This amplifier was built based on Marconi's website, A Design for a 40W broadband VHF RF Power Amplifier for FM broadcast. A few minor tweaks were made to the schematic and a few parts were changed to what I had available (mostly surface mount components). The heatsink is from an old Motorola Mostar 800 MHz radio, and has the perfect heatsink island to match the MRF171A. Also used is a Progressive Concepts external LPF7002 low pass filter because it was also on hand. Since the MOSFET uses 28 VDC, I had to homebrew a 28 Volt / 5 Amp power supply using the schematic found in the ARRL handbook.
Tune up went exactly as stated in the how-to, with the RF power output hitting 57 Watts when driven with a stock Broadcast Warehouse 1 Watt LCD PLL transmitter at 98 MHz. Liberal use of ferrite beads and feedthru capacitors are on all RF detector, fan control and SWR control circuits.
Schematic Reference Value Description Package Supplier Supplier Part Number C16 47 µF Electrolytic, 20%, 35V SMT or leaded Any Any C9, C10, C13, C15 0.1 µF Ceramic, X5R, 50V 0805 SMT Digi-Key PCC1864CT-ND C1, C3, C8, C12, C14 1000 pF Ceramic, 5%, NP0, 50V 0805 SMT Digi-Key PCC102CGCT-ND C11 1000 pF Mica, 100V 2220 SMT Digi-Key 338-1008-ND C17 1000 pF Ceramic Feed Thru Any Any C20 300 pF Mica, 100V 1812 SMT Digi-Key 338-1026-ND C6 100 pF Ceramic, 5%, NP0, 50V 0805 SMT Digi-Key PCC101CGCT-ND C2 1.5 pF Ceramic, +/- 0.25 pF, NP0, 50V 0805 SMT Digi-Key PCC1R5CNCT-ND C4, C5, C7 4.5 - 65 pF Plasitc dielectric trimmer Leaded Digi-Key SG3009-ND C18 16 - 100 pF Mica compression trimmer Leaded Circuit Specialists 423 C19 25 - 150 pF Mica compression trimmer Leaded Circuit Specialists 424
Schematic Reference Value Description Package Supplier Supplier Part Number L1, L6, L7 700 Ohm @ 180 MHz Wide-band ferrite choke Leaded Digi-Key M2204-ND L5 210 nH Air core Leaded Handwound Note 1 L2 64 nH, 5% Air core Leaded Coilcraft 132-05 L8 41 nH, 5% Air core Leaded Coilcraft 132-03 L3 25 nH, 5% Air core Leaded Coilcraft 132-01 L4 21 nH Air core Leaded Handwound Note 2
Schematic Reference Value Description Package Supplier Supplier Part Number R2 10,000 Ohms Cermet potentiometer, 1/2 Watt Leaded Digi-Key CT6P103-ND R3 1800 Ohms Film, 1%, 1/2 Watt Leaded Digi-Key BC1.82KZCT-ND R1 33 Ohms Film, 1%, 1/2 Watt Leaded Digi-Key BC33.2ZCT-ND R4 10 Ohms Film, 5%, 2 Watt Leaded Digi-Key BC10W-2CT-ND
Schematic Reference Value Description Package Supplier Supplier Part Number D1, D2 1N4148 Silicon diode SMT or leaded Any Any D3, D4 1N5232 5.6 Volt Zener diode, 500 mW Leaded Digi-Key 1N5232BDICT-ND D5 1N5401 Silicon diode Leaded Digi-Key 1N5401GICT-ND
Schematic Reference Value Description Package Supplier Supplier Part Number Q1 MRF171A Motorola N-channel enhancement mode MOSFET Case style 211-07 Richardson Electronics MRF171A
 8 turns 18 SWG enamelled copper wire on 6.5 mm diameter former, turns length 12 mm
 3 turns 18 SWG tinned copper wire on 4 mm diameter former, turns length 10 mm
Part numbers and suppliers are mostly for reference only. Surface mount components are not needed, but highly recommended.
Select a picture for larger image.
Original Motorola Mostar 800 MHz RF power amplifier.
RF input match, bias tee and RF input detector.
The FET. Drain to the right, sources grounded.
RF output match. Here is starts getting cramped for space. The RF output goes to the old 800 MHz low pass filter under the PC board. The filter was isolated from the RF connector.
I cut the back of the radio off to use as a cover.
New version with a tin plated PC board and slightly different layout.
RF input, detector and bias tee.
Amplifier's case. It's from an old Motorola repeater.
Case internal view. Low pass filter is on the top.
Amplifier's heatsink close-up.
Directional coupler for the SWR protection circuit. It's from an old Motorola Mocom 70 mobile radio.
Here is a simple SWR protection circuit you can easily build. The directional coupler and detector components are from an old VHF SWR meter (one of those $10 hamfest cheapies). You may want to replace the meter's existing RF bypass capacitors with higher quality (mica) ones. You may also want to replace the coupler line's termination resistors which higher wattage ones. Be sure to use the exact same value, or the SWR meter will be inaccurate. Also, be sure to use non-inductive resistors (carbon-film or metal-oxide will work). This all then feeds a LM3914 bargraph display to light the LEDs, indicating the current SWR ratio. When the SWR ratio reaches approximately 3, it will engage the relay, cutting off the RF input to the power amplifier. You could also just light a LED or sound an alarm if you want to.
SWR protection circuit schematic
Here are a few pictures of the prototype I was working on, one and two.