TV Fool Need help understanding relationship between NM db and Pwr dbm
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 5-Aug-2015, 4:05 PM #1 ZippyTheChicken Member   Join Date: Jun 2014 Posts: 60 Need help understanding relationship between NM db and Pwr dbm My area seems to get pretty weak and unstable signal I am 60 miles out and although the terrain is pretty flat here there are two metropolitan areas between me and the towers I am trying to understand the relationship between NM and Pwr from the reference PWR is the predicted signal power and NM is the predicted Noise Margin My strongest station at this time is showing a NM of 32 and a PWR of -58 the weakest station that I can sometimes pickup is showing NM of 15 and a PWR of -75 Being so far out and in the middle of nowhere and everything at the same time I am surrounded by 4-5 markets and I have a number of CoChannels I would guess that they also reduce or cancel out each other but the stations above are not CoChanneled and they are from one market. From reading the reference and I don't know if this is correct I must have a NM at the TV of over 0 to receive the station and the PWR is the strength of the signal to my home from the tower? so I think the PWR is more where I am getting confused but I am not sure. If I want to pickup a station that has NM of 15 and a PWR of -75 How do I understand what the Antenna and Amp should be theoretically rated at? Figuring I have a 10db rated uhf Antenna connected a RCA 22db uhf /16db vhf preamp then to 50' of coax to a 8 way distribution amp with 3-4db gain out and runs of 30' to each tv. guessing the Distribution Amp should cancel out most wire and splitter loss. from what I understand now between the 10db antenna and 22db of amp a station of NM of 15 means I should be good to go to watch that station But the PWR of -75 I am not sure how I account for that. the -75 seems to suggest that I probably shouldn't be able to get that signal very well if at all Since PWR is so low here with a conventional antenna I guess the most i can expect is about 15db gain at best which would compensate to bring the -75pwr up to -60 ? still not real great I am not sure ... anyway thanks my tvfool because I know you're gonna ask for it. http://www.tvfool.com/?option=com_wr...8e034e42ebcb68
5-Aug-2015, 5:57 PM   #2
Antennas Direct Tech Supp

Join Date: Jan 2010
Posts: 2,942
Simple answer: NM=Rx Power - (-91.0). You can simplify that to be NM=91- absolute value (RX power) for RX Power < 0. You'd have to be less than a mile from a 1 MW station for Rx Power to be calculated to be greater than 0.

Signal power and ATSC:

Doing some math (elsewhere), it can be calculated that the theoretical noise floor for a 6 MHz-wide channel is -106.2 dBm. ATSC specification requires a signal to noise ratio of 15.2 dB. Therefore, in a theoretical circuit, at the input to the decoder portion of the tuner, there must be a signal power > -91.0 dBm. The site's software establishes this as a 0 dB Noise Margin (NM) in that there's no margin for any additional reduction in signal power as that would cause decoding to fail.

See FAQs for expansion of below. http://www.tvfool.com/index.php?opti...d=57&Itemid=78

Quote:
 These values tell you if you are above or below the detection threshold for each station and by how much. Since these values represent the amount of signal "in the air" at your location, you need to have enough margin to account for building penetration, cable loss, splitters, tuner sensitivity, and other factors specific to your setup. If you take the initial NM value for a given channel, add your antenna gain, subtract all the other system losses, and still end up with a value above 0, then you should be able to detect that channel.
Emphasis added to the above quote. One factor not emphasized enough in the above is the tuner's sensitivity known as its 'noise figure'. For a non-amplified system the noise figure of the system would include EVERYTHING from the antenna's terminals to the decoder chip inside the tuner. Balun loss, cabling loss, connector loss, splitter loss, and the input to the tuner (noise figure) must all be summed and then subtracted (by the end user) from the calculated noise margin. As an example, assume a balun loss of 1 dB, 100' of RG6 (about 5.5 dB @ 700 MHz, it's less at lower frequencies), a two-way splitter of 3.5 dB insertion loss per leg, 0.5 dB of connector loss, and a tuner with a noise figure of 6 dB at the channel of interest. That means the system noise figure at the decoder is 16.5 dB and that you need a NM of 16.5 or higher in order for reception to take place. This is without any adjustments for antenna gain (antenna gain improves NF if gain is positive), amplifiers, or real-world signal impairments such as an elevated noise floor due to background noise, multi-path, penetration losses due to trees or building materials, and the like.

For an amplified system, the noise figure of the amplifier dominates the setting of the system noise figure. If you have a preamp with a 2 dB NF, a balun with 1 dB loss, and miscellaneous connector loss of .5 dB before the amp, the noise floor is raised by 3.5 dB at that point, all assuming that the gain of the amp is sufficient to cover the downstream losses by several dB. At the input to the decoder, the noise floor is a bit higher, you'd have to run the cascaded noise figure calculations to get a final estimate.

I've found that 1-edge and 2-edge calculations in the simulator can be wildly inaccurate. My field tests have show results that vary by as much as 20 dB for 2-edge distant signals. For signal locations that "right behind" a significant terrain object like the back side of a hill or mountain, they're next to useless. Therefore, take those types of calculations to be worth less than a grain of salt. LOS signal calculations are usually close enough for government work.

If you Have ANY impairments such as trees or buildings, the calculations cannot include them so any number calculated must be reduced by some "fudge factor" which one can develop after a while.
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Last edited by ADTech; 5-Aug-2015 at 6:49 PM.

5-Aug-2015, 6:31 PM   #3
rabbit73
Retired A/V Tech

Join Date: Aug 2012
Location: S.E. VA
Posts: 2,738
Hi, Zippy:

I see that ADTech beat me to it while I was typing, but I'll give you my answer anyway to supplement what he said. Maybe between the two of us we can turn on the light bulb for you.
Quote:
 I am trying to understand the relationship between NM and Pwr
I'll try to help you with what I've learned. Since you mention the reference, I assume you mean the TV Signal Analysis FAQ so I will not quote it but will try to give you my understanding of it based on my limited knowledge of your situation.
Quote:
 My area seems to get pretty weak and unstable signal
Yeah, I see that from your report, even if it isn't your exact location. Signals that are 1Edge and 2Edge have terrain interference, and should not be expected to be as reliable as LOS signals. All OTA signals constantly vary in strength, 1Edge and 2Edge more so than LOS signals, so you need a "fade margin" for SNR (Signal to Noise Ratio) above the minimum ~15 dB at the "Digital Cliff."

You can compensate for the Edge signal deficiency by using an antenna with a little more gain, but then the antenna will have a narrower beamwidth and must be aimed more carefully. That is the trade-off between gain and beamwidth. The antenna must be aimed at the transmitter for best signal quality to reject multipath reflections.

Quote:
 I am 60 miles out and although the terrain is pretty flat here
Well, seems to be flat, but it isn't. The curvature of the earth starts to affect signals at about 60 or 70 miles. If you click on a callsign in your tvfool report you will see a dark curved shadow at the bottom of the terrain profile. That's the curvature of the earth beginning to interfere with your signal. The transmitter is at the left end and your location is at the right end.
Quote:
 PWR is the predicted signal power....the PWR is the strength of the signal to my home from the tower?.
Correct; the signal power outside in the air at the height you specified without trees or buildings in the signal path. We often call it signal strength, but it is more accurate to call it power because it is stated in terms of decibels as dBm.
Quote:
 NM is the predicted Noise Margin.....I must have a NM at the TV of over 0 to receive the station
Also correct. The predicted NM is based on an antenna with 0 dBd gain (like a dipole). To that you can add the antenna gain and the preamp gain (if you are using one) to make the NM more positive, but you must subtract the NF (Noise Figure) of the preamp because it adds its internal noise that degrades the SNR of the signal.

I made this diagram to help me understand NM when I was learning about it:

And Calaveras made this NM chart:

Interpreting Noise Margin in the TV Fool Report
http://www.aa6g.org/DTV/Reception/tvfool_nm.html

When you are making an estimate of signals, you can make a similar one using signal PWR by adding your antenna gain and preamp gain to the dBm PWR in the report. I often like to do that because tuners drop out a signal around -85 dBm.

Notice that the difference between NM dB and dBm PWR is a constant of 90.9 dB, which is another way of looking at the ~91 dB in ADTech's post.
Attached Images
 NoiseMargin.jpg (60.1 KB, 2962 views) NMChartC.jpg (71.3 KB, 2974 views)
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Last edited by rabbit73; 5-Aug-2015 at 8:30 PM.

5-Aug-2015, 7:38 PM   #4
rabbit73
Retired A/V Tech

Join Date: Aug 2012
Location: S.E. VA
Posts: 2,738
You now have a feeling for the upper and lower limits for NM; here are the limits for dBm PWR:

ATSC Recommended Practice:

Document A/74:2010, 7 April 2010

5.1 Sensitivity

Quote:
 A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from –83 dBm to –5 dBm for both the VHF and UHF bands.

Quote:
 The DTV receiver should accommodate more than one undesired, high-level, NSTC or DTV signal at its input, received from transmission facilities that are in close proximity to one another. For purposes of this guideline, it should be assumed that multiple signals, each approaching –8 dBm, will exist at the input of the receiver.
Quote:
 the weakest station that I can sometimes pickup is showing NM of 15 and a PWR of -75.....If I want to pickup a station that has NM of 15 and a PWR of -75.....But the PWR of -75 I am not sure how I account for that.....the -75 seems to suggest that I probably shouldn't be able to get that signal very well if at all....which would compensate to bring the -75pwr up to -60.....still not real great
Under ideal conditions you should be able to get that signal (WMAR?) if your antenna was outside, aimed directly at the transmitter, with nothing in the signal path, with no multipath reflections, no co-channel interference, no adjacent channel interference, no electrical interference, no spurious signals created in the tuner or preamp from partial overload from stronger signals, and the tvfool report is accurate. A NM of 15 dB is 15 dB more than the minimum required for reception. And a signal power of -75 dBm is about 10 dB more than needed by most tuners.

But your signal doesn't have a PWR of -75 dBm if the antenna is still in the attic.

All of the impediments I listed can reduce the signal quality (as defined by SNR and errors), which increases the digital errors in the signal. The FEC (Forward Error Correction) has a limited ability to correct errors. Once that limit is reached, the signal reaches the "Digital Cliff" where you have pixilation, picture freeze, and finally dropout.

If you are going to continue to do antenna experiments, you really need a way to measure signal strength and signal quality. My first way to do that was with an Apex DT502 converter box that has dual signal bars, one for signal quality and one for signal strength. Cost: a coupon and \$10.

I now use the Diagnostics Screen of my Sony KDL22L5000. Calibration chart in attachment 5.

Bad signal, picture freeze, SNR less than 15 dB and errors:

Good signal:.

My latest Sony, a KDL32R400A, has a much wider range signal strength scale, which is very useful for antenna experiments. See attachment 6.

Some people use a USB tuner, but the software isn't user friendly for me:

Attached Images
 DualSigBars.jpg (43.9 KB, 4969 views) SonySigBad_1.jpg (159.7 KB, 4781 views) SonySigGood_1.jpg (120.5 KB, 6418 views) HauppSigMon.JPG (28.2 KB, 2773 views) SONYSSVSdBmVchart2.jpg (150.2 KB, 608 views) SSCHART KDL32R400A.JPG (118.4 KB, 742 views)
__________________
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; 6-Aug-2015 at 12:40 AM.

 5-Aug-2015, 10:31 PM #5 ZippyTheChicken Member   Join Date: Jun 2014 Posts: 60 thank you very much ADTech and Rabbit73 you both gave me very in depth answers that are very helpful in understanding signal power and noise. I will think about this for a while before I ask another question thanks again
 5-Aug-2015, 10:33 PM #6 ADTech Antennas Direct Tech Supp   Join Date: Jan 2010 Posts: 2,942 Feel free to ask whatever interests you whenever you wish. __________________ Antennas Direct Tech Support For support and recommendations regarding our products, please contact us directly at https://www.antennasdirect.com/customer-service.html Sorry, I'm not a mod and cannot assist with your site registration.

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