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General Scanning Discussion - For general questions not specific to a model of scanner or general discussion of use of a scanner. Manufacturer specific posts should be directed to the appropriate forums below and location specific posts should go in the appropriate regional forum..

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Old 01-13-2010, 10:19 PM
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Default Distance per Watt?

I have always heard that for radios without the use of repeaters, the general rule (for FM) is a range of ONE MILE for every HALF WATT (500 mw) of transmitter power. Centainly, tall landscapes will interfer, but is this a good general rule?

Charles
Smyrna, TN
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Old 01-13-2010, 10:33 PM
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I wish it was that simple, no. If you applied that logic a 100W VHF radio would reliably talk 200 miles.
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Old 01-13-2010, 10:45 PM
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That is why I wasn't sure. I know unaltered CBs, at 4w, can be heard for a couple of miles with no problem but the little FRS radios, at only 0.5w, struggle to be heard a mile away.

Charles
Smyrna, TN
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Old 01-13-2010, 11:44 PM
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Power is only a small factor in the distance that can be covered. On VHF and above (generally) Height and how clear the path is much more important than power, at least once you get pased a 'minimum power level'. Antenna design is also a key factor in most situations (please note that this is 'antenna design' and not just 'antenna gain').

I used to regularly talk over 60 miles using my 4 watt power out HT using the rubber duck antenna. This was when I was on the roof of a the tallest building for 20+ miles (just under 300') to a repeater at the 1100' level which was 60+ miles away. Hams on the International Space Station use an HT attached to a rubber duck antenna stuck in a window for their "hams in space" program and this is easy to hear when they're overhead as well.

Another example on how transmitter power isn't the key factor is on the AM broadcast band (often called MW for Medium Wave). Radio stations are often required to reduce power or change antenna patterns at local sunset to reduce interference with other stations on the same or on close frequencies. Some run 5000 watts of power during the day and have trouble covering their local area fully, but are reduced to running 100 watts (or even less) to keep them from interfering with a station 1000 miles away at night.
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Old 01-14-2010, 1:39 AM
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Line of sight is a critical factor. I understand that only 1 watt of power was used to communicate with spaceships at the moon. But 1 watt down in a canyon has no where to go.

Another thing to consider is "antenna pattern". Is the transmitted beam being sent omni-directional? Or focused in a specific direction?

There is also atmospheric conditions. Some amateur radio operators are able to "talk around the world" via skip or similar means with very little power. As a matter of fact there is a forum here on that topic where people post distant police/etc calls they've received.

Power is over-rated.
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Old 01-14-2010, 1:54 AM
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Quote:
Originally Posted by cpsTN View Post
I know unaltered CBs, at 4w, can be heard for a couple of miles with no problem but the little FRS radios, at only 0.5w, struggle to be heard a mile away.

Charles
Smyrna, TN
CB radios operate in the HF band, not the VHF band and the biggest factor there is propagation.
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Old 01-14-2010, 12:19 PM
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Relaying this information from reliable engineering sources the rule of thumb is one mile per watt effective radiated power direct line of sight on VHF FM. Of course there are plenty of other factors to consider like frequency (directly related to atmospheric absorption), mode, etc. which tend to complicate things but why dwell on it? Why not answer a simple question with a simple answer and leave the complications to the physics professors?

Now if you want a constant in the realm of physics, EM radiation in free space like gravity diminishes by half at the square of the distance. Ground control to Major Tom...
(;->)
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Old 01-14-2010, 1:58 PM
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Once you have 1-2 watts of power at VHF, and especially at UHF (Like the FRS radios mentioned), power is almost meaningless.

Line of Sight is everything.

Let's see, if it's 1 watt = 1 mile, my, that means that a nice FM broadcaster, with 5000 watts, can get 5000 miles... nope. 40 miles, maybe. Maybe 90, if they have a good high antenna site.

Meanwhile, I had no trouble talking to a station 150 miles away, with 2 watts... oh, it happened that the other station was on the Space Shuttle, though...

I had no trouble talking to/through a ham VHF repeater 16 miles away, with 100 milliwatts... because it was at a good high mountain, and I'm at a 250 foot level, with no obstructions between.

The only rule of thumb I've seen from Motorola was one using antenna height, not power.

Take the height of your antenna in feet. Find the square root. That's your expected reliable range in miles. Do the same for the antenna on the other end, add the two results.

A pair of handie talkies: Antenna height, 5 feet. Range, 2.24 miles (each), for (call it) 4.5 miles expected reliable range. This is only over flat, unobstructed ground, though. Buildings and hills obstruct. At UHF, trees aren't helpful, either.

Now have one end stand on a 200 foot hill. Combining the two again, 16.5 miles.

Now, in reality, you may see as much as double this range, in perfect conditions. In not so perfect conditions, you may see half... or less.

Typical urban setting range between handie talkies is 1/2 to 1 mile, maybe 2 miles if you're lucky.
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Old 01-14-2010, 6:35 PM
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So, help me out here real quick.

Why the public service freqs (urban) tend to congegrate around the 460MHz band?

And what about the 800MHz?

Lastly, why the highway patrols use low ~40MHz freqs?
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Old 01-14-2010, 6:55 PM
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Smile It's not the power, it's antenna height!

Quote:
Originally Posted by kb2vxa View Post
Relaying this information from reliable engineering sources the rule of thumb is one mile per watt effective radiated power direct line of sight on VHF FM.
That seems unrealistic. Consider that NOAA transmitter not too far from here, high on a mountain top, with 100K of power, on 162.400 or 162.550 MHz broadcasting on a 15khz wide channel. They don't go 100 miles, let alone 100,000.

Quote:
Originally Posted by SkipSanders
Let's see, if it's 1 watt = 1 mile, my, that means that a nice FM broadcaster, with 5000 watts, can get 5000 miles... nope. 40 miles, maybe. Maybe 90, if they have a good high antenna site.
Another thing to consider is bandwidth. Commercial FM broadcasters are using 200khz wide channels, not 10,15,or 25khz wide.

Quote:
Originally Posted by SkipSanders
The only rule of thumb I've seen from Motorola was one using antenna height, not power.

Take the height of your antenna in feet. Find the square root. That's your expected reliable range in miles. Do the same for the antenna on the other end, add the two results.
I totally agree.
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Old 01-14-2010, 7:16 PM
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Originally Posted by Ablice View Post
So, help me out here real quick.

Why the public service freqs (urban) tend to congegrate around the 460MHz band?

And what about the 800MHz?

Lastly, why the highway patrols use low ~40MHz freqs?
All depends on where you are, and what the terrain is like in the area. Also on what's already installed, and how much of your money the local government feels like spending on replacing entire radio systems.

Public Safety use pretty much all bands. VHF High, UHF, and 800 MHz all get used. Now we'll begin to see 700 MHz used, as well.

VHF Low Band tends to get used by agencies that need long range systems, in uneven terrain, without spotting hundreds of radio towers every few miles. This is why Highway Patrols, that tend to have large coverage areas, often stick with Low Band. Sheriff's Offices have this tendency, too.

UHF 450-512 tends to be used in cities, mostly conventional systems. UHF (and 800 MHz) tend to penetrate buildings better than VHF, and much better than Low Band.

800 MHz, and now 700 MHz, tend to be the newest, trunked systems.

San Diego, for instance, was VHF High Band for a long time. So was the San Diego Sheriff's Office. SDSO then moved to a UHF 460 MHz system (because there weren't enough VHF frequencies available for the expansion they wanted to do). Eventually, both the city and the county installed seperate 800 MHz trunking systems.

There are rumbles now that eventually, they might combine their systems, perhaps as a brand new 700 MHz system.
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Old 01-14-2010, 7:20 PM
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There is really only one constant which Warren touched on but I believe he got backwards.
In free space, if you double the distance the signal will drop by 6dB or 75%. In other words if you have a usable signal at 100mi in free space with 1w you will need 4w to keep the same signal level at 200mi. This is not frequency dependent however you cannot rely on this constant for terrestrial communications, even if its completely line of sight. Ground bounce and signals adding or subtracting in or out of phase can skew the results from +6dB to -20dB or more.
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Old 01-14-2010, 7:20 PM
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Quote:
Originally Posted by Ablice View Post
So, help me out here real quick.

Why the public service freqs (urban) tend to congegrate around the 460MHz band?

And what about the 800MHz?

Lastly, why the highway patrols use low ~40MHz freqs?
While this may not be 100% correct, it's a close approximation and you did ask for an answer "real quick".

The public service folks quickly ran out of channels on VHF-Hi (150 MHz band) and moved to UHF quickly when it opened up to add the necessary channels.

800 MHz works better in an urban hi-rise environment than UHF or VHF. Also, the trunking systems were generally available for the 800 MHz band earlier (at least more reliable there earlier) so when they needed additional channels they moved to 800 MHz to get additional channels (although these were virtual channels from trunking).

VHF-Lo (40 MHz) has generally good coverage between mobiles and base stations over the long distances required by a state wide organization like the highway patrols and power companies. Many have gone to trunked systems now, but often keep their old VHF-Lo systems as a backup and for secondary uses.
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Old 01-14-2010, 8:40 PM
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Gotcha. Thanks.

My scanner has a range from 1240MHz to 1300MHz (1.3GHz), what would that stuff be used for? A quick Googile search revealed nothing.
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Old 01-14-2010, 8:55 PM
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Quote:
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Gotcha. Thanks.

My scanner has a range from 1240MHz to 1300MHz (1.3GHz), what would that stuff be used for? A quick Googile search revealed nothing.
Amateur Radio
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Old 01-15-2010, 10:18 AM
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As a very broad generality, talk distance on a VHF or UHF FM system is affected by three factors:

Attenuation due simply to distance, which is known as free space propigation. There are some software packages that will calculate this for you. The limiting factor is when the power of the transmitter is diminished simply by distance through a medium to the point where the signal strength at the receiver is below the receiver's minimum sensitivity point.

Attenuation due to the curvature of the earch. This is a macro form of intervening terrain. As a broad rule of thumb, the distance to the radio horizon of a transmitter or receiver, in miles, is 1.34 times the height of the antenna, in feet. The talk distance is the sum of the radio horizons of the transmitter and the receiver.

Location specific terrain: mountains, buildings, and the like. This is obvious and location specific, and there are no general rules.
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Old 01-15-2010, 10:25 AM
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the distance to the radio horizon of a transmitter or receiver, in miles, is 1.34 times the height of the antenna, in feet.
-----------
Er, no. Make that the square root of the height of the antenna, in feet. And it's in excess of what Motorola considers the reliable range, which leaves off the 1.34 multiplier.
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Old 01-15-2010, 10:56 AM
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Where do you get the "what Motorola considers the reliable range" from?
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Old 01-15-2010, 11:16 AM
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The frequency, ERP and height above avg terrain are all your factors They all come into play.

I couldn't find a link offhand, but there are equations and calculators kicking around that do estimates.
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Old 01-15-2010, 11:32 AM
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Your leaving out ground reflections, they are always present in terrestrial links and can affect the outcome as much if not more than the other factors.
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The frequency, ERP and height above avg terrain are all your factors They all come into play.

I couldn't find a link offhand, but there are equations and calculators kicking around that do estimates.
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