[opendtv] Re: Reception range

This is the funniest thing that I've seen on this list in months!  More funny 
than the assertion that all HDTV is square pixels -- which isn't the case.

My responses by way of interlineations


>This handy site provides the specifics of TV station transmitters:
>location, HAAT, ERP, current and planned configurations.

>http://www.fcc.gov/mb/video/tvq.html

>Enter the call sign and ask for the detailed output. Or enter your coordinates 
>and a radius, and it will spit out all >stations in that area.

> This site says whether a station is still operating under a special temporary 
> authority.

>http://www.fcc.gov/mb/video/files/dtvstas.html

>I wanted to see what marginal reception translates to for me, in terms of 
>received signal strength.

>The most marginal station I receive is WBAL-DT (NBC 11 Baltimore). It operates 
>over Ch 59, at 513 KW ERP, >antenna 1023.6' HAAT. Decent output, decent 
>antenna height.

Someone actually familiar with the FCC and engineering would report that the 
station's maximum authorized power was 513KW ERP.  Actual power may differ, 
even from moment.  Also, an engineer would have reported the antenna pattern 
(omni, or in not omi, the direction of the maximum lobe, and the amounts of 
beam tilt (mechanical and electrical, and the maximum azimuth),  and any null 
fill.  ONLY then are you prepared to understand the propagation issues.

>Using free space propagation, and the measured 15 dB margin I need for 
>reception, total channel loss should be >142.3 dB, and signal strength at the 
>receiver, assuming unity gain receive antenna, -55.2 dBm.

And, why would you use free-space?  Can you see the transmit site from your 
receive location?  Assuming not, you need to examine the signal path (topo, 
man-made obstruction, land uses), plot it and examine if you are in an odd or 
even fresnel clearance zone.  The difference is significant: in an even zone, 
the signal strength is doubled, an in an odd zone, it is close to zero.

>Not so impressive. Should come in strong.

Sez you.

>Using the Hata model, which takes into account antenna heights as well as 
>clutter, and feeding in minimal clutter, >distance, exact freq, etc., I get a 
>total 160 dB channel loss and a received strength of -73 dBm.

I'm not familiar with the Hata model?  How long has the FCC been using it?  It 
seems to only address one issue I discussed above, "clutter."  Just why did you 
assume minimal clutter?

>The marginal reception makes a little more sense now, but I would think I'd 
>get solid reception more frequently.

Sez you.

>ABC 2, 602 KW vs the 513 for NBC 11, so just slightly higher power, over Ch 
>52, comes in much more reliably. >Although not very strong in the signal 
>meter, the difference translates to solid reception most of the time.

No, it's essentially the same power, since you are talking about a long signal 
path with unknown intervening terrain.  Also, it's important to note that 
fresnel zone clearance is frequency dependent, so ONLY if you have done fresnel 
zone clearances on both stations can you BEGIN to compare signals of different 
frequencies.   Also, just because a station is authorized at a maximum power, 
that doesn't mean that it was operating at that power when you are watching.  A 
call to the station's engineering department can help with the actual operating 
power at a particular point in time, but you would most likely encounter 
someone who knows more about this subject area than you.

>The surprise was one of Mark Aiken's stations. WBFF-DT, operating on a 
>temporary authority at 46.6 KW ERP, or >so the FCC says (as of 13 July 2006), 
>1223' antenna HAAT, comes in solid. The Hata model claims I should see >about 
>-80.3 dBm at the receiver. Even free space loss would result in -64.7 dBm. So 
>this should be a pretty >weak signal, yet it beats WBAL, for some reason. And 
>this is determined over the long haul.
>Same behavior consistently, since Christmas 2005. Is the FCC wrong about the 
>STA?

Only idiots use free-space outside of the laboratory (and space communications) 
and the Hata model is not useful unless and until it has been vetted against 
all the other models.  I doubt that it is ever used by the FCC.  Also, you have 
now way of knowing at the moment you were watching whether the station was 
operating it's facilities pursuant to a CP, or under an STA, or when they 
changed.

Also, this station could have been operating at maximum facilities.  It also 
may have a different tx site, hence a different transmission path, different 
obstructions, and due to the different frequency, different fresnel zone 
clearnance.  

LET ME REPEAT IT:  No fresnel zone clearance: 100% signal.  Even fresnel zone 
clearance: 200% signal.  Odd fresnel zone clearance: 0% signal.  

Bert, this is just one example of why TV stations are engineered by broadcast 
engineers.  You just barely have a clue here, and there is so much you don't 
know -- heck there is much I don't know -- that it's comical to see you making 
statements and not asking questions.

>Another data point is WMPT (PBS 22 Annapolis). It shows marginal in the signal 
>strength meter, though reception >is consistently solid. Antenna is 948' HAAT, 
>at 150 KW ERP, works out to 140 dB path loss with the Hata model, >or total 
>155 dB loss with the required 15 dB margin. Received signal strength comes out 
>to -73.2 dBm.

I have yet to see a single data point from you in this post. Actual ERP, tx 
path, fresnel zone, proper model, etc. etc.

>As a comparison, the analog version of WMPT is 5 MW. It's a good analog 
>signal, but obviously not the equal of >150 KW WMPT-DT in terms of image and 
>sound.

You might have a comparison, were they the same tower, and depending on the 
differences in the height and pattern of the various antennae.  I see no 
information on that.

>So aside from whatever magic Mark Aitken is working, looks like -73 dBm, or 
>slightly lower than that, is what I >can expect for marginal reception.

Why Mark Aitken?  The Chief Engineer of the station is on this list, and he's 
communicated with you at least once that I know of.  Mark is the Director of 
Advanced Technology for Sinclair; people with much smaller purviews are 
responsible for this.

>My antenna should provide several dB of gain, but my low elevation doesn't 
>help. Also, the antenna aim should >be no better for WBFF-DT than for the 
>other Baltimore stations. So this is a bit of a puzzle.

You mean you've previously peaked your receive antenna and you know how many 
degrees difference there is between the optical pointing direction and the 
electrical pointing direction?  Do you know this in the H and V planes?  Have 
you tried the illogical (but sometimes helpful to minimize the effects of 
ground clutter) practice of turning the antenna 90 (on it's side) to see what 
that does?

John Willkie


Bert
 

 
 
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