Following on from my 23cm Masthead system I decided G3PYE needed a similar system for 70cm. The system will mirror the 23cm version with a TX amp and a Preamp in the box, but in addition I fitted a band pass filter for 432MHz.
John G4BAO kindly donated a UHF PA which has an MRF-185 (28v 85w) device in it. The board is mounted on a substantial heatsink and has a circulator on the output. The input has a Motorola PHW2723 which means the amp board can be driven from 25mW (14dBw).
The Motorola PHW2723 has an output of 5w (12w max) and as the system I plan to use this amp in will have more drive than 25mW I disconnected the PHW2723 pins from the board and wired the RF input direct to the MRF-185 via an existing capacitor which I tomb-stoned on the board, it might not be pretty but it works!
I also removed the circulator, as useful as they are they are lossy and are for systems where an antenna failure is a potential, and so wired the RF Output to where the circulator input was.
Testing on the bench with 27v I get 80w output with 2w input. That’s the TX amp sorted.
I originally used a G4DDK SPF preamp which claims 21dB gain for 0.68dB noise at 432MHz. More than ample for terrestrial use.
The SPF Preamp is all SMT so is small and has good performance. After fitting the components and modifying the board slightly I managed to fit it into an old Micro Circuits box with RF in, RF out and +12v – all on SMA connections.
I found the preamp didn’t perform as good as we expected. This is not a fault of the preamp, the issue needs to be investigated. As such I got a G4DDK PGA103+ preamp which has a better IP3 performance over the SPF version.
The PCB was too big to fit in the same type of box as the SPF version so I needed to mount it in a Tin box.
Again I had to cut the board down at the input side so it could be mounted in the box as close to the input connector as possible. The ground plane side was then soldered to the board.
The output connector needed to be extended to the tin box wall, so a short length of coax with an SMA connector on it was used as seen below. The box is a one for one replacement for the current SPA preamp.
One thing which needs consideration for such a masthead system is how to keep it waterproof. The first thing is to get the right type of box. There are lots at rallies and also good deals on eBay.
To mount the amp I needed to cut a hole in the box to get the heatsink through, which was then sealed with “Wet Water Sticky Stuff” which is used for ponds and the like. A hole was cut in the lid of the box for this.
First a bead of sealant was put round the amp where the PC met the heatsink and the heatsink mounted into the lid.
Once fitted then more “Wet Water Sticky Stuff” was added round the heatsink on the outside of the box to form the waterproof bond.
Once the amp was mounted in the box and the preamp made it was time to connect everything up;