Line of sight

[WSGH675]

I been told that a gmrs handheld can reach line of sight to a repeater ok but there has to be a limit how many miles a gmrs handheld can reach a repeater. I know Hawaii is a long ways away but there is nothing but ocean between West Coast and Hawaii but there has to be a range how far a handheld gmrs radio can reach to a repeater 

[Socalgmrs]

Well first it does not matter repeater or simplex.  The radio is going to transmit the same to a repeater or simplex.   Second it’s line of sight.  You can’t see Hawaii from the west coast.  The earth is round.   But a 5w hand held has talked to the space station.  My $20 5?w hts do 50miles simplex each and every day. And 100miles or so to the local repeater (only more miles because it is up 2000’ higher and on a mountain.     
 

so how far a 5w ht can go is limited by line of site long before miles.  

[73blazer]

You'd never reach Hawaii from the west coast because there is no line of sight, the ocean itself gets in the way due to curvature of the earth. Unless your a flat earther, then you'd have no problems!

There really isn't a hard limit, it's all on what gets in your way, aka line of sight. With the earth aligned right, you could hit a repeater on Pluto as long as the sun or other planets don't get in your way. Radio to radio I can't get more than 2miles in the heavy woods, but from the same woods I can hit repeaters 30-40mi away. Once the signal gets out of the canopy, it will travel unimpeded to those towers.

If you really want an answer, you can read this paper. But generally speaking curvature of the earth is going to wreck any UHF (what GMRS is) signal if nothing else gets in your way first. So mabey 70-ish mi on flat open ground...if one side is higher, like on a mountain,  mabey more!

[WRHS218]

Well, unless you prescribe to the flat earth theory there is the curvature of the earth between the West Coast and Hawaii. UHF is a little more obstruction dependant than VHF but but are line of sight. 

[SteveShannon]

1 hour ago, WSGH675 said:

I been told that a gmrs handheld can reach line of sight to a repeater ok but there has to be a limit how many miles a gmrs handheld can reach a repeater. I know Hawaii is a long ways away but there is nothing but ocean between West Coast and Hawaii but there has to be a range how far a handheld gmrs radio can reach to a repeater 

People successfully talk to the International Space Station using a five watt handheld on 70 cm. That’s a couple hundred miles. 
 

[WRNU354]

2 hours ago, WRHS218 said:

Well, unless you prescribe to the flat earth theory there is the curvature of the earth between the West Coast and Hawaii. UHF is a little more obstruction dependant than VHF but but are line of sight. 

 I just visited the Flat Earth Society Discussion Board to see how they explain limits on radio propagation and me now stupider cuz it.

[73blazer]

27 minutes ago, WRNU354 said:

 I just visited the Flat Earth Society Discussion Board to see how they explain limits on radio propagation and me now stupider cuz it.

I spent some time looking through threads there back in the late 2010's, i'm convinced it's a place people go to practice their debating skills. Because they can't possibly believe half of what they say, the disc earth, the perimeter ice ridge protected by the worlds govt's  ...and the list goes on. Hilarious reading if you get bored in a meeting.

[Jaay]

4 hours ago, Socalgmrs said:

Well first it does not matter repeater or simplex.  The radio is going to transmit the same to a repeater or simplex.   Second it’s line of sight.  You can’t see Hawaii from the west coast.  The earth is round.   But a 5w hand held has talked to the space station.  My $20 5?w hts do 50miles simplex each and every day. And 100miles or so to the local repeater (only more miles because it is up 2000’ higher and on a mountain.     
 

so how far a 5w ht can go is limited by line of site long before miles.  

[Jaay]

3 hours ago, SteveShannon said:

People successfully talk to the International Space Station using a five watt handheld on 70 cm. That’s a couple hundred miles. 
 

Absolutely ... With NO obstructions in the way!

[WRYZ926]

3 hours ago, SteveShannon said:

People successfully talk to the International Space Station using a five watt handheld on 70 cm. That’s a couple hundred miles. 
 

 

1 minute ago, Jaay said:

Absolutely ... With NO obstructions in the way!

Though you do have to take into account the Doppler shift as the ISS moves across the sky. The Doppler shift causes the frequencies to shift as the ISS moves.

I know I know, too geeky for most 

[Jaay]

1 minute ago, WRYZ926 said:

 

Though you do have to take into account the Doppler shift as the ISS moves across the sky. The Doppler shift causes the frequencies to shift as the ISS moves.

I know I know, too geeky for most 

Nope, not too geeky at all, and absolutely Correct. I've got 9 ISS contacts under my belt, so far.  5 from my FT991A and a Comet SBB5 antenna at 50 foot, and 4 contacts from my FT70 talkie with a Signal Stick antenna . Of course the ISS app helps with tracking it overhead. 

[AdmiralCochrane]

3 hours ago, WRYZ926 said:

 

Though you do have to take into account the Doppler shift as the ISS moves across the sky. The Doppler shift causes the frequencies to shift as the ISS moves.

I know I know, too geeky for most 

Doppler shift maximizes contact time and clarity, but you can listen to most high passes without worrying about it

[Jaay]

1 hour ago, AdmiralCochrane said:

Doppler shift maximizes contact time and clarity, but you can listen to most high passes without worrying about it

I find that if I track the ISS 15 mhz. up and down from 437.800mhz I can hear it awhile longer before it's out of range.

[SteveShannon]

12 hours ago, Jaay said:

I find that if I track the ISS 15 mhz. up and down from 437.800mhz I can hear it awhile longer before it's out of range.

I’m pretty sure you don’t mean 437.800 +/- 15 MHz. 

https://www.qsl.net/ah6rh/am-radio/spacecomm/doppler-and-the-iss.html

Edited Sunday at 02:17 PM by SteveShannon
Emphasis to MHz added.

[Jaay]

19 minutes ago, SteveShannon said:

I’m pretty sure you don’t mean 437.800 +/- 15 MHz. 

https://www.qsl.net/ah6rh/am-radio/spacecomm/doppler-and-the-iss.html

That's Exactly what I meant. Listening from 437.815 down to 437.785 mhz. as the shift changes. This works well using the vfo on my 991a.

[GrouserPad]

My 50watt gmrs won’t talk 5 miles in my part of the country. 

[AdmiralCochrane]

2 hours ago, Jaay said:

I find that if I track the ISS 15 mhz. up and down from 437.800mhz I can hear it awhile longer before it's out of range.

That is exactly what "maximizes contact time" means

[SteveShannon]

9 hours ago, Jaay said:

That's Exactly what I meant. Listening from 437.815 down to 437.785 mhz. as the shift changes. This works well using the vfo on my 991a.

437.815 to 437.785 is 30 kHz, much less than 1 MHz.  So you’re shifting +/-15 kHz not MHz.

[WRUI365]

Math plays a big part in frequencies. That's like saying you paid $15,000,000.00 for something that is only $15,000.00

[WRTC928]

For an average-height adult, the horizon would be about 3 miles away if the earth were perfectly spherical and smooth. UHF waves aren't 100% line of sight; they do bend slightly due to the earth's electromagnetic field, but it wouldn't add much. So, maybe 3.5 miles if the earth were smooth. Barring anything else, that is the practical limit of a UHF radio. However, the earth isn't smooth and even a little difference in elevation can alter the line of sight substantially. From my house, I can't talk to the WalMart 2 miles to the southwest, but I have talked to another HT 15 miles away to the southeast, a GMRS repeater 25 miles to the northeast, and a 2 meter ham repeater 33 miles to the northeast. Two days ago, I had a conversation on a 2m repeater 20 miles to the north with 1 watt. The difference is elevation -- all the repeaters are on towers and my house is in a relatively high position. To my southwest, there is a hill about 30' higher than my house. I suppose there's some practical limit to how far you can communicate with 1 watt due to atmospheric and electromagnetic interference, but for any practical purposes, it's terrain that limits your range. From one mountaintop to another mountaintop with nothing higher between, 100 miles is attainable with an HT. Unfortunately, most of us aren't in that position. 

[Jaay]

7 hours ago, SteveShannon said:

437.815 to 437.785 is 30 kHz, much less than 1 MHz.  So you’re shifting +/-15 kHz not MHz.

I have my vfo set to 1 khz. increments, unless I use memories then it's 15 khz. at a time.

[Jaay]

7 hours ago, SteveShannon said:

437.815 to 437.785 is 30 kHz, much less than 1 MHz.  So you’re shifting +/-15 kHz not MHz.

Damn typos ...

[WSFF627]

Line of site theoretical vs practical, in theory in space with nothing to attenuate the signal, very, very far. ISS contacts happen all the time with 5w or less. In practical application on earth, it boils down to terrain, geology, height above average terrain, occasionally ducting, and the equipment in use. There are simply too many variables in play to say it will go X miles.

All transceivers and repeaters are not created equal. My Motorola's have the best receivers, my Baofeng's the worst. A repeaters performance varies greatly based on its antenna height above average terrain, antenna type, quality and gain, overall losses from feedline, duplexer and jumpers, if there is an RX pre amp, RF environment (other RF at repeater site) etc.

I can have a conversation on my repeater at 30 miles from some locations with a 4 watt Motorola portable. Standing in that same spot, a 5 watt, $15 Baofeng sometimes works, sometimes does not. I can also move 10 feet and it will no longer work. 

[nokones]

On 3/29/2025 at 7:34 PM, GrouserPad said:

My 50watt gmrs won’t talk 5 miles in my part of the country. 

Why? Do you live in Crater Lake or the Grand Canyon, or are you using a POS Pandaland antenna and cable with poor VSWRs?

[WRUE951]

Steve Shannon put it in perspective..  The ISS is a mere 250 miles above earth, not thousands that many may think..  Still,  250 miles is quite a distance for a 5W HT to reach..   I'm sure there are other factors transmitting into space but i can tell you with certainty, i have never been able to talk 250 miles with a UHF HT on earth.   The best i have done is roughly 80 miles hitting the El Paso repeater from Palmdale.  A line of site shot with that repeater sitting on a 4500' mountain top.    Has anyone really talked 250 miles on earth on UHF with a 4-5 watt HT?   If so you are the King.