this is done, the outbound (downlink) coverage prediction can be made using maximum; medium and low repeater transmit power. 

Then, by comparing the inbound and outbound coverage plots for the three power levels, the ideal repeater TX power can be extrapolated.

Lastly, validate the coverage by driving (or walking) to points in the coverage area, predicted signal strength in known, and measuring the RSSI using a radio. Based on this, tweak the TX power and other radio parameters until balanced coverage is achieved.