Signal Strength differences between 13E and 19.2E

Bamboccio

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Hello,

I have a Visiosat multifeed 75cm dish pointing at 13E and 19.2E with a TM 5402 M3 receiver. The signal for TPs from 19.2 is in the region of 14.5dB and when it goes below 13dB (in case of very heavy rain / if I move the skew of the LNB I get a no signal message on the TM. Instead 13E TPs are in the region of 13.5dB and I can still see channels when the signal goes below 10dB. Any ideas of why there is this big difference?

Thanks!
 

ozumo

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The amount of signal required for lock is determined by the transponder's DVB standard, modulation and FEC:
 

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Bamboccio

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Thank you - so in simple terms that difference on when the channels lock is expected.
 

Bamboccio

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Thanks again. I am trying to understand the third column and the values are different for QPSKand 8PSK. Are they signal levels or noise slowed?

thanks!
 

ozumo

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South Durham
The figures are the required signal level above noise required for lock (carrier to noise ratio). So for DVB-S2 8PSK 3/4 you need a minimum of 7.9db more signal than noise in order to lock the transponder.
 

Bamboccio

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Thanks a lot - how does that relates to the signal strength shown by the receiver?
 

ozumo

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South Durham
If you mean dB figures then generally they should be relatable, though not all receivers are the same. With a motorised dish it's easy to see at what point the transponder loses lock and compare the figures on the table to see if the numbers are similar. On a fixed dish you could use skew to reduce the strength.

I've seen a Technisat receiver that shows a dB figure which represents the margin above lock, which may be more useful for general viewing. Very few receivers show dB numbers below lock, and many receivers don't give dB readings at all.
 

Bamboccio

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Thank you. I am seeing very different values on different TPs in HB with some showing 16dB and other 12.6dB.
So I wonder if there is something wrong with my skew.
 
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ozumo

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My Location
South Durham
Variations in signal strength between transponders is not unusual.
 

Paolo71

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It's called LM aka "Link margin" or NM aka "noise margin". It's the minimum level guaranteed to lock a signal, it depends on FEC, modulation and standard. With some combinations of these factors you can lock a signal with just 1 or 2dB, but in other cases it's not enough to lock the signal even with 12...13...14dB ! That's why is important to have a dish not so small (at least 80-90cm for the most satellites), a very good low noise LNB and of course a not so long excellent coaxial cable.
 
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