[opendtv] Re: Reception range

Bert;

You appear to grossly misinformed, but you think you are informed.  This will 
be my last attempt to help you in any form on this.

There are two stations in San Diego that operate on second adjacent channels, 
at the same ERP, with the same antenna patterns, on the same hill.  One's 
center of radiation is 20 feet above the other.  The antennae are across the 
street from one another, about 200 feet.

They are KGTV and KFMB-TV, and they are not co-owned or otherwise linked.   I 
suspect that these are a much closer match of operating parameters than you 
will find in Baltimore or just about any other market.

They are about 12 miles distant from my current location.  There is an 
intervenng hill between us, about 500 feet laterally from my current location.  
This hill peaks out about 200 feet above my current location.

So, one would expect that the received signal from both stations -- using your 
idea of analysis -- would be similar.

Let me highlight the differences between the stations.  200 feet between tx 
sites laterally, 20 feet in height, 12 mhz in the respective frequencies.  (I 
cannot recall if either is on a precise offset, but they're so old I doubt it.)

So, how is my reception?  One station is entirely in the noise on any tv 
receiver, and the other ranges between TASO grade 1 and 3, most often at 2.  
(Meaning barely usable.)  It's been this way for four decades of which I am 
aware.

Which station comes in better?  The one with the lower center of radiation.

If one were to engage in deep analysis of the various differences (which I have 
done on a few occaisions), one would come to realize that what changes the 
reception condition is: the lateral difference between these two stations, and 
how the signal path encounters the obstructions (houses) on that nearby hill.

I just now checked to make sure that the current reception matches the 
historical situation.  It does.  And, I checked to make sure both stations are 
at their maximum operating power.  They are.

You say that in your situation, that the azimuths are close.  How many seconds 
of arc do you think separate the lateral distance between KGTV and KFMB-TV?  
(I'll let you do the math: 12 mile path, 200 foot difference laterally, and 20 
feet vertically).

At this end, the difference could be that the direct path of the station that 
we -- using models -- should get better reception of travels through something 
as simple as the vertical bar of a swing set, and the other does not.  Maybe 
it's a 2x4; perhaps it's a vertical run of Romex, in or out of conduit. It 
could even be that one travels through the hole created by the "chicken wire" 
upon which stucco is blasted, and the other travels right through the 1 mm 
chicken wire.  I don't know, but I do know that the same situation has 
prevailed since 1959.  Back in the day, KGTV carried NBC.  We could passably 
watch CBS on KFMB-TV, but we only watched NBC when we got cable.  

You don't know what you don't know about which you don't know.  Models help 
minimize what you don't know. But, to draw conclusions from ignorance is very 
entertaining.  Keep it up.

I could provide other details, from other receive sites, using reception of 
similar stations from Mt. Wilson that require many more levels of analysis than 
the above to understand -- to the extent one can understand them.  

You simply lack enough data points to begin to understand your reception of 
Anna and Balmer stations. Let me know when you have done the 4/3 earth contour 
analysis and plotted the terrain and man-made obstructions at the knife-edge 
points.

Some years back, Cliff had a somewhat similar situation vis a vis several 
Philly stations.  When one did the 4/3 earth contour analysis, the intervening 
hill became clear.  One station went through it better than the others due 
either to tx location, or the knife-edge diffraction properties of that station 
against the others.

Over and out

John Willkie


Bert wrote

Richard Hollandsworth wrote:

> 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.

Lots of interesting info in your post, Richard. What's missing, though, is that 
I wasn't looking for an absolute prediction of receivability of a TV station. I 
was trying to figure out why one station, WBFF-DT, was so different from all 
the others. This was a comparative analysis. Any number of shaky assumptions, 
especially at the receiver end, cancel out.
They are equally right or equally wrong for all data points.

Aside from the one Annapolis example, all other stations were from Baltimore, 
located very close together in Baltimore. Which means that a lot of assumptions 
about the terrain, trees, etc. between source and destination also cancel out. 
We're talking almost 50 miles away, as the crow flies. Azimuths don't change 
that much from that distance. As confirmed also by antennaweb.

I already knew that multipath was not a major issue for these stations, by 
checking their analog channel. The distant stations do not suffer too much from 
multipath, probably because the reflections are just too weak.

What I showed fairly convincingly, I think, is that all the marginal stations 
from far away resulted in almost identical signal strength at the receiver, 
which was about -73 dBm threshold level. In absolute terms, this number could 
be off by several dB. It could be -80 dBm in reality, for example. But what 
matters is that it was consistent for all
-- except WBFF-DT. Interestingly enough, even the Annapolis station resulted in 
that signal strength at the receiver. So the Hata model did quite well.

Mark Aitken's response is the one that mattered. The FCC had not updated its 
table of stations operating under a STA. With the ERP for WBFF corrected, 
everything is consistent for the marginal stations. Not only that, but the 
correlation is also excellent for the stronger stations, when comparing their 
transmitter configurations against the Accurian's signal strength meter. So 
that pretty much settles it.

Bert
 
 

 
 
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