Re: Off Topic -- Warning re surplus Nokia Tetra base station Power amps on 70cms

Christopher Bartram <cbartram@...>

Having designed TETRA PAs, I assume that these surplus amplifiers are designed
to deliver 50 - 60W pep, and will produce >200W simply because they were run
'backed-off' in order to get good large signal linearity with a relatively
poorly-performing lineariser.

I should stress that I don't know these amplifiers, but the problems which Mark
and Rob describe aren't black magic!

The issues which will arise from running the amplifiers at higher powers will
(probably) be due to the choice of substrate for the microstripline output
matching elements. If, as I suspect, the amplifiers are built on 0.62 or 0.31mm
FR4 to save money, the major issue will be resistive heating due to dielectric
losses, not dielectric breakdown. While you can't do a lot about the latter,
the heating due to resistive losses in the substrate can be dealt with in the
same way as you'd deal with losses in a resistor: heatsinking!

The power dissipated in a microstripline can be calculated, but as a rule of
thumb, those in a 200W PA will probably only dissipate a few watts.

The characteristic impedance of a microstripline is relatively uneffected by
heatsinking structures placed on top of the conductor. A U-shaped heatsink of,
say, 0.25mm copper sheet soldered to the conductor will radiate the heat

One tool which anyone designing, building or modifying a solid state power
amplifier should possess is an infra-red thermometer. These are cheap and being
non-contact, can be used to look at a the temperatures of PA components while
it is running. Thermal design is often neglected in amateur designs.

Vy 73


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