Since my Motorola service monitor is "on the Fritz", I am using my Singer-Gertsch FM-10C service monitor. The FM-10C uses 2-each 20 dB external attenuators in series when measuring the sensitivity of receivers. Unfortunately, these attenuators are both long and fairly fragile (they stick straight out from the front panel). Therefore, I decided to build an external attenuator that uses switches to select the amount of attenuation needed to measure the sensitivity of receivers.
Basically, I chose to incorporate 2-each 6 dB, 1-each 20 dB, and 1-each 40 dB sections. Since we are dealing with voltage 6 dB attenuation cuts the signal in half and 2-each 6 dB sections cut the voltage to 1/4. A single 20 dB section reduces the signal by 10 and a single 40 dB section reduces the signal by 100.
Most of the time spent building the attenuator was selecting resistor values that came within at least 1-percent of the "ideal" values. To accomplish this, I measured individual resistors with a digital multimeter and selected, in virtually all cases, 2-individual resistors which, when put in series, came the closest to the "ideal" value. In all cases, the total resistance was well within 1-percent and in a couple of cases within less than 0.1-percent.
The "ideal" value of the resistors was calculated using the "on-line" calculator located at
Just "throwing" a switch is much easier than adding and removing the external attenuators.
The aluminum "mini-box" and the slide switches came from Radio Shack. The truth be known, Radio Shack has the best value on aluminum mini-boxes these days at $3.29 each. They were $2.99 but the price went up around 6-months ago. I usually "buy out" the supply of these mini-boxes at my local Radio Shack whenever I am there. I know that some day they will no longer stock them and, therefore, I try to keep a number of them "on hand" for all sorts of projects.