A few years ago we built a solid state amp (tubes are still my
favorite, but sometimes you cannot even find them to do what you
need) that outputs 2 Kw CW into 50 Ohms, at 805 MHz. It never has
been connected to an antenna, instead it drives a 1/2 mile long 3.125
inch EIA coax line, which drives a dummy load at the far end. Along
the line are 44 tap points with directional couplers, each feeding a
small solid state amp to the input of a klystron. For 20 years we
used an RCA cermalox 8501 tetrode to do this, but they discontinued
the bottle. Its very difficult to find tetrodes for 5 kw levels at
800 MHz ranges anymore. Low Power TV is all solid state for this
range.
It took 32 transistors, bipolar MRF899's, to do the job. Running at
22 VDC, this class AB amplifier is water cooled, in one 19 inch full
height equipment rack. Each MRF899 contains a pushpull bipolar pair.
The first unit has been running continuously since about 1996,
without any failures (except one solder joint opened on one
transistor tab, which was hardly noticible to the output). The second
unit went online in 1997 and also has an exceptional record of
reliability.
I am convinced that a solid state amplfier can be built to replace
all but the largest tube system (50 kW and up), at a cost.
Reliability can be high, if protection and conservative design is
followed. For instance, the '899s were rated for 150 W PEP, and we
ran them about 80 Watts. Each is combined with a Sage Wireline
hybrids. A radial 14 way combiner takes it up to the final output
port. Lots of sensors on temp, water flow, collector current, VSWR to
make sure that there isn't a fault to eat all that silicon at once.
The amplifier design (with photos and block diagram) was presented at
the 1998 RF Expo in San Jose, also at the 1998 Linear Accelerator
conference in Chicago. Those proceedings are available. I will email
an 85K PDF version of the paper to interested amateurs.
John
K5PRO
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