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Old 12-Dec-2016, 3:25 AM   #10
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,288
Quote:
But for the signal strength seeming to be misleadingly strong, let's take a super-clear example: channel 2 in Sacramento. The model indicates I should have a -2.8 dB NM...but this is for a 3 kW transmitter 76 miles away with the terrain blockages as shown?? Seriously?? Now...that signal has a 5-meter wavelength, but I don't think low-VHF bends that much.
KFTY is 45.6 miles away; do you receive it?

For the same coverage area, VHF-Low requires less power than VHF-High, ignoring the higher noise level on CH 2-6, which the FCC did when it calculated power allowed by a transmitter. The FCC also assumed everyone would have a receiving antenna outside, in the clear, at 30 feet AGL.
Quote:
Is NM the only determinant of "viewability", or is the dBm equally important? Is NM stated in terms of a dipole reference? In that case, stacked Yagis should give you something better than10 dB extra martin, right? (Sorry for the newbie questions, but NM is a new term for me in this context.)
Viewability is determined by the signal strength and signal quality at the input of the tuner. Signal quality is defined by SNR and uncorrected errors.

Most tuners will drop out a signal weaker than -85 dBm at its input.

I recently did a tuner sensitivity comparison between a 32" Sony and a Channel Master 7003 converter box.
http://www.avsforum.com/forum/186-co...erter-box.html

Yes, NM is stated in terms of a dipole reference. NM 0 is at -91 dBm, but if you don't have a preamp, you must have a NM +6 dB, to allow for the 6 dB average NF of a tuner. The report assumes the dipole is outside and in the clear. You should read the FAQs here:
http://www.tvfool.com/index.php?opti...ask=view&id=57

Quote:
Is this a common experience for rural, somewhat hilly areas?
YES
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Last edited by rabbit73; 12-Dec-2016 at 3:36 AM.
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