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Old 22-Jul-2010, 5:25 PM   #1
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 123
Differing Heights for 2Edge to 1Edge Transition for Same Coordinates

Thank you for TVfool & FMfool websites.

Noticed that the transition from:

2Edge diffraction to 1Edge diffraction occurs at differing heights when observed through three different options:

- 'Online TV Maps' (changing height while at same coordinates)

- 'Online TV Maps' (select coordinate, select height, then select 'Make Radar Plot')

At specific heights, the display indications can change from 1-edge to 2-edge by selecting "Make Radar Plot"

- 'Signal Analysis' (entering coordinates, selecting a specific height, then repeat, changing height until transition from 2Eedge to 1Edge occurs)

It seems that the lowest heights for 1Edge-2Edge transitions are displayed when using only 'Online TV Maps'.

The highest heights for 1Edge-2Edge transitions are displayed when using Online TV Maps then, select 'Make Radar Plot'
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Old 22-Jul-2010, 9:37 PM   #2
andy.s.lee
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Join Date: Dec 2009
Posts: 54
OK. I'll try to look into it. Can you give me a coordinate to try?



Hypothetically speaking the differences should be minimal. All of the tools are using exactly the same data sources (transmitters, terrain, etc.) and the same algorithms.

I won't rule out any possibilities until I've had a chance to investigate this, but off the top of my head, I'd guess that the most likely source of error is in slight round-off difference when going back-and-forth between the different tools.

In Google Maps, moving the pointer around lets you control the latitude and longitude very precisely (double precision floating point values), and the signal analysis is done using the full precision available. When you press the "Make Radar Plot" button or directly generate a radar plot report, the coordinates are rounded off to 6 decimal places. This might move the actual point being analyzed by up to about 1 foot.

In most situations, a small change like this would have almost no impact on the results. However, under the right circumstances (e.g., maybe steep terrain, very "pointy" peaks, or areas with sudden drop-offs), a small coordinate change might be enough to make a noticeable change in the results because some of the critical points in the terrain profile might have moved up or down a little.

Another thing to consider is that the Noise Margin is the one value that really matters. It does not matter whether the path type comes out as 1Edge or 2Edge as long as the estimated NM comes out right.

I hope that in all of your experiments, the Noise Margin numbers don't change by much. There is supposed to be a "soft" transition between 1Edge and 2Edge, so the variations in NM should be gradual. If there are sudden jumps in the NM, then this may be the symptom of a more serious problem (e.g., errors in the terrain database or the way in which terrain profiles are derived).

If the NM numbers are pretty consistent, then I wouldn't worry too much about whether it comes out as 1Edge or 2Edge.
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Old 23-Jul-2010, 1:52 AM   #3
tripelo
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Join Date: Jan 2010
Location: Dallas, TX
Posts: 123
Thanks Andy, for your information and explanation.

Quote:
...Can you give me a coordinate to try?...
In a fairly smooth but hilly terrain, these are a couple of coordinates along with example heights for a 2Edge to 1Edge transition for station WCTE-DT.
37.068331, -84.967819
In: 'Online TV Maps'
Antenna height = 46 feet
Callsign = WCTE-DT
Path = 2Edge
NM (dB)= -7.5

Antenna height = 47 feet
Callsign = WCTE-DT
Path = 1Edge
NM (dB)= -1.7
Note: The 2Edge to 1Edge transition occurred between 46 and 47 feet AGL.
Then, press: 'Make Radar Plot'
** Note the differences (below) from the above result.**
Antenna height = 47 feet
Callsign = WCTE-DT
Path = 2Edge
NM (dB)= -10.6

Continuing in 'Online TV Maps'

Increasing Antenna Heights (one foot at a time) & Press 'Make Radar Plot'
Antenna height = 48 feet
Callsign = WCTE-DT
Path = 2Edge
NM (dB)= -9.8

Antenna height = 49 feet
Callsign = WCTE-DT
Path = 2Edge
NM (dB)= -8.9

Antenna height = 50 feet
Callsign = WCTE-DT
Path = 2Edge
NM (dB)= -8.0

Antenna height = 51 feet
Callsign = WCTE-DT
Path = 2Edge
NM (dB)= -7.0

Antenna height = 52 feet
Callsign = WCTE-DT
Path = 1Edge
NM (dB)= -2.0
Note: The 2Edge to 1Edge transition occurred between 51 and 52 feet AGL.

The following results starting with 'Signal Analysis' is in agreement with the second group of numbers listed above.
Antenna height = 51 feet
Callsign = WCTE-DT
Path = 2Edge
NM (dB)= -7.0

Antenna height = 52 feet
Callsign = WCTE-DT
Path = 1Edge
NM (dB)= -2.0
Earlier, I thought I saw other differences when starting in 'Signal Analysis', but now it is in agreement with starting in 'Online TV Maps' & Press 'Make Radar Plot'. At that time, a five decimal place number was used for latitude.

Quote:
... There is supposed to be a "soft" transition between 1Edge and 2Edge, ..
In the first case above, the 1Edge to 2Edge transition signal level change is 5.8 dB (for one foot change).

In the second case (transition), it is about 5.0 dB.

The differing displayed values between 'Online TV Maps' and 'Make Radar Plot' or 'Signal Analysis' has been noticed at another location, namely Garland TX (northern suburb of Dallas) looking further north to station KXII-DT.
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