I'm not sure why you'd want to use a linear. 1.5 watts ! This is heaps for a balloon.
You might see I mentioned using FRS in the other thread. This is much the same as our UHF CB.
These little transmitters are more than capable of being heard from over 100,000ft and higher if you can get your balloon up there.
Strip them down, throw away the plastic case, and use lithium batteries and reduce the TX duty cycle.
The antenna is ok for the task, but I prefer to make my own. A quarter wave inverted is best for balloons with a 45 degree radiation pattern.
Now, why you'd want to use a linear I have no idea. Building a linear is VERY easy. JimB has said it's 'not trivial' for UHF. But he's thinking of doing it ... the hard way
First is that it really depends on how much TX power you want. Second is that the amplifier doesn't need to be 'linear'. It's FM, so a class C will get the most bang for your buck. But if it must be linear... then there are dirt cheap module which do the task so simply even a moron can make them work.
Any of the Motorola black chip modules will do the job, but a Motorola MHW710-2
http://www.chinaicmart.com/uploadfile/ic-circuit/200971415639747.gif
This circuit will work with a fraction of the components shown, it's that tollerant. They typically have about 20dB gain, but I've had some modules with up to 25dB.
So with this circuit, you'll have to attenuate the output from the FRS radio (~10dB) to get an overall gain of +10db and an output of ~15watts which is massive overkill for a balloon payload. You'll have more trouble trying to power it and the heatsink is going to blow your weight budget.
We've tested these modules to destruction and up to 25 watts can be squeezed out of them.