RF Fox Hunt & Transient Telemetry Lab

K-12 STEM Direction Finding, Raspberry Pi 5 Beacons & RC Circuit Analysis

Project Overview & Objectives

Welcome to the RF Fox Hunt & Transient Telemetry Lab. This module bridges foundational radio frequency (RF) direction finding with practical analog circuit analysis using modern single-board computers. Students build a directional waveguide antenna (the “Cantenna”) to track down a hidden Wi-Fi beacon powered by a Raspberry Pi 5, then transition to data acquisition by logging DC transient charging curves using an ADS1115 16-Bit ADC.

SSID: The_Hidden_Fox (2.4 GHz)

Figure 1: The Autonomous Raspberry Pi 5 "Fox" Beacon broadcasting 2.462 GHz frames.

Bill of Materials (BOM) & Hardware

To assemble and deploy this complete lab workstation, the required hardware components and recommended equipment are outlined below.

Affiliate Disclosure: Some links below are affiliate links. If you purchase hardware through these links, this site earns a small commission at no additional cost to you, helping fund our K-12 STEM outreach programs.

Component Specifications / Model Purpose / Role Source / Affiliate Link
Microcontroller / Server Raspberry Pi 5 (4GB or 8GB RAM Starter Kit) Acts as the hidden Wi-Fi beacon (“Fox”) and telemetry data server View on Amazon
Power Supply Official Raspberry Pi 27W USB-C Power Supply Clean benchtop power for the Pi 5 server View on Amazon
Portable Power Bank High-Output USB-C Power Bank (10,000+ mAh) Powers the Raspberry Pi 5 when hidden during field fox hunts View on Amazon
Wi-Fi USB Adapter TP-Link USB Wi-Fi Adapter (or Alfa Network adapter) Directional RF reception mounted inside the Cantenna View on Amazon
Analog-to-Digital Converter ADS1115 4-Channel 16-Bit I2C ADC Breakout High-precision voltage sampling for RC transient data logging View on Amazon
Prototyping Kit 830-Point Breadboard, Resistors (10\,\text{k}\Omega), and Capacitors (100\,\mu\text{F}) Analog RC circuit construction View on Amazon
Test Instrument 2-Channel Digital Storage Oscilloscope (e.g., Siglent / Rigol) Visualizing square-wave transient step responses View on Amazon


Part 1: Student Beacon Setup Guide

What is a Fox Hunt?

A “Fox Hunt” (officially called Amateur Radio Direction Finding, or ARDF) is a classic engineering game. One person hides a small, battery-powered radio transmitter (the “Fox”) somewhere in a building or park. The rest of the teams (the “Hunters”) must use their knowledge of signal strength, decibels (\text{dBm}), and directional antennas (like our Pringles Cantenna) to track down the hidden transmitter.

To play, we first need to build our Fox. Today, you will program a Raspberry Pi to act as a standalone Wi-Fi beacon that automatically starts broadcasting the moment you plug in a battery!

What is a Wi-Fi Beacon?

Every wireless router in the world constantly shouts an invisible radio packet into the air about 10 times every single second. This packet is called a Beacon Frame. It contains the network’s name (the SSID) so your phone knows the network exists.

Even if you don’t type in the password to connect to the network, your phone’s Wi-Fi antenna can still feel the physical power of the Beacon Frame hitting it. We will measure that power (the RSSI in \text{dBm}) to play our game.

Step 1: Creating the Beacon

We will use nmcli (NetworkManager Command Line Interface) to command the Raspberry Pi’s internal wireless chip to stop acting like a normal computer and start acting like a wireless router.

  1. Open the terminal on your Raspberry Pi.
  2. Type the following command exactly as written, then press Enter:
sudo nmcli device wifi hotspot ifname wlan0 band bg channel 11 ssid The_Hidden_Fox password catchmeifyoucan

What did we just do?

  • ifname wlan0: Tells the Pi to use its built-in Wi-Fi antenna.
  • band bg channel 11: Forces the transmitter to use the 2.462\,\text{GHz} frequency (a long enough wavelength to measure easily).
  • ssid The_Hidden_Fox: This is the network name the Hunters will look for!

Step 2: Making the Beacon “Persistent” (Auto-Start on Boot)

If we hide the Raspberry Pi in a backpack or above a ceiling tile, we won’t have a screen or keyboard to type commands. The Pi must automatically start shouting its beacon the instant we plug it into a portable USB battery bank.

When we ran the command in Step 1, NetworkManager saved our new beacon as a connection profile named Hotspot. We need to modify this profile so it automatically connects on boot with the highest priority.

  1. Tell the Pi to auto-connect to the Hotspot profile:
sudo nmcli connection modify Hotspot connection.autoconnect yes
  1. Give the Hotspot a high priority (so the Pi chooses to be a beacon instead of trying to connect to the school’s normal Wi-Fi):
sudo nmcli connection modify Hotspot connection.autoconnect-priority 100

The Ultimate Fallback: Using Cron Sometimes, NetworkManager gets confused if it sees familiar networks nearby. To absolutely guarantee our Fox wakes up howling, we can add a scheduling rule to the Linux cron system. This tells the operating system to force the Hotspot on every time the computer boots up.

Type crontab -e in the terminal (select nano if it asks for an editor). Scroll all the way to the bottom of the file and add this exact line:

@reboot sleep 10 && nmcli connection up Hotspot

Press Ctrl+O (then Enter) to save, and Ctrl+X to exit. The sleep 10 command gives the Wi-Fi chip 10 seconds to warm up before firing the beacon command.

Step 3: Test Your Fox

Let’s make sure it works!

  1. Restart your Raspberry Pi by typing sudo reboot in the terminal.
  2. Wait 60 seconds for the Pi to fully boot up.
  3. Take out your smartphone, go to your Wi-Fi settings, and look at the list of available networks.
  4. If you see The_Hidden_Fox in the list, congratulations! Your beacon is actively broadcasting.

Part 2: Directional RF Detection & RC Transient Telemetry

System Architecture: Directional RF Detection

This schematic illustrates the hardware data flow. The cylindrical waveguide physically filters out off-axis scattered radio waves so your internal USB Wi-Fi adapter only reads the plane wave arriving from straight ahead.

2.4 GHz Plane Wave Cylindrical Waveguide ("Cantenna") Metallic Foil Liner 1/4 λ Probe USB Data Bus iwconfig wlan0 RSSI: -42.0 dBm Logging CSV...

Figure 2: Directional Waveguide Hardware & RSSI Telemetry Architecture.

Stationary Triangulation Geometry

By measuring the compass bearing (\theta_1, \theta_2) of the peak RSSI signal from two separated baseline stations (Station A and Station B), students plot intersecting rays to pinpoint the exact physical coordinates of the hidden Fox transmitter.

Known Baseline Distance (d) θ₁ θ₂ Peak Vector A Peak Vector B Station A (x₁, y₁) Station B (x₂, y₂) Hidden Fox (Pi 5)

Figure 3: Stationary Triangulation Geometry using Peak RSSI Bearing Intersection.

Lab Instructions: The Fox Hunt

In this lab, you will use your directional waveguide to detect a hidden 2.4 GHz Wi-Fi signal broadcasted by a Raspberry Pi 5. By measuring the Received Signal Strength Indicator (RSSI) in decibel-milliwatts (\text{dBm}), you will triangulate the physical origin of the signal.

Step-by-Step Procedure

  1. Connect the Hardware: Plug the USB Wi-Fi adapter into your laptop. Ensure the adapter is mounted securely inside the uncut Pringles can waveguide.
  2. Launch the Terminal: Open a new terminal window on your laptop.
  3. Select a Tracking Method: Choose one of the exploration methods below.
  4. Find the Peak Signal: Stand at your assigned station and slowly rotate the cantenna 360^\circ. The signal is at its strongest (peak) when the negative number gets closest to zero.
  5. Log the Vector: When you find the peak value, use a smartphone compass to record the exact heading the open end of the cantenna is pointing.

Exploration Method 1: Quick Radar (Terminal)

This command refreshes your screen every 0.5 seconds, turning your terminal into a live radar dashboard without saving the data. Press Ctrl+C to stop.

watch -n 0.5 "iwconfig wlan0 | grep -i 'signal level'"

Exploration Method 2: Data Logging (Python)

For advanced analysis, this script continuously logs the RSSI values to a CSV file along with precise timestamps. Save this as waveguide_telemetry.py and run it:

import subprocess, time, csv

INTERFACE = "wlan0"
OUTPUT_FILE = "waveguide_telemetry.csv"

def get_rssi():
    cmd = f"iwconfig {INTERFACE} | grep -i --color=never 'signal level'"
    res = subprocess.check_output(cmd, shell=True).decode("utf-8")
    for part in res.split():
        if "level=" in part:
            return float(part.split("=")[1])
    return -100.0

print("Logging RSSI (dBm)... Press Ctrl+C to stop.")
with open(OUTPUT_FILE, "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["Timestamp_s", "RSSI_dBm"])
    t0 = time.time()
    while True:
        rssi = get_rssi()
        elapsed = round(time.time() - t0, 2)
        print(f"t = {elapsed:5.2f} s | Signal = {rssi:6.1f} dBm")
        writer.writerow([elapsed, rssi])
        time.sleep(0.5)

Data Collection Log

Measurement Node Coordinates (x,y) Peak RSSI (dBm) Compass Heading (^\circ)
Station A
Station B
Station C (Optional)

Data Acquisition: DC Transient Telemetry

The analog RC circuit connects directly to an ADS1115 16-Bit ADC, which communicates with the Raspberry Pi 5 over the \text{I}^2\text{C} bus.

Vin (3.3V) Step Switch R = 10 kΩ V_c(t) C = 100 μF Common System GND Analog In ADS1115 16-Bit I2C ADC A0 GND 3.3V SDA SCL GND Raspberry Pi 5 GPIO 2 (SDA) GPIO 3 (SCL)

Figure 4: Complete RC Transient Schematic & ADS1115 I2C Telemetry Wiring to Raspberry Pi 5.

I2C Bus & Power Wiring:

  • Red Wire: VDD (3.3V Power)
  • Blue Wire: SDA (Data Line to GPIO 2)
  • Orange Wire: SCL (Clock Line to GPIO 3)
  • Black Wire: Common System Ground

Software Implementation: ADS1115 Data Logging

Once the physical circuit is wired, use this script to record capacitor voltage over time to a CSV file:

import time
import board
import busio
import csv
import adafruit_ads1x15.ads1115 as ADS
from adafruit_ads1x15.analog_in import AnalogIn

i2c = busio.I2C(board.SCL, board.SDA)
adc = ADS.ADS1115(i2c)
chan = AnalogIn(adc, ADS.P0)

print("Logging RC Voltage to CSV... Press Ctrl+C to stop.")
with open("rc_transient_data.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["Time_s", "Voltage_V"])
    
    t0 = time.time()
    while True:
        elapsed = round(time.time() - t0, 3)
        volts = round(chan.voltage, 3)
        print(f"t = {elapsed:5.3f} s | V_c = {volts} V")
        writer.writerow([elapsed, volts])
        time.sleep(0.05)

Assembly Notes & Instructor Guidelines

  • Ground Referencing: The analog breadboard ground, the oscilloscope ground, and the Raspberry Pi digital ground must share a common reference, otherwise the ADC will read floating, erratic voltage values.
  • Time Constant (\tau): For R = 10\,\text{k}\Omega and C = 100\,\mu\text{F}, the theoretical time constant is \tau = RC = 1.0 second. In the DC circuit, the capacitor reaches approximately 63.2\% of V_{in} after 1.0 second.