Thread: Noise Threshold
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Old 24-Jan-2016, 2:06 PM   #8
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,266
Here is a Noise Margin diagram for an indoor antenna with and without a preamp with moderately strong signals. Note that the preamp not only compensates for distribution losses but also allows reception of weaker signals than without a preamp. In this case the improvement is 5 dB, from a +13 dB NM signal to a +8 dB NM signal. The components of the improvement are preamp elimination of the 3 dB coax loss, 3 dB NF improvement of the preamp NF over the tuner NF, minus the 1 dB coax loss between the antenna and the preamp input.

3 + 3 -1 = 5 dB improvement in signal sensitivity from +13 NM to + 8 NM

If the tuner NF is more than 6 dB, or the preamp NF is less than 3 dB, or the loss between the antenna and the preamp input is less than 1 dB, then the improvement will be more than 5 dB.



Here is a Noise Margin diagram for an indoor antenna with a preamp and two weak signals. Note that neither signal is below the thermal noise floor. The antenna gain makes the Noise Margin more positive and improves the SNR.

Attached Images
File Type: png NMdiagIndoorAmp3a.PNG (59.8 KB, 1662 views)
File Type: png NMdiagIndoorAmp4.PNG (71.1 KB, 556 views)
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Last edited by rabbit73; 11-Sep-2017 at 10:10 PM.
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