Use a 12v linear regulator, should be able to supply at least one Amp.
With 18 Volts in, the regulator will be dissipating (18v-12v)*1Amp = 6 Watts, so you want a fairly big heat sink on it.
Modern 12v linear regulators have reverse polarity protection (in case you reverse the leads from the battery)
and a very low dropout voltage (so 12.5v from the battery still gets 12.0v to the radio).
Needs caps nearby at input and output as specified in the datasheet.
Shuts down with an internal current limit if rig has a short, also shuts down if regulator gets too hot.
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An LM2940CT-12 should work
I'd bolt the tab to the aluminum case as a heatsink after first buffing off the anodized finish, use heatsink compound.
Costs about $1, heatsink tab can be grounded, maximum recommended current of 1 Amp
A wimpy regulator like that is a good thing, provides an inherent current limit.
If you need more current, the more expensive MIC29150, MIC29300, and MIC29500 are capable of 1.5A, 3A, and 5A respectively.
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A switch mode power supply sync'd to a 2.5 MHz oscillator would be more efficient
and have no harmonics in the ham bands except at 50.0 MHz.
But that would be a small project.
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Jerry, KE7ER
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