In term of effect on electrical power input or radio-frequency power output, there is no difference between the two mounting locations. Electrical power to the radio and radio-frequency power from the radio will not be affected by a change in physical location of a foot or two.
In terms of operating the radio, there are many differences.
In order to operate a radio, the operator must have access to the controls. The operator needs to be able to adjust:
- channel of operation
- volume
- squelch
- transmitter power
- radio on-off power
- scan functions
- and many other functions related to digital selective calling (DSC)
If the VHF Marine Band radio is in the cabin, it cannot be operated from the helm position.
Modern digital selective calling (DSC) VHF Marine Band radios--which means essentially ALL modern VHF Marine Band radios--have information displays that should be visible to the operator. Operation of the radio will be difficult from the helm position if the radio is in the cabin and the display on the radio front panel cannot be seen.
In terms of environmental factors, mounting a radio in any location protected from sunlight and water will be better than a radio mounting in the open. Flush mounting a radio onto a helm dashboard panel is a very good method to limit the effects of sunlight and water on the radio. Generally the front panel of a modern radio will be designed to tolerate some water splashing. Long-term exposure to sunlight can be reduced by using a molded cover over the front panel of the radio when the boat is not in use.
Another option for radio installation may be to buy a VHF Marine Band radio model that supports a remote microphone with LCD display built in. The radio base unit can be mounted below deck, and only the remote microphone needs to be accessible to the helmsman at the helm controls.