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Repeater elevation to distance

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ChitheadDeSo

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Louisiana
If this is in another thread please point me in the correct direction... However I know somewhere in one of these threads someone posted about an equation that took the elevation and some other stuff and gave you a rough range that you may get. I don't remember this and I found a website that did it for you. Now the questions I have are... The website said that if I have the antenna at 300' I would get 21 miles of coverage, is this correct? The other question is will my actual power on the radio/ antenna gain affect that also, and by roughly how much?

I thought I remembered that equation having something about watts but I could be wrong. Any help would be greatly appreciated.
 

kb2ztx

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Feed Provider
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South West Virginia
There is alot more to this than watts. Terrain is a huge deciding factor. I have installed lots of radio systems over the years and have seen repeaters with 150 watts ERP only cover certain area's and areas 5 miles away have nothing depending on the terrain.

Can a UHF repeater at 300' talk 21 miles. Of course it can. But it depends on antenna make, model, direction, and ERP of the antenna. Most online programs factor this stuff in so its not a simple question to answer without knowing all the facts.

As example I have 1 GMRS repeater 150' up a tower. I get 30-40 miles coverage on my mobile radio. The repeater is putting out 25 watts but out of the duplexer I am getting about 20 watts. Now add in feed line from duplexer to antenna port, loss's in the poly phaser, then feed line up the tower to a DB404 omin directional antenna. The antenna has gain, but i have loss's of about 3.5 db of loss in the feedline, connectors and surge arrestors. Laslty as said the antenna is 150' up a tower...but its on a 2000' high hill.

Repeater 2 is on a 20' tower at my house, is hitting the antenna at 50 watts with a DB408 (double the gain of the 404) and I can't talk to the repeater from 10 miles away.

Ideally you want a balanced system. There is absolutely no need to install a 100 watt repeater at 500' up a tower if all you plan to use on it are bubble pack portables.
 

ChitheadDeSo

Member
Joined
Jul 20, 2016
Messages
48
Location
Louisiana
I do have my GMRS license so that is the classification I am planning to fall under. So, the information I am sitting on now is that I will probably go with the GR1225 all in one style repeater, I am not looking to use the repeater constantly just kind of a hobby with friends and for emergency situations. The antenna I was looking at an omni-directional with some gain on it (don't know how much yet I haven't really looked into that yet.) and it will hopefully be 20' off a 220' building that is 56' above sea level. and the surrounding terrain is all lower lying area pretty much flat. Do have some big trees haha, but they aren't tall really just big around... but anyway, I was thinking I'd push around the 25 watts on the repeater since I have read some stuff about if they are maxed out things will start to fail on them. I will be using a mixture of bubble packs specifically designed GMRS handhelds and a couple of mobile units (both in vehicles and as base stations)

I am always open to suggestions and all since I am still new to the radio world.
 

NC1

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Feb 8, 2014
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631
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Surry County, North Carolina
This site will show you your coverage on a Google map with reasonable accuracy just how far and where your signal will reach:

Radio Mobile WEB Site

1) Click "Radio Mobile Online"
2) Then click "Create New Account" (on left side of page)
3) Create your account with a Username and Password and follow their instructions.

I don't remember if I had to find my location by zooming in on the map to find my house to set a location, or if I entered my GPS coordinates.

You only have to set your location once and next time you log in you are ready to go with the "New Coverage" tab.

This program takes into account the following: (it already knows your terrain)
Base antenna Height, Type, and Gain.
Mobile antenna Height and Gain.
Frequency
Power
TX cable loss
RX cable loss
If you don't know your cable loss, this will figure it out for you: Coax Calculator

Enter all that information and click on "Submit". Wait a few seconds (could take up to 30 seconds to generate a map) and you will have a coverage map that you can modify any time you want. Keep in mind that it only uses Amateur radio bands and frequencies, but is close enough for other services as long as you enter the closest frequency possible.

This should keep you busy for days! I use it quite often. Hope that helps.
 

N4GIX

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Hammond, IN
It is CRITICAL to keep in mind that this "Radio Mobile Online" uses METERS only!
 

RFI-EMI-GUY

Member
Joined
Dec 22, 2013
Messages
4,323
If I am not mistaken POWER, ANTENNA HEIGHT, and EFFECTIVE RADIATED POWER, or ERP is controlled by your FCC LICENSE. It does not matter if it is GMRS, PRIVATE SECTOR or PUBLIC SECTOR. Below is a chart to help you GMRS Power Limits by Frequency and Station Class
you can actually exceed 200 feet AGL by having the tower comply with FAA regulations and comply with FCC ASR requirements. However In some cases, near an airstrip, you cannot put an antenna up at 200 feet.

Sent from my SM-T350 using Tapatalk
 

wa8pyr

Technischer Guru
Lead Database Admin
Joined
Sep 22, 2002
Messages
5,411
Location
Ohio
If this is in another thread please point me in the correct direction... However I know somewhere in one of these threads someone posted about an equation that took the elevation and some other stuff and gave you a rough range that you may get. I don't remember this and I found a website that did it for you. Now the questions I have are... The website said that if I have the antenna at 300' I would get 21 miles of coverage, is this correct? The other question is will my actual power on the radio/ antenna gain affect that also, and by roughly how much?

I thought I remembered that equation having something about watts but I could be wrong. Any help would be greatly appreciated.
Radio Line of Sight Calculator for use on VHF/UHF Ham Bands

This will give you an approximate "line of sight" distance. It's designed to figure the distance from your radio to the line of sight horizon, but if you put in the height of the repeater antenna and assume yourself as the horizon, you'll get about the same effect.

For example, a repeater antenna at 100 feet would give an approximate line of sight distance of 12.3 miles.

VHF/UHF Line of Sight Distance Calculator

This one will be a bit more accurate, as it allows you to put in the antenna height of both stations (you and the other guy). Assuming 100 feet for the repeater and 6 feet for you, the total line of sight distance is 17 miles.

On both of these, your mileage may vary; they both assume flat ground with no obstructions and don't take into account repeater TX power, terrain differences or antenna gain, but they're useful to get an approximate distance.

The formula that adds the other stuff is quite a bit more complicated and would be better figured out with something like Radio Mobile. However, if all you're looking for is a ballpark figure, the ones above should do the trick.
 

SCPD

QRT
Joined
Feb 24, 2001
Messages
0
Location
Virginia
Hey Chit….. As expounded by others, there is a lot that goes into determining how far a radio signal will go.
.
But if I read you correctly, you are looking for a simple rule of thumb to roughly gauge this range.
.
A simple formulae I have used for years is:
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Distance in Miles equals the Square Root of the Height above terrain, in Feet times 1.41.
.
This gives you roughly the distance to slightly beyond the optical horizon- which is the ‘radio horizon.’… remember, that the other station will be at an elevation too…
At this, you should expect a 99% chance of communicating.
.
So, for example;
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Station A’s antenna is 100 feet above ground, talking to Station B‘s, at 144 feet **
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The square root of 100 is 10, of 144 is 12 ( I like simple numbers)
Station A’s horizon is 14.1 miles, Station B’s is 16.9 miles.
Combine the two… 31 miles between the two will give expected coverage.
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You can see why an aircraft at 37,000 feet has such a tremendous VHF range. :)
.
.
…………………………CF
.
____________________________________________________-

.
** Let’s not factor in any other variables- consider this example to be over flat plains, or an ocean, and we are talking V/UHF.
 

RFI-EMI-GUY

Member
Joined
Dec 22, 2013
Messages
4,323
The problem with the estimators, versus those that use a terrain model, is that unless you live on a mountain top or your planet has the terrain of a bowling ball, your mileage WILL vary.

Sent from my SM-T350 using Tapatalk
 
Joined
Jun 16, 2013
Messages
3,577
Location
Texas
As suggested, using a tool to plot terrain is the best way to do it. Myself, I prefer a tool called Splat! over Radio Mobile.

However, if you live in a relatively flat area you can approximate range using the LOS formula and simply add 10% of the height for UHF. So the height you calculate is your height multiplied by 1.1. (VHF is a 1.25-1.3 multiplication factor).


Sent from my iPhone using Tapatalk
 
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