GaAs FET preamp by Kevin Custer W3KKC

Build a good GaAs FET preamp for
2 meters, 222 or 440 MHz.

General Description:
This circuit will work on 2 meters, 220 or 440 MHz. Some part values are different for the 3 bands, however general layout and construction are similar.

Concept:
To build a GaAs FET preamp that has good gain and extremely low noise from readily available components. This preamp is stable with differing source and load situations.

Construction:
The circuit is built dead bug style on a piece of printed circuit board for a good ground plane. The FET sits on a stand made from a piece of copper or brass shim stock or strap ¼" x 1-¼" bent into shape. This stand is used to support the source capacitors and the FET. Leadless disc's or chip capacitors are used for the source capacitors, and output capacitor. Use generally accepted practices for vhf and uhf work. The power and RF section of the preamp must be kept separated. The use of a shield (screen) or separate compartment for the PS is suggested. The power supply section can be built on the under side of the preamp if double sided pc board is used, however this practice is not recommended if board is placed on the lid of a "Bud Box" which is common construction of this style preamp. The transistor stand is bent at these intervals, ¼", 1/8", ½", 1/8" to produce a __------__ (a little tough to produce with text, but you get the idea)

The parts that have values listed on the schematic do not change from band to band. Q1 is a Mitsubishi GaAs Fet from the MGF 1200, 1300, or 1400 family. My favorite is the MGF-1402.
Other GaAs devices may also work well.

The completed unit should be placed into a shielded enclosure, especially if it is going to be placed at a repeater site. Alternately, the circuit can be placed into the receiver cabinet itself.

Noise Figure tests were made by professional equipment to determine NF. A HP 8970 A and 346A were used for the tests. When constructed properly the units will be less than .5 dB with .40 to .45 being typical. Gain is about 24 dB on 2 meters somewhat less on 220 and 440. The circuit is extremely stable when good quality connectors are used. N type or quality teflon BNC connectors are recommended on the 440 model.

Circuit Schematic: Refer to the circuit schematic image to understand circuit flow.

Parts List, 2 meter preamp:
C1, C2 - 1 to 14 pF Johanson 5400 series or equivalent high Q air variable capacitors.
L1 is 5 turns of #18 tinned copper 1/8" internal diameter.
CS (source capacitors) 330 pF leadless disc, or chip capacitor.
Z1 is 5 volt zener diode.
T1 is bifilar wound 6 turns of #28 enameled magnet wire on an Amidon Associates T-25-12 toroid core.

Parts List, 222 MHz. preamp:
C1, C2 - 1 to 14 pF Johanson 5400 series or equivalent high Q air variable capacitors.
L1 is 3 turns of #18 tinned copper 1/8" internal diameter.
CS (source capacitors) 220 pF leadless disc, or chip capacitor.
Z1 is 5 volt zener diode.
T1 is bifilar wound 4 turns of #28 enameled magnet wire on an Amidon Associates T-25-12 toroid core.

Parts List, 440 MHz. preamp:
C1, C2 - 1 to 10 pF Johanson 5200 series or equivalent high Q air variable capacitors.
L1 is 1 turn of #18 tinned copper 1/8" internal diameter, or ¼" strap type inductor.
CS (source capacitors) 100 pF leadless disc, or chip capacitor.
Z1 is 5 volt zener diode.
T1 is bifilar wound 4 turns of #28 enameled magnet wire on an Amidon Associates T-25-0 toroid core.

If you have further need of information, or
If you have enjoyed this construction article, please e-mail me and let me know.
I may take the time to do more quality construction projects in the future.