02 June 2012

RF Step Attenuators for going QRPp

Today I was discussing the way to drop your signal down to nearly nothing.

An attenuator is an electronic device that reduces the amplitude or power of a signalwithout appreciably distorting its waveform. ...
Attenuators are usually passive devices made from simple voltage divider networks.Switching between different resistances forms adjustable stepped attenuators and continuously adjustable ones using potentiometers. For higher frequencies precisely matched low VSWR resistance networks are used.(From Wikipedia)

First I preferred the the Step Attenuator from qrpkits, but including the shipping costs I reconsidered and looked for other solutions. Bernd (DC3QI) proposed an electronic "Dämpfungsglied" from AATIS, but I feared that if I  forgot to turn my power down before attaching the attenuator, the circuit would go to electronic heaven. So Bernd considered an elegant solution using variable attenuators with BNC-connectors so that you can stack them to your delight.

From Wikipedia

But while things got easier to implement decisions how to do it got more complicated. I found an ARRL Paper with a Step Attenuator and several kits e.g. from MidnightSience.

You can get it from 20 to 500 $. But I am an amateur.

Again I looked into the manual of the Yeasu FT 817 and saw that I can reduce my power in steps from 5 W, 2.5W, 1W and 0.5W. So that is a starter. If I find an attenuator with around 40-60 dB I could go down to 2.5 mW.

Then I remembered that all this was not really new to me. About 15 years ago I had a TenTec Argonaut with a Step Attenuator 290 and I made a CW QSO with an Om who lived in Cologne. That was about 308 km away and I used about 10 mW on 40 m. That makes 30.800 km per watt. Really hard to beat.

So I take it easy in the moment. First I will turn the radio down to 0.5 watt and will see how far I get. If I am successful, there a decision has to be made. But I will not need 15 steps down to the ground.

  • 500 mW = 27 dBm
  • 100 mW = 20 dBm
  •  50 mW  = 17 dBm
  •  20 mW  = 13 dBm
  •  10 mW =  10 dBm 
  •    5 mW  =   7 dBm

So all I need is together 20 dBM of attenuation and this could be done roughly with 3 attenuators:

  • 10 dBm
  • 6 dBm
  • 3 dBm

I will search for a cost-saving solution to have a little bit of fun.

Stay Tuned!