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Old 11-Sep-2020, 2:21 AM   #39
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,747
Measuring Signal Margin to Dropout

When digital TV arrived, I was looking for an inexpensive way to measure the strength of digital TV signals. Signal level meters are made for the task, but they cost more than most people want to spend.

I found this UK site that showed how to use inexpensive attenuators to make comparative signal strength measurements of digital TV signals using the concept of margin to dropout. It's the same link as in my signature:
http://www.me galithia.com/elect/aerialsite/dttpoorman.html

I found two Channel Master slide-switch attenuators, each with a 3dB, 6dB, and 12dB switch. With the proper settings I was able to get 0, 3, 6, 9, 12, 15, 18, or 21 dB for each attenuator; the total for both was 42dB.



The next device I found was the Apex DT502 converter box. It has two scales, one for signal quality and the other for signal strength.



Using my attenuators I was able to get readings for a channel as I brought it down to dropout. Obviously, the greater amount of attenuation needed to bring the channel to dropout, the stronger the signal. Since dropout is at about -85 dBm for most tuners, I was able to calculate the strength of the signal by adding the attenuation to -85 dBm.





Later, if I wanted to compare my calculated strength with a measured strength, I could use my Sadelco 719E signal level meter. Once I knew the strength in dBmV, I could convert it to dBm by subtracting 48.8:



and I was also able to calibrate the DT502 signal strength scale:



You can do the same thing with just your TV tuner. Bring the signal to dropout with an attenuator and add the attenuation needed to -85 dBm; that is the strength of the signal. This is most accurate on UHF. On VHF, the noise level is higher, so dropout will be above -85 dBm.
__________________
If you can not measure it, you can not improve it.
Lord Kelvin, 1883
http://www.megalithia.com/elect/aeri...ttpoorman.html

Last edited by rabbit73; 12-Sep-2020 at 1:50 PM.
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