☰ Rogue Valley Amateur Radio Club

April 1, 2021 Club Meeting

Thursday, April 1, 2021 at 7:00 PM Via Zoom. Login sent via email or upon request.

Program: FT8, Pat Cunningham, KD7MPA, and John Laybourne, AC7S

Pat and John will discuss the FT8 mode that has become very popular due to the ability to work with quite low levels of power and small antennas.

Biography: Pat Cuningham KD7MPA is club president, and John Laybourne AC7S is previous club president.

A related new mode, FT4 is becoming popular for contesting and dxpeditions as it permits approximately 2.5x faster cycle time than FT8 (but not quite as good sensitivity).

Announced in the April 2021 issue of The Repeater.

Meeting Notes

From the May 2021 issue of The Repeater.

2021-04-01 RVARC Meeting Minutes

Meeting was on Zoom at 7pm, with about 14 attendees.

7:06 pm Mike, WB6FFC and Allan, K7GT discussed towers.

7:10 pm 7QP will be held on May 1 from 6am to Midnight, with distributed operations allowed due to COVID-19.

Allan said he has a 5-element high-gain tri-band antenna to give away for free to anyone who will use it, rather than store it, and who has a tower on which to mount it.

Treasurer’s report was that no changes to accounts occurred since last meeting.

7:16 pm Pat, KD7MPA, gave this month’s presentation on FT8, and demonstrated its operation in WSJT-X:

Some excerpts from the slides are given below, along with links to several useful FT8/WSJT documents:

What is FT8?

  • Soundcard digital mode
  • Capable of working weak signals
  • Standardized exchange
  • Time-synchronized transmission

Released June 29, 2017 by Steven Franke (K9AN University of Illinois - UC) and Joe Taylor (K1JT Princeton). 8-frequency shift keying format. Protocol changed a little in December 2018

Fundamental tenets of communication Theory imply trade-offs involving:

  • message length
  • signaling rate
  • bandwidth
  • error-control coding
  • modulation type
  • decoding complexity
  • minimum required signal-to-noise ratio (SNR)

FT8 Technical

  • T/R sequence length: 15 s
  • Message length: 77 bits + 14-bit CRC + 83 bits FEC = 174 bits
  • Each symbol carries three bits (8 = 2^3), total number of symbols is 174/3 + 21 = 79
  • FEC code: (174,87) LDPC
  • Modulation: 8-FSK, keying rate = tone spacing = 6.25 Hz (6.25 x 8 = 50 Hz bandwidth)
  • Synchronization: three 7x7 Costas arrays (start, middle, end of transmission) = 21 symbols
  • Transmission duration: 79*1920/12000 = 12.64 s
  • Decoding threshold: -20 dB (perhaps -24 dB with a priori decoding, TBD)
  • Multi-decoder: finds and decodes all FT8 signals in passband
  • Auto-sequencing after manual start of QSO

Exchange typically consists of

  • Call signs
  • Four-character Maidenhead locators
  • Signal reports
  • Acknowledgments

Because of multi-decoding we need callsigns (or hash) in all messages

Setup Equipment

  • Radio w/ SSB
  • CAT Control is a plus
  • Sound Interface (Ex: Signal Link)
  • Computer

Setup Software

  • Install WSJTX
  • Enter your callsign, grid square, sound card, radio interface *
  • Select frequency / band
  • Adjust levels
  • Ensure Clock Accuracy (NTP/time.is)

Can use https://time.is to check your computer clock setting.

Setup Tips

  • Adjust AF output level - transmit (barely move ALC)
  • Enable logging & reporting to pskreporter.info
  • Turn on auto-sequence

Can use PSKReporter at pskreporter.info to see locations of other stations, as well as predicted skip-zone.

References:

8:50 pm General discussion among attendees including Scott, AJ7S about the Hunter Communications fiber rollout, and Lyle, WB7VUF about Sat Comms.

9:30 pm Meeting Adjourned

John KF7UMK, Secretary

Tags: meeting

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