Jim Tonne stated this correctly, that solid state is quite appropiate
for some applications at medium and lower power levels and
frequencies. Harris did build a 1 MW medium wave TX using plastic
FETs I believe, probably called a DX1000 or something. And Longwave
is quite easy with silicon. The VHF and UHF applications are, at
best, running 20-30 kW with sand devices and thats as high as
economics allow so far. I will be at the NAB in Vegas next month, to
see whats new, but most broadcasters are content with these levels.
Silicon Carbide has some promise as the devices could run much hotter
with higher power densities. But the mainstream hasn't bought that
yet. There is a particle accelerator in France right now building a
190 kW CW solid state amplifier at 352 MHz. It is the size of a room,
with four round frameworks, each one loaded with pallets and
combiner/splitter. We can get a tetrode amplifier for this same
application, or a klystron (or IOT). The 1 MW CW klystron to do it is
more compact than this solid state design being built as we speak. A
single tetrode that can be held in your hands (has handles I will
grant you) will do 200-300 kW at up to 425 MHz pulsed and 100 kW CW
at 200 MHz all day long. The amplifier fits into a standard 19 inch
rack, and only screen, bias, filament and anode power supplies are
needed.
Motorola (Freescale, now Tyco-MaCom) made some of the bigger FETS in
a single package with 600 watt dissipation. APT is carrying that
level further with some parts now. So these devices are roughly on
par with 3-500Z on power, and are less rugged.
For Megawatt applications, such as the systems which generate an
E-beam to sterilize mail, or accelerate particles to high velocity
(energy) for science, or dielectrically heat polymers, all use a big
tube for these reasons. Dollars per watt, it is hard to beat for
these specialties. Hams are hanging on to the older technology, for
reasons of economics, because they are easy to build, and for
nostalga reasons. I don't see us going away from tube-based RF
generation as long as there is surplus used equipment available. It
is true that having a silicon device can be quiet, compact, and high
gain-bandwidth. There is an airborne radar that was going solid state
from big tubes, and this may be reconsidered as the SS is far heavier
and less efficient than comparable tube system.
For all of the other requirements though, tubes will remain supreme.
I have staked my career on this, and I haven't been wrong on that so
far. Next week I may be selling hamburgers if someone invents a super
power transistor or IC.
73
John
K5PRO
>Cliff:
>
>The FM broadcast rigs are mostly solid state up to 20 kW.
>The mediumwave broadcast rigs are mostly solid state up to 100 kW.
>Shortwave rigs, which are 50 kW and above and generally 100 kW to
>500 kW, use mostly tubes.
>It is just not in the cards today to make a 100 kW carrier, 400 kW PEP
>HF transmitter using solid-state devices.
>
>Your statement that
>> the broadcast, commercial and military transmitter industry has long ago
>> switched from archaic tube amplifier technology to solid state devices.
>is stretching it a bit! Below 20 kW, yes, but the "industry has long ago
>switched" is simply not correct. The reason for my sensitivity on this
>issue is my association with high power transmitter design.
>True, hams (who are supposed to stay below 1500 W PEP output)
>should be using a solid-state approach. But the "industry . . ."
is not true.
>- Jim WB6BLD
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