Better than asking in r/gmrs.
Radio direction finding - commonly called "fox hunting" - is the practice of locating the source of a radio transmission using directional antennas and signal strength measurements. In the amateur radio world, it's a competitive sport. On GMRS, the same techniques can help identify the source of intentional interference or rule violations when you need evidence for an FCC complaint.
Important: This guide is for educational purposes. If you're dealing with interference on GMRS, document what you find and report it to the FCC. Do not confront, approach, or make contact with the person you've located. The FCC has professional direction-finding equipment and legal authority to act - you don't.
The basic principle is simple: a directional antenna receives a stronger signal when pointed toward the transmitter and a weaker signal when pointed away. Take signal strength readings from several locations, plot the bearings on a map, and the lines intersect at the transmitter's approximate location.
UHF signals (462 MHz, where GMRS operates) are well-suited for direction finding because they travel in straight lines and don't bounce off the ionosphere like HF signals. What you hear is coming from a direct path between you and the transmitter.
A handheld Yagi antenna is the most common tool for UHF direction finding. A Yagi has a boom with multiple elements - you point the boom at the signal source and watch your radio's signal strength indicator peak. A 3-5 element UHF Yagi is compact enough to hold in one hand and directional enough to get useful bearings. Look for one rated for 440-470 MHz.
The antenna connects to your handheld radio's antenna port (usually SMA). You remove the stock rubber duck and connect the Yagi with a short coax jumper.
As you get closer to the transmitter, the signal gets so strong that your radio's S-meter or signal bars peg at maximum regardless of which direction you point the antenna. An attenuator is an inline device that reduces the signal strength by a fixed amount (measured in dB), letting you continue to get directional readings even when you're close. Step attenuators let you dial in different levels of reduction as you home in.
Some operators build simple attenuators from resistors for a few dollars. Commercial step attenuators with BNC or SMA connectors are also available.
Any GMRS handheld that shows signal strength works. The more granular the display, the better - a numeric RSSI readout is more precise than 5 bars. Some radios accessible via CHIRP have a hidden RSSI screen. An SDR (software-defined radio) dongle connected to a laptop gives you the most precise signal strength measurement, plus you can see the signal visually on a waterfall display.
This is the core technique. Drive to a location where you can hear the interfering signal. Point your Yagi antenna slowly in a full circle and note the compass bearing where the signal peaks. Mark that bearing on a map as a line extending from your location. Then drive to a second location at least a mile away and repeat. Where the two lines cross is the approximate area of the transmitter. A third bearing from a third location narrows it further.
The farther apart your measurement points are, the more accurate the intersection. Bearings taken from locations that are close together produce a large uncertainty zone.
At UHF frequencies, your body absorbs and blocks a significant amount of signal. You can use this to your advantage without any special antenna: hold your handheld radio at chest level and slowly rotate in place. The signal will be strongest when your body is not between the radio and the transmitter, and weakest when your body blocks the path. This is surprisingly effective for getting a rough bearing, especially in open areas.
Mount a directional antenna on your vehicle (or have a passenger hold the Yagi out the window) and drive toward the signal. Periodically stop, take a bearing, and adjust your route. As you get closer, add attenuation so the signal doesn't overwhelm your receiver. When the signal is strong from all directions even with full attenuation, you're very close.
Higher ground gives you cleaner bearings because there are fewer reflections and obstructions between you and the transmitter. A hilltop, parking garage rooftop, or elevated park makes a much better measurement point than a low-lying neighborhood street. UHF is line-of-sight, so anything that gives you a clear view of the horizon improves accuracy.
Listen on the input. This is the most common mistake in repeater-related direction finding. If someone is jamming a repeater, you need to listen on the repeater's input frequency to hear their direct signal. Listening on the output only tells you where the repeater is, which you probably already know.
If manual yagi-and-attenuator direction finding isn't cutting it - maybe the interferer is mobile, transmits in short bursts, or you're dealing with a recurring problem on a busy repeater - there's a step up: the KrakenSDR.
The KrakenSDR is a 5-channel phase-coherent software-defined radio designed specifically for direction of arrival (DoA) estimation. Unlike a single-channel SDR where you manually sweep an antenna, the KrakenSDR receives on all five channels simultaneously and uses the phase differences between them to compute a bearing to the signal source automatically.
The technique is called correlative interferometry. Five antennas are arranged in a Uniform Circular Array (UCA) - evenly spaced around a circle at a specific radius. When a radio signal arrives at the array, it reaches each antenna at a slightly different time, creating measurable phase differences between the five received signals. The software analyzes these phase differences to calculate the direction the signal came from.
The key is phase coherence - all five receiver channels (5x R820T2 tuners with RTL2832U ADCs) share a single clock source so their samples are synchronized. On startup, an internal wideband noise source fires through all five channels simultaneously, and the software correlates each channel against a master channel to calibrate out any residual timing and phase offsets. This calibration runs automatically.
| Receive channels | 5 (simultaneous, phase-coherent) |
| Tuner / ADC | 5x R820T2 / 5x RTL2832U, 8-bit |
| Frequency range | 24 MHz - 1,766 MHz (GMRS at 462 MHz is well within range) |
| Instantaneous bandwidth | 2.56 MHz |
| Clock stability | 1 PPM (shared across all channels) |
| Calibration | Internal wideband noise source with automatic channel correlation |
| Size / weight | 177 x 112 x 26mm / 670g |
| Power | 5V @ 2.2A (11W typical), USB-C |
| Host computer | Raspberry Pi 4/5 or Android phone |
The five antennas mount on a ground plane in a circular pattern. The radius of the circle determines the usable frequency range - a wider circle works for lower frequencies:
| Array radius | Frequency range | GMRS coverage |
|---|---|---|
| 100mm | 510 - 1,275 MHz | Yes |
| 150mm | 340 - 850 MHz | Yes |
| 200mm | 255 - 637 MHz | Yes |
| 250mm | 204 - 510 MHz | Yes |
For GMRS (462 MHz), any of these radii work. The 150mm or 200mm radius is a good balance between array size and performance at GMRS frequencies. The antenna kit includes five identical whip antennas with matched-length coax cables - each antenna connects to a specific numbered channel on the KrakenSDR.
Mount the antenna array on your vehicle roof, connect the KrakenSDR to a Raspberry Pi or Android phone, and drive. The software displays a real-time bearing to the signal on a map. As you accumulate readings from different locations, the system builds a heatmap that progressively narrows the transmitter's location. Hundreds of readings taken while driving produce a much more accurate result than a handful of manual yagi bearings.
This is especially effective against intermittent transmitters. With a manual yagi, you need the signal to be transmitting while you're pointing the antenna. The KrakenSDR continuously monitors and captures bearings the instant a signal appears, even for sub-second transmissions.
Cost reality: The KrakenSDR unit runs around $750, the 5-antenna kit is around $250, and you'll need a Raspberry Pi or Android phone as the host. There's a significant learning curve for setup, calibration, and interpreting multipath reflections in the results. This is overkill for a one-time interference hunt where a yagi and attenuator would do fine. But if you run a busy repeater and deal with recurring problems, or want to build a permanent monitoring station, it's the right tool.
KrakenSDR is the current commercial product from the team that originally created the open-source KerberosSDR. Full documentation, antenna templates, and the Android app are available at krakenrf.com.
Direction finding can narrow the source of interference to a neighborhood or even a specific block. That information is valuable evidence for an FCC complaint. When documenting your findings:
The FCC's Enforcement Bureau has professional-grade direction finding equipment that can pinpoint a transmitter much more precisely than consumer equipment. Your documentation helps them prioritize the case and gives them a starting area to focus their resources.
Outside of interference situations, radio direction finding is a legitimate and fun activity. Amateur radio clubs organize competitive fox hunts where a hidden transmitter is placed somewhere and teams race to find it using direction-finding techniques. The same skills and equipment apply to GMRS. Some GMRS clubs organize their own fox hunts as social events and skill-building exercises. It's a practical way to learn the techniques in a low-stakes environment so you're prepared if you ever need them for real.
| Equipment | Use in Fox Hunting |
|---|---|
| TWAYRDIO 7-Element UHF Yagi (400-470MHz) | Directional antenna for taking bearings. Point and sweep to find the signal peak. 11dBi gain with a tight beam pattern on GMRS frequencies. |
| BECEN 60dB SMA Step Attenuator | Reduces signal strength as you get closer so your S-meter stays useful. Dial in 10-60dB of attenuation in steps. Essential for the final approach. |
| Yaesu FT-60R Dual Band HT | A favorite among fox hunters for its real analog S-meter that responds smoothly to signal changes. Receives GMRS frequencies (ham radio - not Part 95 certified for GMRS transmit, but you only need to receive for direction finding). |
| Wouxun KG-Q10H Quad Band HT | Alternative HT with numeric signal strength readout and quad-band coverage. Good receiver sensitivity and GMRS-ready out of the box. |
| Nooelec RTL-SDR v5 | Software-defined radio dongle for laptop-based direction finding. Gives you a visual waterfall display, precise RSSI readings, and the ability to record transmissions for later analysis. |
| KrakenSDR + 5-Antenna Kit | Coherent 5-channel SDR for automated direction-of-arrival calculation. Drive-and-map automatic triangulation without manual antenna sweeping. Significant cost (~$1,000 total) and learning curve - the right tool for serious, recurring interference work. |