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, which will also be documented (shortly), along with the SWR shutdown circuit.

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.

Capacitors

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

Inductors

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

Resistors

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

Diodes

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

Transistors

Schematic Reference	Value	Description	Package	Supplier	Supplier Part Number
Q1	MRF171A	Motorola N-channel enhancement mode MOSFET	Case style 211-07	Richardson Electronics	MRF171A

Notes

[1]  8 turns 18 SWG enamelled copper wire on 6.5 mm diameter former, turns length 12 mm
[2]  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.

• MRF171A datasheet  (290k PDF)
• Richardson Electronics  MRF171A supplier
• Coilcraft 132 series inductors
• Digi-Key
• Progressive Concepts
• Circuit Specialists

Pictures

Select a picture for larger image.

Original Motorola Mostar 800 MHz RF power amplifier.

My version.

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.

Alternate view of the RF input.

Alternate view of the RF output.

I cut the back of the radio off to use as a cover.

The heatsink is not adequate for this FET so it will require forced air cooling.

New version with a tin plated PC board and slightly different layout.

RF input, detector and bias tee.

Layout overview.

FET itself.

RF output.

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.

SWR Protection

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  (18k GIF, 21k PostScript)

Here are a few pictures of the prototype I was working on, one and two.