Attenuator pads

A resistive pad lowers a signal by a set number of decibels while keeping the impedance matched at both ends. Design one, build it from standard resistors and see which resistor gets hot.

Design a pad

Attenuation 10 dB

Impedance

50 Ω for most radio work.

Input power

Build with

Resistor values and heat
Resistor	Exact	E24	Heat
input series	25.97 Ω	27 Ω	5.238 W
shunt	35.14 Ω	36 Ω	3.243 W
output series	25.97 Ω	27 Ω	531.7 mW

Built from E24 values the pad gives 10.1 dB and an input impedance of 51.53 Ω (SWR 1.03 : 1). The load receives 984.6 mW of the 10 W a 50 Ω source offers.

Open this pad in the circuit bench

For a matched pad the shares are the same for tee and pi designs. Above about 10 dB the input resistor takes most of the power, so it needs the highest rating.

Input resistor
Middle resistor
Output resistor
Delivered to the load

K = 10^(dB / 20) = 3.162series = Z0 (K - 1) / (K + 1)shunt = 2 Z0 K / (K² - 1)

Try this

Six decibels passes about a quarter of the power to the load. The input resistor already takes about a third of it as heat.
Only 0.1 W reaches the load, and the input resistor turns about 8.2 W into heat: size that resistor first.
The resistor values are completely different, but the heat in each position and the attenuation are the same.
Rounding to the coarser E12 series moves the attenuation and input match a little. The readout shows how much, so you can decide if it matters.

Apply it

A receive attenuator keeps strong signals from overloading the receiver (General pool G4A13). On the lower HF bands it costs little signal-to-noise ratio, because atmospheric noise is usually stronger than the receiver’s own noise (Extra pool E4C11).
Use non-inductive resistors. Wire-wound resistors have inductance that can make an RF circuit behave unpredictably (General pool G6A06); a dummy load uses a 50 Ω non-inductive resistor (Technician pool T7C03).
Check each resistor’s power and voltage ratings. Vishay’s 0207 metal film resistor, for example, is rated 0.6 W at 70 °C and 350 V maximum (Vishay document 28766).

How it works

With K = 10^(dB / 20), a tee pad has series resistors Z0 (K - 1) / (K + 1) and a shunt resistor 2 Z0 K / (K² - 1); a pi pad has shunt resistors Z0 (K + 1) / (K - 1) and a series resistor Z0 (K² - 1) / 2K.
The tool also solves each pad as a circuit between a source and load of Z0, so the attenuation, match and heat it reports hold for standard resistor values too.
In a matched pad the input resistor absorbs the fraction (K - 1) / (K + 1) of the input power: a third at 6 dB and more than half above about 10 dB.
Use non-inductive resistors: wire-wound resistors have inductance that makes RF circuits unpredictable.

Learn more: Building your own gear

Sources: Attenuators (Microwaves101); Radio Mathematics (ARRL Handbook supplement) (ARRL); MBA/SMA 0204, MBB/SMA 0207, MBE/SMA 0414 Professional Metal Film Leaded Resistors (document 28766) (Vishay Beyschlag); 2026-2030 Technician question pool (NCVEC Question Pool Committee); 2023-2027 General question pool (NCVEC Question Pool Committee); 2024-2028 Extra question pool (NCVEC Question Pool Committee)

Practice the exam questions: G4 Amateur Radio Practices (General), G6 Circuit Components (General), E4 Amateur Practices (Amateur Extra)

Found something wrong or out of date? Report a problem with this tool on GitHub (a public issue; nothing is sent until you submit it there).