How Far Can I Receive?
A common question from new (and some not-so-new) scanner owners is "How far can I receive "X" system from?"
There are a lot of factors that have to be used to give a definitively exact answer, but for most signals above 20 MHz the limit is usually determined by your line-of-sight to the transmitting antenna. Closer in to the antenna, you'll get some reflected signal that "fills in" areas where there isn't a true line-of-sight. But, once you get a few miles out, terrain (hills) and the natural curvature of the earth are going to be the biggest factors affecting reception range. Basically, if there is any terrain between your receiving antenna and the transmitting antenna, your chances of being able to receive anything from that site are pretty low.
I found a tool that is pretty cool that will answer the question "Do I have a line-of-sight to that antenna?"
http://www.heywhatsthat.com/ provides a Google app that is pretty cool.
To use it for this application, first find the antenna information in this site's database. For example, for the Arlington, TX public safety system, that info can be found at:
http://www.radioreference.com/apps/db/?cs=WNDW499
Note that the altitudes given on that page are in meters, so you'll need to multiply by 3.28 to convert to feet (that the Google app asks for).
Put the antenna's latitude/longitude into the "FIND" blank (I used 32.75806N, 97.11167W). then click FIND, select satellite view, and zoom in all the way. You should see the antenna structure. Click on the base of the structure to fine-tune the location. (The lat/lon blanks didn't work for me, so I just entered the above coordinates into the "address" field.)
Now, enter the antenna's height above ground level. In this case, it is 127.7 Meters, or 419 feet.
Give your map a name, then click "Submit Request." It takes about 2 minutes for the results to be returned.
When you get the results, zoom out to give yourself a good view of the entire area of interest, then click "VISIBILITY CLOAK" at the top of the map. The map is shaded in with all areas that are within a line-of-sight to the antenna.
You can click on any point on the map and the app will draw a line from the source location and provide the distance to that point below the map.
It isn't clear from the application, but I believe that the "other end" of the line-of-sight is assumed to be 6 feet off the ground. So, if you have a receiving antenna on a mast, add the height above ground of your antenna (minus 6 feet) to the height of the transmitting antenna to see if your antenna is going to get a straight shot at the signal source.
This application also does not take into account buildings and foliage, which also block, reflect, or attenuate signals.
It does a pretty good job of explaining why reception of the Arlington system gets spotty on Hwy 360 north of I20 (there is also a coverage "hole" from Arlington's other tower).
Of course, if you are trying to DX such a system from many miles away, you'd need to start considering other factors such as transmit power, receiver sensitivity, antenna gain, band noise floor, etc. But for many (if not most) of us, the line-of-sight model is going to suffice.
Thought I'd share!
A common question from new (and some not-so-new) scanner owners is "How far can I receive "X" system from?"
There are a lot of factors that have to be used to give a definitively exact answer, but for most signals above 20 MHz the limit is usually determined by your line-of-sight to the transmitting antenna. Closer in to the antenna, you'll get some reflected signal that "fills in" areas where there isn't a true line-of-sight. But, once you get a few miles out, terrain (hills) and the natural curvature of the earth are going to be the biggest factors affecting reception range. Basically, if there is any terrain between your receiving antenna and the transmitting antenna, your chances of being able to receive anything from that site are pretty low.
I found a tool that is pretty cool that will answer the question "Do I have a line-of-sight to that antenna?"
http://www.heywhatsthat.com/ provides a Google app that is pretty cool.
To use it for this application, first find the antenna information in this site's database. For example, for the Arlington, TX public safety system, that info can be found at:
http://www.radioreference.com/apps/db/?cs=WNDW499
Note that the altitudes given on that page are in meters, so you'll need to multiply by 3.28 to convert to feet (that the Google app asks for).
Put the antenna's latitude/longitude into the "FIND" blank (I used 32.75806N, 97.11167W). then click FIND, select satellite view, and zoom in all the way. You should see the antenna structure. Click on the base of the structure to fine-tune the location. (The lat/lon blanks didn't work for me, so I just entered the above coordinates into the "address" field.)
Now, enter the antenna's height above ground level. In this case, it is 127.7 Meters, or 419 feet.
Give your map a name, then click "Submit Request." It takes about 2 minutes for the results to be returned.
When you get the results, zoom out to give yourself a good view of the entire area of interest, then click "VISIBILITY CLOAK" at the top of the map. The map is shaded in with all areas that are within a line-of-sight to the antenna.
You can click on any point on the map and the app will draw a line from the source location and provide the distance to that point below the map.
It isn't clear from the application, but I believe that the "other end" of the line-of-sight is assumed to be 6 feet off the ground. So, if you have a receiving antenna on a mast, add the height above ground of your antenna (minus 6 feet) to the height of the transmitting antenna to see if your antenna is going to get a straight shot at the signal source.
This application also does not take into account buildings and foliage, which also block, reflect, or attenuate signals.
It does a pretty good job of explaining why reception of the Arlington system gets spotty on Hwy 360 north of I20 (there is also a coverage "hole" from Arlington's other tower).
Of course, if you are trying to DX such a system from many miles away, you'd need to start considering other factors such as transmit power, receiver sensitivity, antenna gain, band noise floor, etc. But for many (if not most) of us, the line-of-sight model is going to suffice.
Thought I'd share!
Last edited: