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.
Click on picture for larger image
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.
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.
Instrucciones
This is the start of a leet 40 watt amp, with full protection and
computer
control.
Notes
-----
- Use silver mica caps in RF sections
even better are metal clad micas (www.rfparts.com)
- Use non-inductive resistors (metal oxide, most metal film) in RF sections
and for parasitic resistors
- Use a large ground plane
- Tune for EFFICIENCY, not for high rf output
- Don't run parts at their maximum ratings
- Use BNC connectors below 5 watts, N connector above DON'T USE PL-259/SO-239
- nte320 appears to be a replacement for the mrf240
- mrf238 = 2n6084 = blw60
- the led in the ipa does not behave linearly, so dont use it for
power adjust, just as a indicator for rf input
- bipoler RF amps are dead. use mosfets