|Any major improvement in the beacon required a new final amplifier.
Al Katz (K2UYH) helped us obtain this high-power
Kuhne in Germany. It's rated at 500W linear power and runs on
28VDC. Because it will be in continual use, I've derated it to
350W for life and reliability.
This module is an enormous improvement over the original amplifier, which could (episodically) deliver 200W and which required frequent module replacement. The higher operating voltage of the new module allows lower current wiring, which is more convenient and results in less voltage drop. It's also far more efficient.
Four fans seem to provide excellent cooling. I've added a temperature probe (not shown) at the air outlet to monitor operation. It remains largely cucumbric at the current <30% duty cycle. Because the amp is fully linear we will be able to run other modes such as SSB and JT65 in the future.
|Here's a view of the amplifier installed with a pair of driver
amplifiers from Down East Microwave. I call the assembly on top of
the amplifier the "monitor bridge."
The Eventide Argus is being used as a CRT monitor for data display. (Sadly, no moving map is needed in the beacon at this time.)
This view is before the frequency reference and synthesizer have been installed in the open space between the amplifier and the Argus.
|The monitor bridge has three 20dB couplers which are connected
between the various amplifier stages. The BNC connector goes to an
RF "sniffer" connected right at the amplifier output and adjusted for
-40dB pickup at 1296MHz.
Not shown is the "head" for the R&S NAP power/reverse power meter, which is mounted on the rear panel directly behind the "sniffer."
|This is the frequency synthesizer, a modified Maxim IC evaluation
board for a chip normally used in cellphones. By changing the VCO
(not shown, on the bottom), I was able to run it between 1200 and
1400MHz. It can be set in .05Hz increments! I also modified
it to DC-couple the I/Q inputs for power control.
Because it was so settable, I decided to incorporate a normal ovenized 10MHz crystal as the frequency reference rather than use the external atomic standard. That way the exciter/amplifier is completely self-contained, and the frequency control "loop" is closed manually. Whenever I notice that the frequency error at 1296MHz starts to exceed 1 Hz, I just reset the synthesizer frequency. This tiny board replaces a 50lb+ HP synthesizer!
|The HP 3448 switch box sits on top of the amplifier and the monitor
bridge where it can be connected under computer control to the various
amplifier stages. The output of the switch module goes to a
computer-controlled spectrum analyzer which measures the signal
characteristics to make sure they are within power tolerance and that
spurs haven't crept in.
The left-hand module and connector has a number of relays that control amplifier power, select DC voltages to be measured, and perform other control functions.
|Here's the partially assembled amplifier in a test setup with dummy
load and a mess of measuring equipment.
After confirming all was OK, I whittled a front panel out of copperclad PC material and installed it in the final beacon rack.