# Distance per Watt?

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#### zz0468

##### QRT
Where do you get the "what Motorola considers the reliable range" from?
Didn't Motorola hand out one of those cardboard slide rules with a "range calculator" at trade shows years ago?

#### zz0468

##### QRT
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?
I've seen a few general rules of thumb like this, but the general rule of thumb on that is, they're worthless. Look up the inverse square law on Google. That describes free space loss, and is as simple as it gets.

#### RKG

##### Member
the distance to the radio horizon of a transmitter or receiver, in miles, is 1.34 times the height of the antenna, in feet.
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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.
Oops; correct.

#### cpsTN

##### Member
I've seen a few general rules of thumb like this, but the general rule of thumb on that is, they're worthless. Look up the inverse square law on Google. That describes free space loss, and is as simple as it gets.
As astronomers will tell you, the Inverse Square Law also works for gravity.

#### cpsTN

##### Member
I heard the HP is using older post-WWII frequencies and never say a reason, I supppse, to upgrade.

#### S1deband1t

##### Member
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.
Hmmm, seems feasible for UHF.

Is there a formula for SW for this? I have heard as a rule of thumb it's 1 mile per foot of antenna.

I have a magnet mount mounted to a small base plane on my roof. I measured it at 21'-2" to the top of my antenna.I usually get at most on AM a range of 8.5 miles tops. 7 miles is usually my max for "reliable" contact (anything more is a craps shoot). The receiving antenna plays a major role but as a rule of thumb, using this formula I get 21.15 feet should net me a range of 4.6 miles. I am getting over 150% that range. Now I know I am not talking huge distances but, is there another formula for SW frequencies or is this all UHF?

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#### S1deband1t

##### Member
Also, based off these calculations, if I was to put a 102" whip antenna (pure 1/4 wavelength) at the top of my high roof (currently my magnet mount is on the lower roof for easy put-up and take down) which would be about 9' higher not including 8.5 feet for the antenna height itself then...

21'2" + 17'6" = 38'8" which would be 38.66 feet. When the formula is applied I arrive at 6.2 miles would be my range. This seems to become more askew as the height increases.

Also, power IS a factor no matter WHAT frequency or band. For every doubling of power a 1.4x range would need to be accounted for. For every quadrupling of power the range would double. So, someone with 4 watts may have a range of "Z" miles while someone with 16 watts of power would have a range of ("Z" x 2) range all things being equal.

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#### jwt873

##### Member
There are many "line of sight" calculators on the web. Here's one: Line Of Sight Calculator

Of course, these consider the world to be as smooth as a billiard ball.

Google Earth is a good resource. It shows the elevations for most of the earth. You can add the elevation to the formula.

Plus, if you bring up the Google Earth 'ruler tool', and draw a line between two points, you can save the path you created. Once you save it, the path line will change from yellow to red.

Now right click on the red line and choose "Show Elevation Profile". That will bring up a chart showing all the elevations between two points. It will show if there is a hill in the way that might block any signals.

#### ElroyJetson

##### Member
Line of sight, noise floor, obstacles, and receiver sensitivity.

The Voyager probes, currently beyond the orbit of Pluto, are still sending good data and we're still receiving it, and those satellites were launched with 10 watt transmitters that are currently putting out about 5 watts.

Voyager 1 is more than 20 billion kilometers away and is actually in interstellar space now. We still have communications with it. which I think is insanely cool.

#### prcguy

##### Member
All the online calculators are not "line of sight", they are "free space" calculators and are only valid in space away from anything and everything including the earth. The are not accurate for calculating antenna gain/loss and distance for antennas on earth because ground bounce of signals off the earth can and do play havoc with the end result.

As an example, an online calculator shows a 10mi path at 146MHz with a 3dB gain transmit antenna and 0dB gain receive antenna has a path loss of 96.86dB, which you can then add transmit power, receiver sensitivity and other things to get a system performance level, add curvature of the earth and eventually get distance.

What they don't show is signal ground bounce between the transmit and receive antenna and that it can add up to 6dB of "gain" in the system due to multipath signals arriving in phase or 20dB or more loss in the system due to signals arriving 180deg out of phase.

These ground bounce signals are almost impossible to calculate and must be measured in the field, however they can constantly change due to the surroundings, water content and time of year. So keep this in mind when using any online range calculator, unless your at a specially designed antenna range where ground bounce is designed out of the range.

On one of the early Mars missions one of the NASA so called engineers failed to take this into consideration with the Mars rover antenna and range was significantly reduced from what he had calculated.
prcguy

There are many "line of sight" calculators on the web. Here's one: Line Of Sight Calculator

Of course, these consider the world to be as smooth as a billiard ball.

Google Earth is a good resource. It shows the elevations for most of the earth. You can add the elevation to the formula.

Plus, if you bring up the Google Earth 'ruler tool', and draw a line between two points, you can save the path you created. Once you save it, the path line will change from yellow to red.

Now right click on the red line and choose "Show Elevation Profile". That will bring up a chart showing all the elevations between two points. It will show if there is a hill in the way that might block any signals.

#### Jimbnks

##### Member
One the better one I found is this site:

HeyWhatsThat WISP

It allow to set a lot parameters, to get a ballpark ideal.

#### KC4RAF

##### Member
What 'prcguy' posted is what you should

consider. There's just too many variables that can come into play at any given instance. You can not use those on line calculators and say they are facts. Trops, buildings, tree...

#### teufler

##### Member
got a card board slide rule from motorola, gives approximate readins well.

#### S1deband1t

##### Member
There are many "line of sight" calculators on the web. Here's one: Line Of Sight Calculator

Of course, these consider the world to be as smooth as a billiard ball.

Google Earth is a good resource. It shows the elevations for most of the earth. You can add the elevation to the formula.

Plus, if you bring up the Google Earth 'ruler tool', and draw a line between two points, you can save the path you created. Once you save it, the path line will change from yellow to red.

Now right click on the red line and choose "Show Elevation Profile". That will bring up a chart showing all the elevations between two points. It will show if there is a hill in the way that might block any signals.
They are interesting indeed. However, the signal gets diminished from outside sources, interference (both man made and natural), absorption of signal etc...

If I stood on top of my roof with a blister pack of 0.5w FRS transceivers, I sincerely doubt that I could talk to another rooftop "line of sight" at 20 miles away. QRP may be alive and well on far lower frequencies but I doubt once you get into VHF and above that it would count for anything. 1 Watt of power at 27mhz I feel would go ALOT further than 1 Watt of power at the UHF portion of the spectrum.

#### jwt873

##### Member
All the online calculators are not "line of sight", they are "free space" calculators and are only valid in space away from anything and everything including the earth. The are not accurate for calculating antenna gain/loss and distance for antennas on earth because ground bounce of signals off the earth can and do play havoc with the end result.
Line of sight calculators take into account the curvature of the earth. If you're on a ship at sea, you can only see so far before everything disappears 'over the horizon'. The higher you go, the further you can see.

As a note, the calculator link I posted has nothing at all to do with RF. It only lets you know if you can 'see' the other station. It's a good start for calculating a radio path

#### blue5011

##### Member
Google Earth is a good resource. It shows the elevations for most of the earth. You can add the elevation to the formula.

Plus, if you bring up the Google Earth 'ruler tool', and draw a line between two points, you can save the path you created. Once you save it, the path line will change from yellow to red.

Now right click on the red line and choose "Show Elevation Profile". That will bring up a chart showing all the elevations between two points. It will show if there is a hill in the way that might block any signals.