35W PA for 23cm
Prototype 35W PA using two M57762 modules
A further development of the 18W PA has been carried out, to combine two 18W modules. The amplifier uses a balanced configuration, with branched arm couplers to provide the necessary splitting and combining functions. After consultation with interested parties, it was decided to use pcb couplers rather than Sage wireline, which is somewhat fiddly to work with and is not cheap. The couplers work very well, with amplitude balance within 0.1dB and phase balance better than 1 degree. The pcb material used was 1.6mm FR4. The has the advantage of being cheap, and the loss of the couplers (about 0.5dB) was considered a good compromise between cost and performance.
To keep construction as simple as possible a single pcb is used, with one of the modules "embedded" into the board through a hole. Rather than using an expensive heatsink, a simple 6mm thick aluminium plate is used as the "chassis". This may be bolted to a heatsink of the constructor's choice. I am experimenting with water cooling, which keeps the devices cooler (more power, better lifetime) than an air cooled heatsink of reasonable proportions! It also has the advantage of keeping the size down, which is necessary if more than one of these units is to be combined!
The PA produces over 40W output saturated with a power added efficiency of about 30%. The prototype has so far been tested with Vcc=13.8V and 15V (within ratings, but take care if operating into poor VSWR loads - risk of blowing up modules has been reported!).
Data below shows power output, gain and collector currennt as functions of drive, and data across the band at 1W input power.
Performance data
Vcc = 13.8V
Vcc = 15V
Download Excel files if you want full data:
13.8V swept power, 13.8V swept frequency, 15V swept power, 15V swept frequency