Brent,
Typical RF power amplifier modules are most efficient and produce the
least spurious output when operated at the rated output level. The
M57719N is rated at 14 watts on a 12.5 VDC supply, over the 142-163 MHz
range. When you start reducing the drive or lowering the supply voltage,
you will very likely cause the module to become unstable and its
efficiency will plummet.
A case in point: My radio club has a packet node that uses a name-brand
mobile transceiver on the 2m side of the system. In compliance with the
Part 97 dictum to use the minimum power necessary for communications, we
set the power level on the 2m radio to "low," thinking that it would
dissipate much less power at the lowest level, thereby allowing its duty
cycle to be high. After operating flawlessly for several months, the PA
module burned up. A replacement module cost about 80% of the cost of a
new radio, but we felt it was worth repairing the radio. I installed the
new module carefully and ensured that all drive parameters were correct.
I also ran some tests to determine the efficiency of the module at each
of its four drive levels. Surprise! It was most efficient at the
highest (50 watts) power setting, and the module ran hotter at lower
power levels- verified with an infrared thermal scanner. I confirmed the
amount of heat being generated within the PA module versus the power
being transferred to the antenna, by noting the current draw. This was
not what I expected; I really thought that the input current would be
more or less proportional to the output power setting, but that was not
the case. The radio current draw at the 5 watt setting was about half of
the current draw at the 50 watt setting. That means that the input power
that didn't go the antenna was being wasted in the PA module, and it
couldn't handle the heat.
This looks like a fertile ground for experimentation. Moreover, it may
not be prudent to use a 14 watt module when all you need is, say, 5
watts. If the 14 watt module does not lend itself to efficient power
control, then it may be a good idea to install a 5 watt module. You
might also contact Mitsubishi engineering for guidance on controlling the
power of their products.
73, Eric Lemmon WB6FLY
73, Eric Lemmon WB6FLY
Brent wrote:
> Anybody have any suggestions on varing the power on the M57719N Power
> Module?
>
> Would it be best to limit the rf input?
>
> Would it be ok to lower the vcc on the module?
>
> which would be the best approach?
> Brent
>
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