# Converting NM in dB to power in dBm

#### IDRick

##### DTVUSA Member
Playing with the numbers from tvfool, I found that I can convert NM in dB to power in dBm with the following equation:

dBm = -90.99 + (NM in dB x 1.0033)

Does this equation make sense? I'm a Dairy Science guy, not an engineer so go easy... My last physics class was during the Nixon administration and let's just say that I didn't leave a lasting impression with the Physics prof...

Why do I want to do this? Well, one has to be concerned about overloading the tv tuner and/or pre-amp. With my margin to dropout method, I am able to estimate the received signal at the antenna. For example:

Margin to dropout is 45 dB
plus receiver NF ~ 6 dB
plus balun loss 1.5 dB
plus cable loss 2.5 dB

Received signal is 55 dB's. The received signal will be different than the expected estimate (NM from tvfool + antenna gain) using tvfool data because mine reflects what is actually received after subtracting unquantified environmental losses...

What is the power level? Using the above conversion equation and my received signal estimate of 55 dB, power is -35.8 dBm.

Once I have these numbers, I can then use the process I described in this post: http://www.dtvusaforum.com/dtv-hdtv...pre-amplifiers-usage-reviews-3.html#post37171

This is reasonable right? Not to bad for a dairy guy... :becky: And, yes, I am familiar with BS and CS... :behindsofa:

#### EscapeVelocity

##### Moderator, , Webmaster of EV's Antenna Blog
What the !

You lost me.

Edit:

Oh, OK....took me a minute.

Thumbs up! Im all out of Thanks buttons.

Last edited:

#### IDRick

##### DTVUSA Member
Sorry, EV, that was sort of a brainstorm post... I need to hire Don M to write my posts...

Bottom line, if I know margin to dropout, I can then estimate the actual amount of signal at the antenna and estimate power. I need power estimates to calculate max input to the pre-amp. I can do all this with a \$20 converter box, a set of pad attenuators for \$12 and a computer spreadsheet. Cool!

Best,

Rick

#### ProjectSHO89

##### Contributor
Rick,

See Official TV Fool forum - Page 14 - AVS Forum post # 392 and following for the correct explanation of the TVF NM figure.

Roughly, NM+Pwr+15.2=(approx) thermal noise floor of around -106 dBm.
Edit: Approx CORRECTED FORMULA: |-106 + 15.2 - Pwr| = NM
Where:
-106 = Noise Floor in dBm
15.2 = ATSC specified S/N ratio for processing
Pwr = calculated power level (a negative value) in dBm

The Pwr value (in dBm) can be converted to a dBmV (as would be read by a SLM) value by adding 48.8 to the Pwr value.

Last edited:

#### IDRick

##### DTVUSA Member
Thanks projectsho! I greatly appreciate it! I figured it was a constant, just didn't know the origin.

Best,

Rick

Last edited: