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5712Re: [wsjtgroup] Re: FSK441 vs JT6M -- when to use

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  • Bill W5WVO
    Feb 1 6:02 PM
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      Very good point! This is really the same situation as having what you consider "low" or "no" noise and an extremely weak signal is decodable below the noise floor. In your case, you have a very high noise floor and what would normally be a strong signal, yet still below your noise floor. It's all just signal-to-noise ratio, a relative relationship. And as we discussed earlier, JT6M has a better signal-to-noise ratio than FSK441.
       
      Good luck with the power company. I have a very strong noise that comes on every evening at approximately (but not exactly) the same time, and goes away after anywhere from five minutes to an hour or more. Receiver noise blankers don't even touch it. I have NO idea what it is. I need to put together a 2-el handheld 6m yagi (maybe with capacity hats on the end of the elements to reduce size) and go hunting. My large yagi says it is almost due west of me, so I have a general idea.
       
      Bill W5WVO
       
      ----- Original Message -----
      From: k4ymq
      Sent: Sunday, February 01, 2009 5:59 PM
      Subject: [wsjtgroup] Re: FSK441 vs JT6M -- when to use

      There is one other situtation where JT6M far outshines 441. that is
      when a high noise floor is present. I have found that I can decode a
      ping through S5 to 6 noise level with JT6M where 441 decodes zip.
      But it is not a magic pill for noise, as it won't work through the S8
      to 9 that I have so often here. The power company and I are both
      chasing noise sources and I am getting better co operation. I am also
      making a big effort to eliminate sources of noise in my home. Found
      some suprises so far.. my washer and dryer generates noise even when
      off.
      Ira
      K4YMQ

      --- In wsjtgroup@yahoogrou ps.com, "Bill W5WVO" <w5wvo@...> wrote:
      >
      > I hope nobody is upset with my posting these little tutorials for
      HSMS/WSJT
      > newbies. If anyone thinks this is an inappropriate use of this
      list, please
      > email the list owner and let him know. On the other hand, if you
      just don't
      > find it useful or interesting, please simply delete. Thanks.
      >
      > ------------ --------- --
      >
      > Many, many WSJT newbies have asked me what the difference is
      between FSK441
      > and JT6M, and when each mode should be used in preference to the
      other.
      >
      > First, some history and a little technical explanation.
      >
      > FSK441 was developed as a mode expressly optimized for meteor-
      scatter
      > propagation. It is capable of decoding an entire message in
      considerably
      > less than 1 second. FSK441, like all WSJT modes, is AFSK, or audio
      > frequency-shift keying. Your sound card creates four audio tones in
      various
      > sequences to encode the alphabet, numerals, and a few special
      characters.
      >
      > FSK441 is not a "smart" protocol, meaning that it does not analyze
      data
      > patterns to determine message contents (as does JT65), nor does it
      create
      > "most likely" message contents by averaging multiple decodes of the
      message
      > string (as do both JT65 and JT6M). What FSK441 decodes, character by
      > character, is what you see in the decode window. It is a very
      MANUAL mode!
      > And to get the best decoding performance out of it, more operator
      smarts are
      > arguably required than for any of the other WSJT modes. (Advanced
      use of the
      > FSK441 tools in WSJT is beyond the scope of this short article, but
      such a
      > paper should be written.)
      >
      > JT6M came along later. It is not a "dumb" protocol like FSK441
      because it is
      > capable of making some intelligent guesses about the content of the
      message
      > being decoded. For this reason, and because it is a slower protocol
      and
      > therefore requires less bandwidth than FSK441, JT6M is a more
      sensitive
      > mode. On average, you can decode a more-or-less steady strength
      JT6M signal
      > at better than 10 dB below the noise floor. In comparison, FSK441
      requires
      > signals at least 1 or 2 dB above the noise floor.
      >
      > For this reason, a lot of hams jump to the conclusion that JT6M
      must be
      > better for long-distance meteor-scatter contacts because it is "more
      > sensitive". This conclusion is incorrect, and we'll discuss the
      reasons
      > here.
      >
      > As anyone knows who has had any experience with meteor-scatter work,
      > meteor-scatter radio propagation events (variously called pings,
      burns, or
      > bursts) come in all strengths and sizes, from the weak 1-dB, 100-
      millisecond
      > (ms) ping up to the 20-second long, speaker-rattling "blue whizzer"
      caused
      > by a bigger than usual meteor hitting the atmosphere in exactly the
      right
      > place. Now, here's the technical part:
      >
      > The amount of ionization created by a meteor burning up (called the
      plasma
      > density) is determined by a number of variables, including the
      meteor's
      > composition, its mass, its speed, and its angle of entry. When a
      very high
      > plasma density is achieved momentarily, the effective Maximum Usable
      > Frequency (MUF) for that meteor trail is quite high, sometimes as
      high as
      > 432 MHz or more. However, since this extremely high plasma density
      occurs
      > only in a small portion of the meteor's ion trail and begins to
      weaken
      > quickly, pings in the high VHF and UHF ranges are infrequent and
      quite
      > narrow, usually less than 1 second duration. The higher in
      frequency you go,
      > the more narrow and the more infrequent meteor pings are.
      >
      > FSK441 was designed to decode meteor pings as brief as 150 ms
      duration.
      > Therefore, under normal circumstances on 2 meters and above, FSK441
      is
      > really the only mode that will reliably provide meteor-scatter
      > communication.
      >
      > On 6 meters, however, propagation can be sustained with a lower
      plasma
      > density, and meteor pings tend, on average, to be longer in
      duration, often
      > lasting more than a second or two. JT6M requires a burn of at least
      a
      > second, and more reliably two seconds, in order to encode/decode a
      > worst-case length message (both calls plus reports). These longer
      pings
      > allow the more sensitive JT6M mode to be usable for some meteor-
      scatter work
      > on 6 meters.
      >
      > Now, there is a catch. Here is the nexus of the problem:
      >
      > Very weak pings that might seem to make the higher JT6M sensitivity
      > advantageous also tend to be very short. Therefore, even if a 200-
      ms ping
      > several dB below the noise floor could theoretically be decoded by
      JT6M (and
      > could not be decoded by FSK441), the ping is oftentimes too short
      to contain
      > a reliably decodable JT6M message. So your extra sensitivity is of
      little
      > use! Pings that are long enough to contain full JT6M messages are
      also
      > typically more than strong enough to be decodable by FSK441. And
      FSK441 will
      > also decode comparatively weaker messages only a few hundreds of
      > milliseconds long.
      >
      > Therefore, in most cases, FSK441 is still a better performer in 6-
      meter
      > meteor-scatter work than JT6M, though plenty of meteor-scatter QSOs
      can be
      > made and are made every day using JT6M. It's just a matter of the
      > percentages.
      >
      > If conditions are strong and the station you are trying to work is
      in the
      > "sweet spot" for meteor-scatter propagation (500-800 miles), JT6M
      will
      > virtually always work just fine. If, however, you are: (1) working
      under
      > poor MS conditions, or (2) you are trying to work a station at the
      edge of
      > MS range, or (3) you are trying to work a station very close to you
      via
      > high-angle, high-MUF meteors, it is better to use FSK441. Why?
      Because all
      > three of these conditions tend to produce pings that are
      comparatively
      > shorter in duration, and FSK441 simply works better for short pings.
      >
      > JT6M, however, has a wonderful redeeming value. It is an
      outstanding mode
      > for very weak signal work when signal strengths are either fairly
      stable or
      > variable over a period of at least several seconds. Such signals
      can come
      > from tropospheric, D-layer ionoscatter, and weak sporadic-E
      propagation.
      > When F2 propagation begins to return to 6 meters in a few more
      years, we may
      > find that JT6M is an outstanding mode for completing 6-meter QSOs
      when
      > propagation is still too weak to support SSB or even CW
      communication.
      >
      > In general, then:
      >
      > (1) On bands above 6 meters, use FSK441 for meteor-scatter QSO
      attempts.
      > JT6M will generally not be useful on 2 meters and above.
      >
      > (2) On 6 meters, use FSK441 as your default mode for meteor-scatter
      QSO
      > attempts. Use JT6M only when meteor burns are fairly long -- at
      least a full
      > second. More often than not, these conditions will also produce
      burns that
      > are fairly strong as well, again on average.
      >
      > (3) Use JT6M for weak-signal QSO attempts using atmospheric
      propagation
      > modes like tropo, ionoscatter, and sporadic-E.
      >
      > These are not laws, merely suggestions based on observation and a
      little
      > science. Exceptions to these generalizations do occur regularly on
      6 meters.
      >
      > Bill W5WVO
      >

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