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Author Topic: How can I understand signal strength (WSJT-X)?  (Read 51 times)
KE7KXS
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Posts: 21




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« on: Today at 11:56:41 AM »

Hi,

I use WSJT-X in FT8 mode. I'm trying to understand the top left pane.

On a line it shows the (presumably) signal strength in dB.

What is the difference between a negative number and positive?

Is -28dB stronger than say -10dB? or is it in reverse?

Thank you, Caesar.
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K0UA
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Posts: 1348




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« Reply #1 on: Today at 12:19:17 PM »

The numbers shown are signal strength relative to the noise level in a 2.5khz bandwidth. So to answer you question, the more negative a signal strength is the weaker it is.  So lets say you receive a report of -15, your signal is 15dB below the noise level. If you receive a report of +15 your signal is 15 dB Above the noise level.  
A report of 0 would be at the noise level.   The difference between a -15 signal and a +15 signal is 30 dB.  WSJT-x mode FT8 typically has decodes from -24 to about +20.  it may go higher, but I have never seen one higher than +20, and I have never seen a good decode lower than -24.   The mode JT9 will go a bit lower, at least 2dB lower and still give decodes.  A -28 signal would be 18 dB weaker than a -10 signal
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K0UA
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Posts: 1348




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« Reply #2 on: Today at 12:28:09 PM »

Keep in mind these reports are relative to the conditions on the far end.  If for instance a station has a high noise level on his end, you signal will look a lot weaker to him than if he didn't have a high noise level.  An awful lot of Europe suffers from rising noise levels just as we Americans do in urban environments.  A station in a quiet rural environment will easily give out better signal reports than he may receive back when his QSO partner is in an urban environment. Also keep in mind others may be running a lot more power than you are, so you may hear them much better than they hear you.  A good way to get a feel for you station and it antennas is to WSPR.  WSPR mode is included in WSJT-x and you pick a power level you are comfortable with, usually 5 watts or less.  I often use 100 milliwatts, and let it run on the band of interest for at least 2 or more hours.  Then go to the WSPR website and see how you did, and how your antenna patterns seem to run. You can learn much from "whispering".  Much about propagation, and much about your own system, when you do this many times, and over a period of time.

http://wsprnet.org/drupal/wsprnet/map
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