[opendtv] Re: Reception range

In my earlier post, I forgot to mention HAAT as an important component in determining real received signal strength.  I did not mean to imply that Bert (or others) couldn't learn this, just that theoretical models he employed weren't relevant to UHF or VHF. 
 
To the items Richard has offered below, I would add antenna/lead in and leadin/receiver impedance mismatch, and (if any) balun loss/gain.  (Yes, sometimes, it's a gain.)
 
While Richard discusses the theoretical models and then goes into practical matters, there are several seasonal issues that need to be contemplated, and these aren't necessarily included in any publicly-available models that I am familiar with.
 
There is even a way, using ray-tracing algorithms against a GIS database of buildings, to model multipath caused by man-made obstructions.
 
If there are trees with leaves along the signal path, one needs to examine the size of the leaves (minima and maxima) and their moisture content.  I know for a fact that L-R doesn't take this into account; a customer of mine saw that L-R worked only at an Alaska location distant from his tx site when the moisture in the intervening leaves was above or below a certain point (or the leaves were gone.)
 
Leaf issues (thankfully, not much of a problem in So Cal, where the leaves are few and the moisture content low) are frequency dependent, and sometimes they will affect the analog visual carrier, but not th audio (or vice versa.)  Leaf issues can increase after a rainstorm, only to abate somewhat as the leaves dry out.
 
Those of us familiar with LPTV (where signal levels are everything) know that this restriction is reflected in the FCC's part 76 rules, which require tx antennae to be located away from and above trees.
 
These issues will come up in interesting ways, I believe in DTV, since the entire 6 mhz is important for the carrier, not with an emphasis on a few points in the envelope.
 
So, use the right models and follow the footsteps of others.  But, drawing conclusions after using inadequate models improperly makes as much sense as asserting that MPEG-2 HDTV by the specifcation only uses square pixels.  Why, anybody who has ever read the MPEG-2 video spec knows (or should know) that  there are two choices for each code, and only a few are limited to square or non-square.  The relevant section has been posted to this list multiple times, amid much discussion.
 
Even Bert, I believe, can learn this, if he wants to.
 
John Willkie
-----Original Message-----
From: Richard Hollandsworth
Sent: Aug 27, 2006 5:47 PM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: Reception range

The FCC stipulates the Longley-Rice Irregular Terrain Model (LR-ITM), which is available as a free dowload:
http://flattop.its.bldrdoc.gov/itm.html
But the HMI is still living in the '70's....
Guidelines for using LR-ITM are found in FCC OET-69 (Feb2004 edition):
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet69/oet69.pdf#search=%22fcc%20oet69%22
It is a STATISTICAL model, based on AVERAGED statistics for a large number of (mostly rural/sub-urban) locations and environmental conditions in the world, presuming an outdoor antenna at 10 meter height.  A Clutter Loss correction was experimentally found to be needed, esp for urban environments.

The antennaweb.org website uses LR-ITM, but I have been unable to obtain any information on HOW they use it, but I believe that they are including co-channel and adjacent channel interference effects.
 
The SPLAT implemention of LR-ITM, with a modern graphic interface, is available as a free download for LINUX operating systems:
http://www.qsl.net/kd2bd/splat.html

The RADIO MOBILE implementation of LR-ITM, with a modern graphic interface, is available as a free download for Windows op sys:
http://www.cplus.org/rmw/english1.html
However, the clutter loss model is inadequate and does not readily support input of local parameters, such as indoor/attic loss, sensitivity loss due to antenna VSWR and system noise figure calculation.  It is also difficult to define a multiplicity of different TX and RX antenna gain patterns and to calculate co-channel and adjacent channel interference effects.

I've been using R-M for over a year to estimate UHF DTV performance, esp. compared to the generally over-optimistic antennaweb.org:
http://hdtv.forsandiego.com/messages/4364/4900.html?1135634429
I developed an Excel spread sheet calcuator to compare antennaweb.org and R-M predictions to actual performance.  The attached update to the (unavailable archived) file now includes an Overload calculator for alternative Preamps.

As you will quickly see, many line items in the calculation require making some shaky assumptions and tailoring to your specific location...such as FCC's recommendation to assume 7 dB for clutter loss irrespective of your local terrain/buildings....commercial propagation prediction programs use proprietary clutter databases for improved results.

Some of the factors that I know are NOT YET included in the calculation are 1) TX antenna elevation gain/loss, 2) close range (under 5 miles) TX antenna gain irregularities causing equallizer degradation (see Bob Chase's posts in avsforum antenna thread), 3) receiver sensitivity degradation due to intermod products and 4) multipath causing constructive and destructive interference that is different for each frequency in both horizontal and vertical dimensions.

This latter effect can cause some channels to be received reliably and others not at all:
http://www.hdtvprimer.com/ANTENNAS/siting.html

holl_ands

========================================
"Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> wrote:
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.

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.

Not so impressive. Should come in strong.

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.

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

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.

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?

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.

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.

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.

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.

Bert

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