EASY AS "PI" ATTENUATOR

Copyright (c) 1994, 1996 by Clark Conti, N1NVK -

Many thanks to Clark Conti for passing this on to the public.

Attenuators help us fox hunters to determine distance to a target by allowing us to compare relative signal strength, or the correct beam heading when the raw signal is too strong to easily get a peak. An adjustable attenuator can be worth its weight in gold. Unfortunately laboratory types usually are. That is why I developed the "EASY AS PI ATTENUATOR" described in this article. It can be built for under $20 with parts available at any Radio Shack*.

The pi attenuator (shown in figure 1) is so named because it resembles the Greek letter p (PI), two vertical bars connected at the top by a third bar. Although both input and output show equal impedance (for our case 50 Ohm), part of the signal is reduced as it passes through the device. Figure 1 shows a 10 dB step. Looking at the circuit from either end shows a 96.2 Ohm resistor in parallel with a 167.4 Ohm, for an impedance of 61 Ohms ... optimum would be 50, but this is close enough not to throw the antenna and radio into impedance miss-match, so play along. The signal is reduced by R2 / (R2+R3)... OK so that's not exactly 10 dB either, so sue me. The purpose of this project is to build something simple to use for fox hunting, not a lab standard.

I combined 5 Attenuators on a 2 pole 6 position rotary switch in a shielded enclosure with 2 BNC connectors so that I could quickly turn down the signal level coming into my rig. Each successive step knocks a full S-Meter reading to about half scale. The last step (or is it the first?) is a straight feed through with no resistors for a direct reading.

The schematic is shown in figure 2 with a parts layout in figure 3. I suggest using 5% 1/2 Watt resistors, which might even survive if you accidentally transmit on low power. 1/4 W ones won't, and 5W jobs are just too darned big to stuff in the box. A suggested parts list from Radio Shack is in table 1.

After action report on the project is as follows: Accuracy of the steps stinks. Impedance is far from perfect, and on the 100dB setting, you get only slightly more signal as with no antenna at all, ... BUT... I made it cheap and it does exactly what I want it to do, so I call it a huge success. Who needs lab standards anyway? After all we're not professionals, were AMATEURS!

TABLE 1 - PARTS LIST

R1,R2 100 OHM 271-1108

R3 82 OHM 271-1107

R4,R5 68 OHM 271-1106

R6 390 OHM 271-1114

R7,R8 47 OHM 271-1105

R9 2.2 K OHM 271-1121

R10,R11 47 OHM 271-1105

R12 22 K OHM 271-1128

R13,R14 47 OHM 271-1105

R15 47 K OHM 271-1130

SHIELDED PROJECT CASE 270-0235

BNC CONNECTORS (2) 278-0105

2 POLE 6 POS ROTARY SW 275-1385

* Radio shack is a registered trade mark of Tandy Corp... I have to say that or else I get into trouble.