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Old 12-Sep-2010, 11:45 PM   #1
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Join Date: Sep 2010
Posts: 12
Transmitter Profile Detail

When looking at the transmitter profile detail am I supposed to assume my house is located at the right edge at ground level? How accurate is the topography? The reason I ask is that my house is located in an area that is about 1000ft above sea level. Do these calculations take that into account or do I need to add it to my antenna hight?

I am 53 miles 85 deg (west) of the los angels area towers. From what I can tell, there should be nothing blocking the antenna's line of sight to the transmitters yet the report indicates 1 edge for most channels. The towers are at close to 6000 ft and my house (53 miles away) is at 1000 ft.

I seem to get everything I want except KNBC (real channel 36). KABC (ch 7) is marginal but works at night.

Is it possible to know where my antenna is on the transmitter profile detail chart?
geobrick is offline   Reply With Quote
Old 13-Sep-2010, 6:10 PM   #2
Join Date: Dec 2009
Posts: 632
Hello and welcome!

Yes, the terrain profiles are drawn such that the transmitter is on the left and your house is on the right. The terrain between the transmitter and your house is shown in gray. The colored areas above the terrain show the approximate signal strengths in the air passing over the terrain. The vertical dimensions have been exaggerated to make some of the smaller terrain features more visible.

The ground elevation at your house and at the transmitter are already taken into account. You do not need to do anything to compensate for a high ground elevation. The Earth's curvature is also automatically factored into the model.

Note that the scale of the profile plot is several miles in the horizontal direction (your distance to the transmitter, 53 miles is about 280,000 feet). Even though the vertical scale of the profile is exaggerated, the vertical dimension of the image still represents several hundred or even a few thousand feet. If you specify your antenna height as 10, 30, or even 100 feet, it is probably going to be barely noticeable on the profile view. Even a 100 foot tower will be infinitesimal when compared to the mountains and foothills that can get in the way of your signals.

The terrain data is primarily derived from the Shuttle Radar Topography Mission (SRTM) data that was collected in the year 2000. In most cases, the terrain data is accurate to 90 meter "bins" or better. The model does not take into account buildings, trees, or other localized obstructions.

Even if you get "1Edge" diffraction of your signals, it's still possible to get plenty of signal for your receiver. There are some parts of the country where the majority of the people get their signal via "1Edge" or "2Edge" diffraction because there are lots of rolling hills in the way.

The curvature of the Earth can also be a source of blockage. For most stations, the line-of-sight horizon is usually somewhere around 50 to 70 miles away from the transmitter (depends on the ground elevation and tower height of the transmitter relative to the surrounding terrain). At 53 miles, you would be getting close to the optical horizon of a "bare Earth", so even a small hill or terrain bump may be enough to get in the way and cause your situation to show "1Edge" diffraction.

Bottom line is that the Noise Margin (NM) column is what really matters. The Path type may be interesting to know, but usually doesn't make any difference as long as you have enough antenna to pull in the signal.
mtownsend is offline   Reply With Quote
Old 14-Sep-2010, 3:27 PM   #3
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Join Date: Sep 2010
Posts: 12
Thanks for the thorough reply. The reason I'm asking is that I'm getting every channel emanating from Mt Wilson in Los Angeles except for channels 36 (KNBC) and 7 (KABC) which are marginal. I'm doing this with an attic mounted antenna (a 10 year old radio shack VHF/FM/UHF model that's about 101" long). I also added a CM-7777 pre-amp to help with the cable runs and splitter downstream.

I'll provide more detail in the reception section.
geobrick is offline   Reply With Quote
Old 11-Aug-2011, 9:00 PM   #4
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Location: Dallas, TX
Posts: 173
Signal Pwr (dBm) Computation

Hi mtownsend & others,

Thanks for the info in your response to geobrick.

How are the power levels at transition zones calculated?

For example, when transitioning from 2Edge to Tropo there could be at least two major sources of signal power:

- The 2Edge portion, and
- The Tropo portion.

Seems at these regions, the two or more sources could be relatively independent in amplitude and phase (and for some portion of time be interfering with each other).

Does TVfool compute a statistical average of both, over some time (space) region?

Or, maybe, just take the larger of the two and use that value for the signal power?

Or, some other computational algorithm.

Could you elaborate on how these transition regions are handled?
tripelo is offline   Reply With Quote


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