The FARA 2 meter amplifier project

Posted on Feb 5, 2014

This article describes a 2 meter amplifier capable of running 25-30 watts output. More than 35 amplifiers have been procured at a cost of less than $50 each in these quantities. Photo A shows the final version of the circuit board; the completed amplifier is shown in Photo B. It is designed around one of the newer bipolar RF devices from Motorola, an MRF1946A (Q1).

The FARA 2 meter amplifier project
Click here to download the full size of the above Circuit.

This device compares favorably with many of the RF FETs available as the MRF1946 is capable of developing 10 dB gain at 146 MHz, while the older bipolar devices (the 2N6080 series) produce only about 5. 7 dB gain. RF FETs are generally rated at 13 dB gain at 28 volts-. in the 12-14 volt range they also yield about 10 dB. The design presented here is unconditionally stable, while FET amplifiers require a bias supply and careful tuning at the higher voltages to maintain stability. The cost of the MRF1946A is only about two-thirds that of the FETS, yielding the most "Bang for the Buck!" Motorola produced an application note (RF Device Data, Application AN955) for a 150 mW to 30 watt land mobile VHF amplifier in the 160 MHz range, based on the MRF1946. This was the starting point for the design. The schematic diagram is shown in Figure 1. DC voltage into the amplifier is decoupled by C2, C3, L1 C4, C5, and L2. D2 is the reverse polarity protection diode-if the voltage is inadvertently reversed, D2 will limit the reverse voltage to 0. 7 volts and fuse F1 will open, protecting the amplifier. The output stripline (Z1) described in the application note was lengthened for operation at 146 MHz and the output capacitor (C10) was empirically adjusted to yield an efficiency in the 70% range, just about what one would expect of a Class-C amplifier. The input circuit was derived from the formulas given in the RSGB VHF/UHF Manual. This...

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