430 MHz preamp Gain 12dB with the MGF1302

As I had good experience with the 144 MHz preamp build with a MGF1302, I wanted to do the same for 430 MHz (70cm). The UHF Unterlage part I/II was searched and the design of DJ9HO was soon found.
The MGF1302 is a cheap fet and in my view rather strong. It is not easily damaged by static discharge or RF power. The design was slightly changed.
I used a LM317 to control the drain voltage and current instead of a 7805 and a potmeter in the source. The source resistor is fixed 150 Ohms and I used normal 460 pF SMD capacitors instead of the ceramic type described.
The first test was discouraging ....stupid but I used a fet with unknown status ....the gain was 0 dB. After replacing it with a new version the gain was 5dB. Wow ...what progress.
After checking connections I got the conclusion that grounding of the copper print was terrible. I had to make a ground connection between the copper print and the aluminium housing with a M3 screw.
That did the trick and gain was to be tuned up to 16dB. After putting the lid on the box the gain went down to 12dB.
Not a problem but it could be better. I had to drill a hole so I could tune the LM317 adjust port 5K potmeter from the outside. Also 2 more grounding screws were to be added to get a good working stable preamp.
The gain could be more but then I would have to experiment with the output matching. The L2 (1 turn and diameter 4mm form drain to + of LM317 is not needed and can be replaced with a 120 Ohm resistor.




If you want to build a preamp beware of making good ground connections and play with the drain voltage to get the optimum drain current and gain.

Another design I tried was from Dubus and uses Lecher transmission lines. The performance with a MGF1302 was reasonable. Gain of 14dB and a NF of 1.8 was achieved.
This could be better as I had not used a smd resistor at the drain and should have replaced the smd capacitors for trapezium types which lie directly to ground.


If a preamp oscillates you can see the result on a spectrum analyzer .... it looks like a marker generator:



8-09-2009 PA3FXO


Home