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Author Topic: Are there any "Rules of Thumb" for 2 meter FM range?  (Read 4669 times)
ND9B
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Posts: 52




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« on: November 01, 2011, 04:13:40 PM »

There must be a simple way of estimating VHF communication range. When I consult the literature, I always run into "path loss", then things get way too complicated, and I never get an answer. Surely two way radio installers must have some tables or guide lines that tell them how high a customer's tower needs to be.

My particular situation is an 80 ft tower, a 50W 2M FM rig, and a 6dB omni-directional antenna. What range should I expect to a mobil? What range to another base with an identical setup? What range to a handitalkie?

ND9B
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AA4PB
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« Reply #1 on: November 01, 2011, 04:24:08 PM »

It's not that simple - no valid rules of thumb. There is software available but you have to have an actual Terrain map and plot the actual station positions along with the antenna height, gain, coax loss, power output, and receiver sensitivity.

I may be able to get 15 miles range to a mobile on level ground but if his location places tall buildings, hills, or other obstructions in the signal path then I may not be able to make 5 miles.
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K1CJS
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« Reply #2 on: November 01, 2011, 06:11:47 PM »

What really needs looking at is the terrain height--above sea level.  How high is the antenna?  You have to figure in the height of the tower base above sea level then add in the antenna height on the tower.  Now you see if there is anything higher between you and where you need your signal heard.  Figure in other transmitters that may interfere, along with their position and height, not to mention their power output.  Now, figure in the time of day, (or night) whether there are any storm fronts around, what season of the year it is, and so on.

All these things AND MORE will affect how far your signal is going to get out.

You may get clear communications to only fifteen or twenty miles from your station to a mobile station, or you may be able to reach out sixty or seventy miles from your station to another fixed station.  You could well have your signal heard even further away if atmospheric conditions are right.  As AA4PB said, there are no clear rules of thumb.  You may be able to get exceptional coverage one day--and the next day have little to no coverage.  All too often, it's a crap-shoot. 
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KG4RUL
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« Reply #3 on: November 01, 2011, 07:49:47 PM »

From a 17' Dual Bander, mounted on a 40' tower at our EOC, running 50W, we can reliably communicate to a mobile station at a distance of  approximately 26 miles.  Of course, the Low Country of South Carolina is fairly flat.  Our next project is to install a remotely controlled transmitter in the equipment shack under our EOC's other tower (150').  The hardline is already installed to a 4 bay, 2M antenna located at the top of the tower.
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W8JX
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« Reply #4 on: November 01, 2011, 09:03:35 PM »

My particular situation is an 80 ft tower, a 50W 2M FM rig, and a 6dB omni-directional antenna.


Depends a lot on terrain. Assumption below ore based on fairly flat ground

What range should I expect to a mobil?

25 to 35 miles

What range to another base with an identical setup?

75 miles or more

What range to a handitalkie?

10 to 15 miles maybe a it more.
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KA4POL
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« Reply #5 on: November 01, 2011, 11:11:06 PM »

Besides having a good location for the antenna the weather conditions also play an important role. As someone put it, the so called mysteries of propagation. Particularly in autumn you can experience longer distance connections.

As you see there are so many uncertainties that it is almost impossible to set up a rule of thumb.
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WB2WIK
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« Reply #6 on: November 02, 2011, 10:25:52 AM »

Besides having a good location for the antenna the weather conditions also play an important role. As someone put it, the so called mysteries of propagation. Particularly in autumn you can experience longer distance connections.

As you see there are so many uncertainties that it is almost impossible to set up a rule of thumb.

I usually study the terrain profile from the site in question to everything around it for whatever distance I'm trying to achieve.  Google Earth provides reasonably good elevation data if you're meticulous enough in using it.

I can put a 2m vertical on a 150' tower having a ground elevation of 5,000 feet above sea level and have no usable "range" at all if I'm surrounded by mountains that are 6,000 feet high within a couple of miles. Wink

If the working area is flat as a pancake for miles in every direction, it's a lot easier to calculate working radius.
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KA4POL
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« Reply #7 on: November 02, 2011, 11:58:06 PM »

Besides having a good location for the antenna
I can put a 2m vertical on a 150' tower having a ground elevation of 5,000 feet above sea level and have no usable "range" at all if I'm surrounded by mountains that are 6,000 feet high within a couple of miles. Wink
True, but then this would not be a good location per my statement. On the other hand, with that location you still might have a rare chance to get reflections into a duct. For those conditions I'd recommend a circular polarized antenna and also SSB and not FM.
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KA3NXN
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« Reply #8 on: November 03, 2011, 04:23:40 AM »

You have to take terrain into consideration. The ideal terrain is over flat water. Even water that is constantly churning causes issues.  A great example would be the Gulf of Mexico. There are usually no waves unless there is a storm out at sea. Also the angle of the antenna. Is it angled down so it hugs the curvature of the earth or is it just horizontal with no down tilt. It's not up the the manufacturer of the radio to give you "how far" the signal will travel, but the math should be done by the antenna and tower designer / installer. The power output of the radio is a very, very small factor when calculating these distances. Every situation is very different from the next. I have seen repeaters with the same power out and same gain antenna on the same tower. One was UHF and one was VHF and they behaved completely different when it came to coverage and usability.

Also don't believe the gain figures that you get from the antenna manufacturers. They are known to grossly over inflate these figures. Especially the Japanese and even cheaper Chinese multi-band knock offs. The only Ham antenna that I would use for a tower repeater or base antenna is the Hustler G7-144 for 2 meters, the G7-220 for 1.25M and the G6-440 for 70CM. I even don't like their dual band antenna. I tend to stay primarily with single band antennas, but that's a different story. If you really want a banging omni signal consider a commercial grade antenna like a dipole array or a Hustler Spirit series. But mind you they are quite expensive compared to the ham grade $100.00 antennas. I had a Hustler Spirit for 2 meters on a 30' tower in Florida and was able to easily get 50+ miles from base to mobile with only 50 watts. But again I was on the coast and the terrain was flat.

Welcome to the hobby/engineering of radio

I have to laugh when I see those hand held FRS radios on Ebay that the seller claims 30 mile range. They must have tested these out in outer space and using line of sight.

Jaime-KA3NXN
« Last Edit: November 03, 2011, 04:43:38 AM by KA3NXN » Logged
AA4PB
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« Reply #9 on: November 03, 2011, 06:04:16 AM »

All of the calculations cannot take into account the occassional special propogation modes. On occassion you may find that you cannot work a station 15 miles away and yet you can work a station two states away at that time using ducting. There is just no formula that can predict that.
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KM3F
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« Reply #10 on: November 03, 2011, 09:15:21 AM »

According to G.R. Jessop VHF hand book here are three tables.
For free space attenuation at 144 mhz;  
50km -110db,  100km- 116db, 150km- 119db,  200m- 122db, 300km- 125db and up to 1000km- 136db.
.
Tropo scatter path loss over smooth earth at 144mhz;
50km-50db, 100km-52db, 150m- 55db, 200km- 58db, 300km- 64db and out to 1000- 112db.
.
Theoretical performance between two ?good? sites under flat conditions at 144mhz;
with path losses of 247db, range 790km, power=100 watts, noise figure at 3db, 2x 25 element yagies, and 22db antenna gain.
.
My comments; The last performance between two good sites can be extended to EME by increasing the path loss to as high as 260 db.
Take into account the sky temperature (N.F.), scatter loss off the moon, the RF power used at both ends, the antenna gains at both ends and the reciever noise figures and you can see what it takes in terms of DB to overcome the total losses to make a round trip.
This info is also in GRs book.
Last comment on this are that FM may be shorter than SSB due to bandwidth considerations and ability to hear a signal between the two modes and is even better using CW mode just as further compairisons.
Hope this offers some perspective as conditions are all important at both ends.
Ken.
« Last Edit: November 03, 2011, 10:00:22 PM by KM3F » Logged
WB6DGN
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Posts: 619




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« Reply #11 on: November 04, 2011, 05:38:06 PM »

Check out the following sites, they should provide what you're looking for.  While not foolproof, its about as good as it gets in my opinion.  There's a bit of a learning curve, somewhat steep at first (speaking for myself) but not insurmountable.

http://www.g3tvu.co.uk/Radio_Mobile.htm

http://www.cplus.org/rmw/english1.html

Tom DGN

I might add that when I first downloaded this program, the guidebook mentioned in the first link was not available; that should make things a lot easier to get started.  Tom
« Last Edit: November 04, 2011, 07:27:07 PM by WB6DGN » Logged
KB1GMX
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Posts: 828




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« Reply #12 on: November 05, 2011, 01:57:02 PM »

I've always used Range=1.4*sqrt(height) where height is the average height above terrain.

Path loss is only useful when the path is LOS and unobstructed.  It applies to frequencies
like upper UHF more and has to take into account antenna factor.  But in most cases though
the numbers look harsh you allow of antenna factor (gain) and you get a number that says
basically for this TX power you can expect this much signal at this range and with modern
FM or narrower SSB/CW radios this means enough signal is usually there unless the range
is extreme such as EME.  When applied to paths like space station at 10degrees above the
horizon the signals are often big as the range is not all that great.

For my 36ft height LOS (Line Of Sight) is 8.4miles to the radio horizon or about 17 miles to a similar station on the other side of the horizon. This is an approximation and I find it is conservative though useful. 

For the repeater I use (485ft height) that works out to about 30 miles and the mobile antenna
height adds another 3.  This is strict radio line of sight and is often what happens to those
signals are more prone to terrain, atmospheric effects and power used. 

What this doesn't figure on is scattering that adds range.   Things like tropospheric scatter and bending combined with terrain reflections off higher objects those numbers can be very small compared to range I often see.  The difference is what you can bet on and what  commonly
occurs in normal conditions.

That's as close to a rule of thumb but there are calculators for HAAT (height above average terrain)and LOS paths on line to explore this for any two stations with known locations.

In the end height is everything.


Allison
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