50-70 MHz preamp with the BFG135a
As I had build and placed a dualband 4m-6m LFA Yagi (design G0KSC), I did not think I needed a preamp on these low frequencies. But for fun I wanted to build a broadband version which should cover 50 to 70 MHz. The design of YU1AW was found on the internet and I build one on a self made PCB.
This was not a direct succes as it oscillated big time. I made a 2nd version with some adjustments and it was not much better.
Fiddeling with it and making use of better grounded connectors and shielding it was the key to succes.
Grounding, even on these lower frequencies is essential. To get the best bias working point, I made the bias resistor a tunable version and set it to 470 Ohms for best gain and no oscillations.
Here are some photos of the build preamp and a sweep with my VNWA to show the gain S21 and S11 input imedance and VSWR. I used a 30dB attenuator at the output to my VNWA for safety. In the picture the 6m gain is -10dB, so +30 attenuation leaves a nice +20dB gain.
As I wanted to feed the relais and preamp via the coax, I needed to build a bias-T. This is an easy project but took more time than calculated. I used a normal 1nF ceramic capacitor and a 10uH choke as is commonly used. All worked fine but with 100W on 50MHz the capacitor was blown to pieces. Nice effect, but not wat was needed. I found out that a 500V silver mica capacitor was the best way to go. Anything between 470 to 1000pF will work. Do not use 33 or 47pF versions as they influence the VSWR.
In the picture with the open preamp you also see my bias-T build in an old housing with N-connectors on it. Cheap and easy to make and its saves out an extra cable for the +12 volts needed in the preamp box.
20-01-2013 by PA3FXO