Originally Posted by Falcon_77
After I wrote the above message, I saw that it says "relative field values," so presumably that would mean relative net dBu strength, right?
The way I understand it (FWIW) :
DBu(dBµV/m) (electric field intensity) is decibels above one microvolt/meter (dBµV/m). It's something that changes as you move farther away from transmitter site and involves things such as transmitter output power/antenna gain/antenna pattern/etc.
I think of The relative field values as "describing" the antenna's pattern, with values allways between 0 and 1. A value of "1" would equate to the antenna's maximum gain (main lobe) in Dbi or DbD.
In other words, the relative field values shown on FCC site "describe" the transmitting antenna's directional pattern while "limiting" the values to values between 0 and 1 rather than expressing the values for the polar plot in dbi or dbD ...
In additon to Tower guy's post involving using (20 * log V) to convert the relative field values to Db units(decibel units are logarhythmic), you might want to read the text under "figure 2" here :http://www.softwright.com/faq/engine...Y%20UNITS.html
BTW, FCC's OET bullitin 69 (think I provided link to it earlier in thread) also says this about the relative field values :
"In cases where the TV Engineering Data Base indicates that a directional antenna is employed, the ERP in each specific direction was determined through linear interpolation of the relative field values describing the directional pattern. (The directional pattern stored in the FCC Directional Antenna Data Base provides relative field values at 10 degree intervals and may include additional values in special directions. The result of linear interpolation of these relative field values is squared and multiplied by the overall maximum ERP listed for the station in the TV Engineering Data Base to find the ERP in a specific direction.)"
Also, FCC provides some handy calculators (including Relative field values to ERP so you don't have to do the "manual" calculation I provided earlier) here :http://www.fcc.gov/mb/audio/bickel/findvalues.html
Or rather, perhaps it is better to state that digital signals can be received at weaker dBm levels than analog.
Genearlly, I think, Yes, at least if you are talking about relatively "snow-free" NTSC signals. There are several "reasons" why ... One is that DTV power is measured as "average" power, NTSC is expresssed as "peak" power. Another as I understand it is threshold for DTV reception with our receivers can be achieved with signifcantly less signal than it takes to acheive a "non-snowy" analog picture. I can't recall what that "exact" difference is don't hold me to this, but thought I'd read somewhere it was about a 12db difference.
Although, my experience is you can receive/detect or "see" a extremely weak ("snowy") analog signal "above the snow" with signal levels well below those as defined by for the grade B "service contour". I expect probably even below what is needed for threshold DTV reception, at least based on my "rough" and non-verifyable observations that seems to be the case ...
Anyhow, Among other things "related" to your post+inquires, You'll find the values for the "service contours" FCC uses (in Dbu) listed for DTV/analog here.http://www.fcc.gov/mb/audio/bickel/curves.html
You might also find info in this article useful :http://www.tvtechnology.com/features...Coverage.shtml
Converting the FCC's service grade specs to dBm yields about -65dBm for analog and -85dBm for digital for a service area.
That's pretty close to what I get for UHF Using the ".8 Convert dBm to Filed strength in dB/u and mv/meter" option in the RF calculator/excel spreadsheet available here :http://www.rfsolutions.com/rfcalc1.htm
Using a mid-UHF band freqency of about 600MHZ I get :
-89.5dbm = 41dbu (FCC defined service countour for UHF DTV)
-66.5dbm = 64dbu (FCC defined grade B service contour for UHF analog full service stations)
*Do* keep in mind that the FCC service contour "specs" take into account "FCC planning factors" regarding such things as receive antenna gain, feedline loss and receiver noise figure, presumably with the typical equipment a consumer would be utilizing. There is info in FCC OET bulletin #69 on this as well, as discussed earlier in this thread), whereas TVfool predictions isn't "accounting for" things such as antenna gain (which I personally think is a *good* thing) ....
For DTV, taking into account these "planning factors" seems fairly straightfoward .... For instance on UHF, assuming my conversion from 41dbu noise limtied service contour = -89.5dbm is correct, I believe 41dbu "works out" to being fairly close to equaling threshold level for DTV reception if we "factor in" the FCC planning factors in a fairly conservative way along with typical NF of DTV receiver (approxmiately). As we need:
-106.2dbm (thermal Noise floor) + 15db S/N(planning factor for required carrier/noise ratio) + 7db(planning factor for estimate of receiver's noise figure) = - 84.2dbm -- w/o use of a preamp with a lower noise figure, As a "Conservative" Minimum signal level at receiver input required for DTV reception ...
So, for UHF 41dbu/-89.5dbm we "add" 10db of antenna gain and we actually now have -79.5dbm at antenna terminals, but now, we have to subtract the FCC "planning" factor for loss in the Feedline(coax), which is -4db on UHF , then we are "at" -83.5dbm at the receiver input, only +.7db "more" than our required minimal signal level at receiver input ....
I'm not sure how such a "equation" works out for analog, as although I believe the FCC planning factors for antenna gain/feedline losses are the same, If I recall correctly I think they use a "assumed" 10db noise figure for the receiver, and most importantly, I don't know what signal level at input of receiver "exactly" equates to what I'll call a "snow free" analog picture ....
I am trying to ascertain the probability of reception at a site after 2009 and TV Fool does not yet have 2009 data in the output. Of course, the FCC doesn't have much of this data itself yet, so it is more conjecture at this point (at least we have the max ERP's).
I think in many/most cases The 2009 "data" will be the same as most stations current licensed DTV operations -- exceptions are stations that will be "moving" to a different channel(often their current analog channel). And there certianly is a significant number of those. Can't remember exactly at this point and am getting too "lazy" tonight to look it up, but I think it's around 500 or so stations are moving - I think the info on that is posted somewhere in DTV table of allotments thread in reception area ....
Other than what info is currently specifed in the "new" DTV table of allotments(I assume that's what you're looking at) I don't think we'll have the hard "data" on those facilities for those stations which will be moving until they file for Construction permits(and FCC grants them) for their new DTV facilities. You'd think those would soon be "showing up" in FCC database, given we're only 15 months, 1 day, 7 minutes away from analog shut off for full service stations at time of this post ...
I'd think "Theoretically", in most cases, the "current situation" for the TVfool prediction for the analog station involved *should* be the same (or at least roughly similar) to what will be the case for the digital station that's moving to that channel after analog shut off, as the DTV stations service area is (again generally and roughly) *supposed* to match the analog ... And I'd think you might be better off to use that as a "estimation" until the DTV station info gets into FCC database (and TVfool as well) ... Hopefully that will happen by 2/18/09
... OTOH, there may or will be some exceptions to that ...