The midterm uses what you have built since Lesson 4. The fox hunt is a receive problem on the air: a hidden transmitter, a readout of its strength, and your feet. The capture-the-flag is the receive problem in a recording: unknown signals I release as recordings. Both parts are individual and take home. Work alone, keep your flowgraphs, and turn in one PDF on Gradescope by the date I announce, with the evidence each part asks for below.

The fox is a small transmitter I hide inside a search area, sending short bursts at 915 MHz at low power. It might be a PlutoSDR or it might be a LoRa radio, and the receiver below finds either one, because it measures power in a channel and does not need to decode anything. You hunt with your own PlutoSDR and your own antennas, or the ones that come with the Pluto. I announce the search area and the hours the fox is on the air. Nothing you build for the hunt transmits.

MT-foxhunt.grc is the receiver. The Pluto’s LO sits 250 kHz below 915 MHz, so its DC spike from Lesson 5 stays off the fox. A Frequency Xlating FIR Filter moves 915 MHz to the center and keeps a channel as wide as the Channel width slider. Complex to Mag^2 turns each sample into power, and the readout shows it in dBFS two ways: the 100 ms average, and the strongest 2 ms in the last quarter second, which catches a short burst the average would smear out. The ADC level readout shows the largest I or Q sample; 1.0 is full scale.

If the fox is a LoRa radio, each burst is a train of chirps: a tone that sweeps across the radio’s band from one edge to the other, again and again. The figure is simulated and turned on its side, with time across; GNU Radio’s waterfall runs time down the screen. Long chirps draw diagonal stripes. Short ones blur into a block with sharp edges at both sides of the band, which a narrowband signal never makes. If the fox is a Pluto, expect whatever burst I send. You do not need to decode either one. Set Channel width to cover the burst you see.

The Pluto’s antennas hear all directions about equally, so your body is the direction finder. Hold the Pluto close against your chest and turn slowly through a full circle, stopping for each burst. The reading drops by several dB when your body sits between the antenna and the fox, so the fox is behind you at the lowest reading. That null is sharper than the peak, so use it for the bearing. Walk that way, then stop and take a new bearing. The readings jump with every step, so compare several bursts before you trust a change.

Lesson 4 showed what happens when a signal is bigger than the ADC’s range: the samples clip, the spectrum grows false signals, and a stronger signal no longer reads stronger. Close to the fox that is your hunt. When the ADC level readout reaches 1.0, the power readout stops rising, so every step looks the same. Lower RX gain by 10 dB and keep going. A reading means something only together with its gain, so log both. Within a few meters, even 0 dB can clip; take the antenna off and the Pluto still hears the fox.

The second part is a set of recordings of unknown signals, which I release with the midterm. Each is a SigMF pair like the L7 Lab’s test files: the samples in .sigmf-data, and the datatype, sample rate, and center frequency in .sigmf-meta. For each recording, find the signal, identify it, demodulate and decode it in GNU Radio, and recover its flag, a line of text. Each flag is worth points, and a flag counts only with its writeup. Some signals may be ones Module 6 covers in depth; the lesson pages on this site are yours to read ahead.

Work each recording from the outside in. Read the .sigmf-meta first: the datatype tells you how to open it, and the sample rate sets every display. Play it through a SigMF Source (Minimal) and Throttle into a frequency sink, a waterfall, and a time sink, and find the signal. Ask what changes: the amplitude, the frequency, or the phase, and how fast. Then move the signal to 0 Hz with a Frequency Xlating FIR Filter, decimate to a rate the receiver you need expects, and build or reuse that receiver from the lesson that fits.

Module 6 starts with Lesson 12, CW, and works through the signals you meet on the air, from keypad tones to aircraft. Keep every flowgraph you build for the midterm; several will help there.