Antenna height

[rlyons3]

Hi everyone. I’m pretty sure I’m answering my own question but just looking for confirmation.  My base is 130 feet above sea level. I’m trying to reach the other side of my town and the elevation between me and what I’m trying to reach is 380 feet than it dips back down.  Would my antenna need to be 250 feet in the air to do what I want to do? 

[BoxCar]

GMRS operates on a line-of-sight basis. If your transmitter antenna can't "see" the receiver's antenna i a straight line, then they will not communicate. There are websites that will provide you with a path outline base on the gps location and elevation of both antennas showing any obstructions such as hills.

[WyoJoe]

Here is a website where you can enter two points to see the path between them. It also allows you to make adjustments for elevation of the antenna.

https://www.scadacore.com/tools/rf-path/rf-line-of-sight/

In my experience, the RF path for GMRS isn't purely line of sight. I have generally been able to get over or around small hills successfully, but not bigger hills.

Your success in reaching the other side of town will largely depend upon how big the "dip" is on the other side of the high spot. If it dips down to 350 feet, you'll probably be fine, but if it dips back to 130 feet as it does on your side, probably not.

[rlyons3]

42 minutes ago, BoxCar said:

GMRS operates on a line-of-sight basis. If your transmitter antenna can't "see" the receiver's antenna i a straight line, then they will not communicate. There are websites that will provide you with a path outline base on the gps location and elevation of both antennas showing any obstructions such as hills.

Anyway you can send me a link to them sites? 

[rlyons3]

9 minutes ago, WyoJoe said:

Here is a website where you can enter two points to see the path between them. It also allows you to make adjustments for elevation of the antenna.

https://www.scadacore.com/tools/rf-path/rf-line-of-sight/

In my experience, the RF path for GMRS isn't purely line of sight. I have generally been able to get over or around small hills successfully, but not bigger hills.

Your success in reaching the other side of town will largely depend upon how big the "dip" is on the other side of the high spot. If it dips down to 350 feet, you'll probably be fine, but if it dips back to 130 feet as it does on your side, probably not.

Ya it dips back down to 130/150 feet. 

[WyoJoe]

6 minutes ago, rlyons3 said:

Ya it dips back down to 130/150 feet. 

This will probably be tough to overcome from you location. Your best bet would be to use a couple of hand-held radios to check reception in the various areas you're interested in. This would allow you to find out what works and what doesn't.

Putting an antenna 250 feet in the air is not very realistic. The cost to build and maintain the tower, antenna, and feedline would be significant, and even then, it would only get you even with the top of the hill, perhaps not high enough to "see" into the dip on the other side.

A better solution, in my opinion, would be to build a repeater site on the top of the hill (or close to the top), if possible. If you're not familiar with repeaters, the video below explains them pretty well:

https://www.youtube.com/watch?v=b-kR1MSgr9k

 

[axorlov]

Generally, true, GMRS s line of sight. But I can see there are some taller, steeper hills around your location. You may have path around instead of path over. You would need to experiment, as suggested already. Also, if both stations are fixed locations, simple corner reflector may help a lot, or other directional antenna.

[wrjc901]

My repeater antenna 50' in the air at 452' where its planted. So 500 at to the top. 
The kicker to the south of me is a Ridge thats 550' with 60' trees on top of it. 

Dead south to south east over that ridge is terrible. With a 50w radio it gets enough signal scatter to hit the repeater out to 10 miles. 

But to the west/south west it can talk 25 miles. Ridge tops 50 to 60 miles. 
GMRS in some places seems to fold over the hills between here and Conway. 
There is places in town you can hit the repeater but very few because so many ridges. 

Over 200' tower, you need a light, and get approval from the FAA. 
Then a simple pole is not likely to make it that high. 
I'm using heavy top rail for Chainlink fence, and at 50' its a real challenge. 
I got all the hardware 1/2" hardline, etc, I could goto 100' even the extra pipe. 

Now getting it up there is the issue, and can't find an affordable lift to rent. 
In a few weeks I get some more cable, may try to ginpole a 125' up, 
using the guy cables. Ruined a few poles trying in the past. 
I really want to be able to hit the freeway and the city south of me at 15 miles. 

[JohnE]

I would run a propagation map to see what it will take to achieve that and if it can be before pushing any higher

[wtenney24]

New user here. I was wondering how much of the antenna would need to visible in line of sight. The reason I ask is I use CalTopo for all of my off-road mapping and navigation. It’s a very powerful program that I barely scratch the surface of, but it is capable of running viewshed analysis. In short, you choose a location, set an “eye level” height, and the program shades everything that is visible from that point. There’s a repeater near where I’ll be camping soon and I wanted to run the viewshed to see where I’d be able to ping the antenna (if need be). So my question is what “eye height” should I use? Let’s say the antenna in question is 100’. How much of the antenna would need to be visible to successfully receive?

[BoxCar]

In radio communications with a repeater there are 2 measurements that need to be done. From the repeater site you do a "talk out" or coverage map of the repeater taking the terrain into account. The other measurement is the "talk-in" measured from the mobile to the repeater. Again, you need to factor in the terrain. The program I used to use had an annual license fee of $1500 and we used it to coordinate Part 90 public safety frequencies. At UHF frequencies like GMRS, the signal will refract from hilltops and ridges allowing some signal to "bleed" into and out of canyons and valleys. As you are estimating 100 feet for the height of the repeater antenna you can assume any part of the antenna will allow a signal to be received. We normally used 10 feet for the height of a mobile antenna assuming one having gain mounted on a vehicle roof.

6 hours ago, wtenney24 said:

New user here. I was wondering how much of the antenna would need to visible in line of sight. The reason I ask is I use CalTopo for all of my off-road mapping and navigation. It’s a very powerful program that I barely scratch the surface of, but it is capable of running viewshed analysis. In short, you choose a location, set an “eye level” height, and the program shades everything that is visible from that point. There’s a repeater near where I’ll be camping soon and I wanted to run the viewshed to see where I’d be able to ping the antenna (if need be). So my question is what “eye height” should I use? Let’s say the antenna in question is 100’. How much of the antenna would need to be visible to successfully receive?

 

[JohnE]

I have found radio mobile to be pretty accurate once you dial it in correctly.

[wtenney24]

18 hours ago, BoxCar said:

As you are estimating 100 feet for the height of the repeater antenna you can assume any part of the antenna will allow a signal to be received.

 

That’s what I was looking for! Thank you very much! And yeah, the viewshed in CalTopo is a far cry from real radio propagation programs. But it’ll let me identify some key locations that I could send an emergency transmission from if necessary.