Frequency Measuring Test
| Frequency Measuring Test | |
|---|---|
| Abbreviation | FMT |
| Sponsor | ARRL |
| Held | Generally twice a year, in April and November |
| Origins | A tradition dating back to 1931 |
| Recent format | |
| Transmitting stations | K5CM (Oklahoma) and W8RKO (Ohio) |
| Bands | 40 and 80 meters |
| Signal | Unmodulated carrier, one minute of key-down per run |
| Results | Posted at fmt.arrl.org when the entry deadline passes |
| Top listing | The "Green Box", for entrants within 1 Hz on every transmission |
The Frequency Measuring Test (FMT) is an ARRL event in which amateurs measure the exact frequency of a carrier sent by a designated station and submit their result, which is then compared with the true value. It is not a contest with points or contacts but a test of measurement skill, and the best entrants come within 1 Hz on frequencies of several megahertz. The FMT is generally held twice a year, in April and November, and the ARRL traces the tradition back to 1931.[1][2]
How it works
Before each test the ARRL publishes a notice listing the transmitting stations, their approximate frequencies and a schedule in UTC. In recent years the transmitters have been K5CM in Oklahoma and W8RKO in Ohio, each sending on the 40-meter and 80-meter bands, near 7064 and 7065 kHz and near 3598 and 3599 kHz.[1][3] The tests take place in the evening in North America.[4]
Each run begins with a call-up of a few minutes, so that listeners can find the signal, followed by one minute of steady, unmodulated carrier, the key-down period.[1] The call-up frequency may differ from the key-down frequency by up to 10 Hz, so only the key-down period should be measured.[3] The stations take turns, one band at a time, with runs starting 15 minutes apart.[1]
Entrants submit their measurements through the FMT website, along with a note on the equipment and method used, within about three days.[3] When the deadline passes, the true frequencies and everyone's results are posted.[1] Entrants whose readings are within 1 Hz on every transmission are listed in the Green Box, the event's honor roll.[1]
Measuring techniques
Special laboratory equipment is not required. A modern HF transceiver can measure frequency quite accurately once its reference oscillator has been calibrated, and successful entrants have used ordinary transceivers together with PC software such as Spectrum Lab or the frequency-measuring mode of WSJT-X.[1][2]
- Calibrate first. Accuracy depends on the receiver's reference. Entrants check it against a standard-frequency station such as WWV or Canada's CHU, or use a stable 10 MHz reference, such as an oven-controlled oscillator calibrated against those stations.[2]
- Measure the audio. With the receiver in a CW or SSB mode, the carrier produces an audio tone. Spectrum-analysis software measures that tone very precisely, and adding it to the dial frequency gives the carrier frequency. WSJT-X has a frequency calibration mode designed for this kind of work, and its author, Joe Taylor, K1JT, has published instructions for using it in the FMT.[3][4]
- Allow for the path. A signal arriving by skywave can be shifted slightly by Doppler effects as the ionosphere moves, and multipath and fading make the reading wander.[5] Averaging over the whole key-down period helps.
An SDR with a stable reference makes the job easier, but some entrants have done well with a standard transceiver and a little care.
History and purpose
In the early decades of amateur radio, accurate frequency measurement was a practical necessity. Transmitters drifted, and a station that strayed outside a band edge was breaking the rules. The ARRL's early frequency measuring activities, dating from 1931, were promoted as insurance against out-of-band operation.[2] Official Observers and Official Relay Stations were required to take part, and W1MK, the league's headquarters station before W1AW, was one of about a dozen transmitting stations.[2]
Today the FMT is mainly a test of precision. The bands and transmitting stations have varied over the years; a November 2016 test, for example, also included a 20-meter transmission from California.[5] Practice tests are run through the year and announced on an online group for FMT enthusiasts.[1] The event has also been publicized by HamSCI, the citizen-science group, since the readings capture small frequency shifts along the propagation path.[5]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 ARRL FMT, ARRL.
- ↑ 2.0 2.1 2.2 2.3 2.4 Frequency Measuring Test Success Does Not Require Sophisticated Equipment, ARRL, April 2019.
- ↑ 3.0 3.1 3.2 3.3 November 2026 Frequency Measuring Test, ARRL.
- ↑ 4.0 4.1 April 2026 Frequency Measuring Test, ARRL.
- ↑ 5.0 5.1 5.2 ARRL Frequency Measurement Test, HamSCI, November 2016.