How Far can I Receive?

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UPMan

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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!
 
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UPMan

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It would be the same regardless of whether it was a repeater system or a non-repeater base/tower (or, for that matter, two people talking direct with no repeater or antenna tower, although buildings/foliage becomes a bigger factor).
 
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Twister_2

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I don't understand it. I typed in the information for WXL40...look at it. Is it working?

EDIT I GOT IT!!!
 
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adamsdawson

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Well my sheriff , fire , and ems are using anlog kenwood LMR radios that are on the 154.340-154.995 VHF freq's you know. But i can get about 15 miles from a repeater at the jail. Thats were all of the main repeaters are. But i am using a radio shack pro 70 right now. Hope this helps.
 

APTN

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I did this for my area. It looks like a large area is within "line of sight". Although, the aforementioned factors require consideration. Also, I just used the closest antenna site. I know that multiple sites are in range of my receiving location.
 

hotdjdave

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I think the amount of output power from the transmitter would also come into play; the higher the power, the farther the distance.


That is a pretty neat tool you found there, Paul.
 

benbenrf

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I think the amount of output power from the transmitter would also come into play; the higher the power, the farther the distance.


That is a pretty neat tool you found there, Paul.
To double the Tx distance (while maintaining the same signal strength on the Rx end) you need to multiply the Tx output 4 x, or increase the antenna gain 4 x

... the rule changes from HF thru to VHF, UHF ... and onwards (i.e. the higher the freq, the more the power required to maintain the same Rx signal strength) as distance doubles - but for all practical purposes, its a fair estimation.
 

N4DES

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I think the amount of output power from the transmitter would also come into play; the higher the power, the farther the distance.


That is a pretty neat tool you found there, Paul.
And also another item is if the system utilizes directional antennas. Many, if not all NPSPAC Systems (866-869 MHz) have to utilize some sort of RF pattern control at their juristictional boundaries. If your outside of the agencies system foortprint you might not be happy with the reception and could even get interference from an agency on an adjacent frequency.
 

benbenrf

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Directional Rx antenna - yes, the more directional your receive antenna the more gain it has, but as important, if not more important than directional gain, is that the greater the "directionality" of your Rx antenna the less the "noise" figure will be that your receiver stages (IF conversion stages, filter stages, Demod stages and amp stages) will have to deal with and process out of the bandwidth contaaining the signal you wish to demodulate.

Reduced noise input will offer any receiver (but the cheaper the receiver, the bigger this advantage will be) a big big plus point when it comes to the baseband info recovery (e.g. audio, video, data).
 

UPMan

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Most frequencies we are interested in (not shortwave) are line-of-sight. More power will slightly increase coverage if there are objects (buildings, mountains) that can reflect a signal back down into a dead zone. However, if the signal is blocked by terrain, 10x power won't gain the range reliably (or much). The limiting factor is the curvature of the earth, not RSSI.
 
E

Eielson

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Conditional!

The range is changing constantly and it is up to three major factors.

Weather

Stratospheric condition

Time of the day
 

UPMan

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In that case, you are talking primarily about HF, not 30 MHz and above. While those factors have some slight effect, they are not major factors in the public safety bands.
 

greenthumb

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Those factors can have influence above 30 MHz, and in fact do at irregular intervals. I have seen atmospheric conditions affect propagation all the way up to (and including) 800 MHz. Extended propagation during the summertime is not all that uncommon. Some of this seems to be tied to a time of day, generally in the evening and overnight, but it's not a regular (daily) occurrence as it is below 30 MHz.

Weather can have an affect on the n-GHz bands for microwave paths and radar, particularly above ~15 GHz.
 
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UPMan

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I think that is consistent with what I said...but going through all the backed-up posts I wasn't able to get to while I was at APCO, I opted for brevity. :)
 

milf

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Here is your list of every major factor-
Height/Elevation of TX antenna
Maximum TX power.
Height/Elevation of RX antenna.
Selectivity/sensitivity of the RX unit.
Type of antenna, whether its just antenna, or high gain, or yagi etc...
For rubber duckies- Structure your in at the time can be a HUGE variable.
Amplification on RX end. (GRE super amp etc)
WX/atmospheric conditions as as they do effect maximum ranges....
And then you calculate the max LOS basics for the band your talking about.
Thats for those wanting to get extremely detailed info...
For the basic average scannist- Slightly farther than actual LOS is what you get. for analog tx... Less than actual LOS for digital tx....
Thats called better safe than sorry basics...
 
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E

Eielson

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My Area

>50% of my hometown's actitivies are hourly updated taperecorded information.

I don't have problem of distance though ~10-12 miles easily.

I tried several antennae but with only mimimum advantage because the next tower will be across the valley.
 
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