Folks...
Just picked up the new ARRL handbook (2001 version, I'd guess, with a
brownish colored cover). Something the prospective '817ers here may
be interested in:
There's an nifty little 1.8-30MHz power amp recipe in there; it's
based upon two power MOSFETs that seem more intended for switching
power supply use than RF use - nevertheless these little buggers work
fine thru 30MHz (there's a tad of drop-off on 10M but it's still a
useful unit) without spectral problems, distortion, etc. [The output
of this amp has switch-selected lowpass filters for the HF bands to
cutout harmonics/TVI.]
However, because of the TO-220 packaging of these power MOSFETs (TO-3
variants apparently aren't available) and the heatsink design of the
project, extended use of continuous carrier modes (FM, FSK & perhaps
AM) may fry the transistors or shorten their lives somewhat. [Do
remember that these transistors go for around $3 each, though.] But
for SSB & CW - with appropriate usage cycles - it probably is fine.
BTW, the project's "article" in the handbook has a nice bit of
thermal analysis treatment: I won't ever think of heatsinking so
cavalierly anymore.
Better (more surface area, better coupling to transistor) heat sinking
and perhaps fan assist or use of piezoelectric cooling modules could
help here. Or perhaps a rejiggering of the design using *four* of
these transistors and a power combiner setup might be interesting,
not pushing any transistors near/over their limits over time.
Parts kits, according to the project specs, are available from Mouser
Electronics.
{But by the time all's said & done, I am not sure this design may be
much cheaper than one of CCI's (Communications Concepts) offerings
using a more standard application of Motorola MRF-series power
transistors.)
Just a thought, anyway....
73 Bill N6AOT
