How About a Little Power on the 13 cm Band (2304 MHz) ?

by Dave W6OAL

I have found that by perusing the Internet specifically Ebay there are treasures to be found. One vendor I have found, “Pyro Joseph”, a supplier of surplus microwave amplifier boards to have been able to fill my needs and save me a lot of money in elevating my power on the microwave bands. On this particular band (13 cm) he offers 75 W amplifier boards that are pulls from commercial 13 cm Spectrain brand amplifiers. These boards are shipped with a set of basic instructions such as the control pin on the installed 10 pin DIP connector and voltage to be used plus what pins to monitor for the setting of bias of the amplifiers’ stages.

This is all good and well but some basic procedures need to be followed in building these boards into ‘plug & play’ amplifier units. An adequate heat sink is required as these 75 W boards idle at around 7 amps. I have used sinks that are 8 X 10 inches with ½ to ¾ inch high fins. The purist may want an exact square inch per watt device and the equations for such can be found in Down East Microwave ‘Notes’. Whatever heat sink used, a fan atop the sink is advisable. But foremost and prior to anything else, CONNECTORIZE the board! The board can/will accommodate (SMA-f) edge mount connectors (horizontal) or board mounted connectors (vertical). I stress this so much here as at microwave frequencies impedance matching into and out of the boards is paramount of importance. You don’t want to burn up a board due to sloppy ingress/egress!

If edge mount (horizontal) connectors are employed a heat spreader will be required as two of the connector legs of each connector will be beneath the board and raise it about a 32nd of an inch. I suggest, in this case, the use of a copper heat spreader at least a 32nd to 1/8th of an inch thick and the same size as the board. Use a sparing amount of thermal grease (Thermolox or equivalent) as to form a thermal junction and not a thermal barrier. Use this between both the board and spreader and board and heat sink. If top mounting of the connectors is contemplated, remove two of the legs of the connector, shorten the remaining two legs and center pin, then butt solder the connector to the input and output pads provided on the board. Make sure there are adequate fillets on the legs and center pin.

A box or sides of sorts may be constructed around the board leaving adequate space around the board, an inch or so. The walls will also serve as the front and back panels. On the front panel I like to employ a couple of LED’s; a green to indicate power to the amplifier and a red to indicate a transmit condition. A power switch? Forget it, unless it is to be so configured on the front panel to activate an ‘automotive relay’ that would carry the current. The board runs on 24 to 28 volts so why not just use a Power Pole connector and let the power switch be on the 24 to 28 VDC power supply? The rear panel will need to be fitted with input and output bulkhead connectors. I prefer SMA’s with rear coaxial configuration, not solder pot and shield tab. The impedance must be as possible as it can be made to 50 ohms in order to obtain the specified 75 W output for ~1.5 W input.

Consider a relayed 24 – 28 V ‘output’ on TX port for TX/RX external relay control via a single ‘test point’ jack. I use RCA phono jacks for PTT. This fairly much completes my suggestion menu to get these boards on the air. My main concern has been the input and output configuration in order to maintain a constant impedance in and out of the board. I have built up several of these boards and have learned a little more each time which is a good thing. Never stop experimenting and learning.

NOTE: 13 cm amplifier boards are available on Ebay, One Spectrain RF Amplifier Boards.