1---
2app_version: 0.1.0
3precision: 3
4form_title: Resistive Attenuator Pad (Pi / T)
5form_description: Design a matched resistive attenuator — enter the wanted attenuation and line impedance, get both Pi-pad and T-pad resistor values.
6form_inputs: Attenuation (dB)|attenuation_db, Line impedance (Ω)|z0
7form_outputs: T-pad series R (Ω)|r_series_t, T-pad shunt R (Ω)|r_shunt_t, Pi-pad shunt R (Ω)|r_shunt_pi, Pi-pad series R (Ω)|r_series_pi
8---
9# Resistive Attenuator Pad (Pi / T)
10// "I need to knock 10 dB off this signal without upsetting the match."
11//
12// Both pads are symmetric and matched: looking into either end with the
13// other terminated in Z0, you still see Z0 — handy ahead of a sensitive
14// receiver input or a power meter that expects 50 Ω.
15
16attenuation_db = 10 // wanted attenuation
17z0 = 50 // Ω, line impedance (50 Ω is standard for ham gear)
18
19k = 10^(attenuation_db / 20) // voltage ratio for this attenuation
20
21// T-pad: two series resistors bookending one shunt resistor to ground.
22r_series_t = z0 * (k - 1) / (k + 1)
23r_shunt_t = z0 * 2 * k / (k^2 - 1)
24
25// Pi-pad: two shunt resistors to ground bookending one series resistor.
26r_shunt_pi = z0 * (k + 1) / (k - 1)
27r_series_pi = z0 * (k^2 - 1) / (2 * k)
28
29"Voltage ratio K": k
30"T-pad series R (Ω), ×2": r_series_t
31"T-pad shunt R (Ω)": r_shunt_t
32"Pi-pad shunt R (Ω), ×2": r_shunt_pi
33"Pi-pad series R (Ω)": r_series_pi
34
35# Nearest resistors you can actually buy
36// The exact values above rarely exist off the shelf — e24() rounds each one
37// to the nearest standard value from the E24 preferred-number series.
38"T-pad series R, nearest E24 (Ω)": e24(r_series_t)
39"T-pad shunt R, nearest E24 (Ω)": e24(r_shunt_t)
40"Pi-pad shunt R, nearest E24 (Ω)": e24(r_shunt_pi)
41"Pi-pad series R, nearest E24 (Ω)": e24(r_series_pi)
42
43// Power rating check: the pad dissipates the difference between what goes in
44// and what comes out. For a transmitter pad (as opposed to a receiver-line
45// pad), size every resistor generously above its share of this total.
46input_power_w = 100
47output_power_w = input_power_w / k^2
48"Total power dissipated in the pad (W), at 100 W in": input_power_w - output_power_w
49