This is a power amp that is great for giving your 2m HT a bit more grunt.
(Reproduced with the permission of the author)
This is a very simple 2m power amplifier that will produce 30w or more of output with 3 - 4 watts of drive. It can be modified for different drive levels by changing R1, R2 and R3. (They form a PI attenuator network, and in this case, a 10dB attenuator is used)
I chose to build this amplifier as I had a surplus Mitsubishi M57737 module lying around that was not being used. I have "Milled" out the PCB for this unit using a dremel tool and a fine cutting wheel. It certainly is not pretty, but it definitely works.
This circuit can be divided into 4 main sections:
1) The input attenuator - 10 dB in this case, so 4 watts of drive will give 400mW to the hybrid module. (400mW is the maximum input rating for this module) R1, R2 and R3 form this circuit.
2) The amplifier - A Mitsubishi M57737 VHF 30w Hybrid module - 300mW in = 30w out, however, it is a class C module and is not linear, so it is only good for use in FM or CW modes. U1 forms this part of the circuit.
3) The low pass filter - this attenuates the harmonics produced by the hybrid module to an acceptable level. (2nd harmonic was 50+dB below the carrier at full drive, 3rd harmonic was un-measurably low at full drive). L3, L4 and L5, along with C5, C6 and C7 form the low pass filter.
4) The T/R switching circuit - this part is optional, only needed if you want to use this amp as an outboard rather than part of your project.
The supply to the amplifier is isolated from RF by L1 and L2, along with C1, C2, C3 and C4.
L1 and L2 are made by winding 8 turns of 1mm - 1.2mm enameled copper wire onto a 5mm drill bit, and the turns are wound tightly with no space between them. L3, L4 and L5 are made by winding 4 turns onto a 5mm drill bit, and spacing the turns about 2mm apart.
It is as simple as that!
This is the circuit for the beast....
This is a revised schematic, with T/R relay switching included, along with adjustable control voltage for power output adjustment
Here is a shot of the "PCB" with components on it
This is a shot of the inside of the unit after assembly.
This is a shot of the unit with the T/R switch in it - it isn't pretty, but it works!
....and here is the unit under test...note that the meter is on the 200w FSD range.
A couple of notes:
1) The M57737 is now obsolete, but the Toshiba SAV7 is not, and is a drop in replacement.
2) This unit is for FM/CW/DSTAR modes only. As it is class C (non-linear), it is not suitable for any AM modes.
3) I ended up deciding on a DPDT relay for T/R switching, for simplicity and cost effectiveness. You can use two coaxial relays instead.....
4) R1, R2 and R3 must be carbon or metal film type resistors, of a rating of at least 1w each. Do not use wire wound resistors here.
5) When checking the unit on a spectrum analyzer, use a dummy load with a sensing port, do not rely on the "Sniffer" antenna on the analyzer - you will get false results if you do - it is also a good idea to have the lid on the box prior to taking the final readings.
6) A full sized schematic is attached. (The assembled prototype also has a polarity protection diode installed, along with 4 more capacitors connecting directly to the supply input. The extra caps are not necessary, but the polarity diode is highly recommended, as is putting a fuse in line with the DC input of a 7.5A to 8A rating)
If you have any questions about this project, just contact me via the Blue Mountains Amateur Radio Club.
73 de Jamie,
VK2TIM
