18 February 2013

Hendricks Fabulous 41dB Step HF Attenuator

Life is full of adventure and the straight way is not always the fastest lane. And: you need patience like a fisherman to achieve your goal.

Die Geduld des Fischers  © Joachim Lehrer
So it was with me. If you have just a wire and not an antenna, you have nothing but 5 watt output and you think that this is enough and you make contacts with (nearly) everyone you can hear in your radio. Then everything is normal. But you are missing the spice.

In the last months I discovered a lot of other Blogs, were Oms wrote about the new dimension. QRPp and QRPpp. Ok, from time to time I turned the power down to 2 watt, 1 watt, 500 mW and I was able to make contacts. Here and Then. But when your read the Blog of Bert, PA1B or Mike VE3WDM carefully you will notice a difference: They are not talking about Watts or hundreds of Milliwatts, they are talking of 80 mW, 36, 8 and 1.8 mW. And they were able to make real QSOs.

So I decided to give it a try. And of course I had already a source for the big power eliminator: Hendricks attenuator. The circuit comes from the ARRL handbook and I could have done it myself. But building the ugly style is possible, but I like to have a case and I am not a tinkerer. So I ordered it over there and it took only some days until the parcel arrived here in Tuebingen. 

Contents of the parcel from  Doug Hendricks QRP Shop
Of course I could not resist to make an inspection of the parts. You know, when ordering a kit from the USA - these guys are very friendly, but if a part is missing, they have to ship it to you and that is only trouble ... and I don't like trouble. But as always the kit was complete and even some parts were doublets. So if the builder does something wrong he has a spare part. Well done Doug.

Next I sorted the resistors. I don't trust my eyes so I measured them, just to be sure. 

Sorted parts of the kit.

And I checked and double checked the values with the excellent construction manual which you have to download from the web page. 

Checking the parts

Now came the part which was not so much fun: Soldering the parts of the attenuator on the pcb board. It went too fast and made no problems. There were no mistakes, all parts went into there holes, there were no rewirings and after 30 minutes everything was on its place. It would have been much more fun, if there would have been more parts and it would have taken 20 hours to solder. But you get what you pay for and that was satisfactory.

Assembled board
Before closing the case I had to apply the decals. (That was really hard .... to find the word in the dictionary. Never heard it before ;-) I thinks I will later on cover it with a transparent foil for I don't trust my sweating fingers.

The finished attenuator.
So now that everything was done, I switched it between the K2 and the QRPOmeter and gave it a first try.
Of course you can say, what's the heck: Just turn down the power of your rig and you are QRPp. Thats right. But soetimes you want to be proud and really be sure that you have not cheated.
Now the operation is clear:
If I let the K2 rest with the output power of 5 watt and I switch the Bypass out and put
- 1 dB - I get 3.9 Watt
- 2 dB -       3.1 Watt
- 3 dB -       2.4 Watt
- ...
- 7 dB -       1 Watt
- 10 dB        0.48 Watt
- 20 dB        51 mW
- 30 dB        5.5 mW
- 35 dB        2,0 mW (Ben congrats again for your 1.8 mW)

And if I set the K2 to 1 Watt output I can go down with -40 dB to 200 uW. (But I will not try that ;-) So when you here me on the bands, please give me a shout. Perhaps you are the one to brake that mileage record of Ben or Mike.

And to please your mind, let's hear some music which starts in QRP-mode and ends in a crescendo when the PA goes into overdrive:

Stay Tuned!