Re: Will a hand held radio perform better with a 8 foot antenna?
The length and particularly the height of the antenna makes much more difference than the output power difference between 5 and 25 watts.
I completely disagree. An increase in power to 25-watts from 5-watts represents a fivefold increase, which in decibel measurement is +7-dB. There are few--if any--VHF Marine band antennas which are usable from a small boat which can provide 7-dB of actual gain, and, if the do, it is only in a narrow lobe of their main beam. On the other hand an increase in power to 25-watts from 5-watts provides an actual 7-dB increase in power all the time and in every direction. This is much more important than the often much over-stated gain of any antenna.
In the case of a hand held device, the antenna probably has less than isotropic antenna performance, so it could be considered to have a gain of -3-dB, that is about half the power of an isotropic. Connecting a real antenna, even one with only unity gain, would be an improvement.
We used to have a saying about antenna gain and transmitter power: with antenna gain you are just hoping you get some actual gain, but with transmitter power increase you know you are getting real gain.
On the other hand, antenna gain is reciprocal and will improve reception, too.
I would dispense with the notion of trying to use a hand held transmitter and wire it into a fixed antenna. The cost of a fixed Marine Band VHF radio is very low, only about $100 for an excellent radio, and it would make more sense to get one, rather than fiddle around with a hand held. When you really need the hand held radio, its battery will be dead.