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Old 13-Sep-2020, 6:51 PM   #51
Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,677
Using an Attenuator to Measure the Strength of a Digital TV Signal

Here is an example of using an attenuator to measure the strength of a digital TV signal. I picked RF channel 20 to measure.

This is my signal report in dBm:

This was my setup:


                                  TV for SNR
                 0 to 10 dB          /
Ant > 25ft RG6 > Attenuator > A/B Switch 
                  at 0 dB            \
                                  Meter or SDR
The Sadelco DisplayMax 800 signal level meter reading was -7.3 dBmV = -56.1 dBm

I added attenuation, for a total of 30 dB, to bring the signal down to 15 dB SNR at the "Digital Cliff."


                                                TV for SNR
                 0 to 10 dB      Fixed             /
Ant > 25ft RG6 > Attenuator > Attenuators > A/B Switch 
                  at 7 dB        20+3 dB           \
                                                Meter or SDR
This is what the scan looked like at dropout. Note that I had to increase the SDR gain for the proper display:

So, to calculate the signal strength, the basic rule says to add the amount of attenuation needed for dropout to -85 dBm, which is the average dropout point of a tuner:
-85 dBm + 30 dB = -55 dBm

The signal level meter reading for the signal was -56.1 dBm; close enough for a good estimate to determine the signal strength.

The 30 dB of attenuation needed to bring the signal down to 15 dB SNR is sometimes called Margin to Dropout.

Channel 20 is listed at -26 dBm on my report. The trees are causing a lot of signal loss.

Note that I really only needed the TV and the attenuator to determine the signal strength. I added the SDR and the signal level meter to verify the method.
If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 3-Dec-2020 at 8:11 PM.
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