The MHW 612 by Motorola, it's a VHF Broadband power module amplifier for commercial mobile transmitters giving a maximum Output Power of about 20 Watts (50 Ohm). The Module is mainly for operating between 146-174 MHZ, but
our 2m amateur Band (144-146 MHz) is very close to the lower edge of module bandwidth.
I bought a piece from eBay for about 10 Euros(!) and I tried to find some description, a diagram or "data sheet", without succes. Just the operating voltage (12 V), the Bandwidth (146-174 MHz) and the output power (20 W) was known.
Anyway, after few experiments on my bench, I achieved to find the correct connections into the module's pins ( without "smoke" ! )
With this thought, I decided to construct a PA amplifier, knowing that the module has a very big Gain and on the other hand, is usually a pretty simple construction, using just a few external components.
Picture above shows my circuit. The module has just 4 pins. The 2 across Pins are for RF Input-Output respectively, the 3rd for the main Power-supply (+Vc) and the 4th pin for the Output-Power handling. I have designed a simple circuit by using the BD435 NPN transistor,
in order to control the output. This way, I achieved to adjust the output between 0,1 and 15 Watts, just by using 0,25 Watts driving input-power. The Power output adjusting is done through the 1K potentiometer. The result was good enough. The total Gain was ~17 Db, fully controlled.
The module can accept in the ADJ-pin up to 12 V directly from +Vc, but in this case the current into +ADJ pin strongly increased, without
any spectacular difference in the Output-Power. Just 1-2 Watts more (probably was close to the "saturation-point"). So, I decided to keep the +ADJ up to 10V maximum, for safety operation of module.
+ADJ pin Volts |
+ADJ pin I (mA) |
+Vc pin I (A) |
Output (W) |
2,7 | <50 | 0,3 | 0,25 |
3,0 | ~100 | 0,85 | 0,5 |
3,4 | ~120 | 1,0 | 1 |
3,7 | ~150 | 1,33 | 2 |
4,8 | ~200 | 2 | 5 |
5,8 | ~250 | 2,65 | 10 |
9 | ~500 | 3,5 | 12 |
10 | ~600 | 4 | 15 |
Output measured by Hansen FS-5 SWR-Power meter, terminated into a BIRD 8130 Dummy Load. |