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RF-switched power attenuator, the MilliWatter Extreme

Explore the World of extreme QRP!

The MilliWatter Extreme will let you explore the amazing potential of extreme QRP "QRPp". The MilliWatter Extreme is an RF-switched transmitter-attenuator that opens up the world below 1 Watt. These low power levels are not available for many transceivers. Few power meters are accurate at these levels either so the MW Extreme opens up a new world of challenge; a world that is inaccessible for many transceivers used for QRP.

Because the MW Extreme's attenuators are RF-switched, you retain full receiver performance - essential for the higher HF bands. The MW Extreme has an adjustable hang-delay time for convenient use on different modes.

Using the MWEXTREME is much more convenient than using conventional attenuators, especially for the multi-band MilliWatter who needs to tune an antenna on band changes. With the MilliWatter Extreme you can tune on full power at the flick of a switch and then immediately join the fun on QRPp.


The MilliWatter Extreme gives you two accurate new power levels: 1/10th power and 1/50th power. We spent a lot of time designing the attenuators to give exceptional performance across the entire HF band.

The following examples show the power levels the MWEXTREME opens up for you:

  • Input power 5 Watts, output (low) 500 mW, output (extreme) 100 mW.
  • Input power 2 Watts, output (low) 200 mW, output (extreme) 40 mW.
  • Input power 1 Watts, output (low) 100 mW, output (extreme) 20 mW.

You will be amazed at what can be done with such extreme QRP. Set yourself some new challenges, worked all states or worked all continents on 100mW perhaps?

The MilliWatter Extreme gives you three convenient power levels at the touch of a switch "full", low and extreme.

The MilliWatter Extreme is also great for WSPR and datamode experiments. It can also be used to reduce power for transverter drive.



If you are using an antenna that is not matched the MW Extreme must be between the transceiver and antenna tuning unit.  Power is supplied by a standard USB socket (5 Volts), just hook it up to your shack computer or any USB charger. Lead not supplied - any USB-B printer lead will do. 


Input and output impedance 50 Ohms

Through loss < 0.15 dB

10 dB attenuator +/- 0.1 db 0 - 30 MHz

17 dB attenuator +/1 0.15 dB 0 - 30 MHz

Maximum power input 10 Watts

BOXA Products

All BOXA products are available in three configurations:

1. Board kit: a high quality PCB with all on-board components. 

2. Board kit plus enclosure: adds a high quality rugged extruded aluminium enclosure complete with laser-cut front and back panel. No drilling, no metalwork needed!

3. Fully built and tested: the complete product with enclosure, built and tested at our factory. Ready to use.



Construction instructions

Circuit diagram - see download page

6 Reviews Hide Reviews Show Reviews

  • 5
    An Effective Kit for QRP operations

    Posted by G0VNJ (Mike) on 3rd Oct 2015

    If one is interested in WSPR or similar QRP transmissions then this is an excellent kit by any standard and one easy to build. It only needs a rudimentary understanding of 'how to solder' to complete the board and have it operational in short time. A helpful sheet of instructions with explicit detail and photos can be downloaded from the SOTABEAMS site if required. The kit is enhanced by having included a bespoke aluminium case which the board, when completed, will fit perfectly and guarantee a professional looking piece of equipment for the shack. To cap all, a great price too! Forget the ready built unit, go for the kit, create some solder smoke and have fun. What's not to like. Tnx Sotabeams and best 73

  • 5
    Nice attenuator automatic switched and RF sensed too!

    Posted by Hans Hopstaken on 21st Mar 2015

    Hello fellow Hams! I bought this unit ready built and tested. Very satisfied with it and using it together with my Yaesu FT-817nd. Most of the time running 100 milliWatts RF power out and already been heard in VK2,3 and 5 as well as ZL2 and 3. So what do you want more ... Antenna here is a 2 x 10 m. inverted Vee with apex at 9 m. agl. 73 Hans, pa0hop

  • 4
    Great gadget for WSPR

    Posted by Matt G8XYJ on 5th Mar 2015

    I brought this with the intention of using it for WSPR with my Yaesu FT950 or 817. Wanted to see just how effective QRPp can be and just how far it will go. I bought the kit version, all very straightforward however my complaint is that the Transistor has to of course be inserted the correct way. Richard makes this clear in the instructions, but alot of people don't read them and would simply solder the Transistor in to the board based on the white lined of the PCB! I feel it prudent to say that it might be worth modifying the board to make the assembly of the transistor far easier for less experienced kit builders. All in all, very straightforward and a nice kit to try out QRPp. [ADMIN: thank you for your feedback Matt. The transistor will be corrected in the next version.]

  • 5
    it works great

    Posted by Unknown on 11th Feb 2015

    Bought the already assembled version. getting 20mw out with 2w in from my TenTec jupiter. 73 Paul WE2M >>>>>>>>>>

  • 5
    QRPpp is fun and working !

    Posted by Unknown on 10th Feb 2015

    Hi folks... Got and built the MilliWatter Extreme two weeks ago. Very easy and fun to build. Since then, I had very good results. Like that one on Fev 07 2015: CT7AEQ at 5,104 km (14,039.0 MHz CW) from my QTH. I used the KX3 at 2,5 W and with the 17 dB attenuation, the real "power" was 0,05 W. Got a 589 . This represents 102,080 km per Watt !! The only change that I made: I prefer to use 9 V battery for portable operations. So I terminated the printer cable with a LM7805 voltage regulator which transforms the 9 V to the required 5 V. Works fine. Thanks SOTABEAMS for that nice product ! 73 de Pierre VE2PID

  • 5
    Easy build, works as stated.

    Posted by Thaire Bryant W2APF on 2nd Feb 2015

    I put the MilliWatter together yesterday. Great instructions, all parts accounted for, well designed kit. Case is very sturdy and well designed. It worked on start-up. NH to France on 10.105 MHz with only 20 mw with KX3 and yagi antenna. Thsnk you for a nice kit.