Here's another post inspired by a search hit. Some fine soul hit my blog today on a search for "expected range on a 2 meter handheld radio". This search would have brought our feckless reader to this post, which does not fairly answer the question. So, in the hopes that the next person to do this search actually learns something, I'll essay to answer this question.
The answer, of course, is "it varies". The amateur radio two-meter band is a VHF band, and as such is almost entirely line of sight. (Sporadic E skip, tropospheric ducting, meteor scatter, auroral skip, and earth-moon-earth are all quite difficult with an HT, so I won't dwell on them here.) So, basically, the range of any 2-meter radio is going to be limited by the horizon. However, because the atmosphere refracts radio waves considerably more than it does light, the effective radio horizon is about 15% further than visual horizon. So that establishes one of the limits on range: the receiver at the other end must be above the effective horizon. This is a function of four things: the altitude above ground of the transmitter, the altitude above ground of the receiver, the distance between the two, and the terrain between the two. The effective radio horizon for a given location, assuming flat terrain, is about 4.11 kilometers times the square root of the height of the antenna (in meters) above ground. So two stations with their antennas each one meter above ground will be in each others' horizons if they are closer than about 8 kilometers (a typical handheld-to-handheld case). If one of the stations is, instead, a repeater with its antenna 60 meters above ground, the range is nearly 36 kilometers. If one of you is atop the John Hancock Center in Chicago (344 meters), the range would be a whopping 80 kilometers. Of course, all of these are assuming flat terrain; if one of you in on top of a mountain, then that will also increase range, and if there's a mountain between you then you will be out of luck.
This isn't the end of the discussion, though. Not only do you have to have to be within each other's radio horizons, you also have to have enough power to survive path loss. Path loss represents the reduction in strength of the radio wave as it travels through space. Much of this is simply due to the inverse square law: as the wavefront grows in size, it occupies more space without having any more power, and thus has a lower power density. The receiving antenna's size doesn't change, so if it's moved further away it will receive less of the transmitted field. This is "freespace path loss", and if this is all you were facing loss would simply double twice with each doubling in distance (a loss of 6 dB for each doubling in distance, or octave). However, in real situations, other factors also contribute to loss, and in average conditions the effective loss in VHF is closer to 7 dB per octave, with a base at 1 km of about -71 dB. Most HTs will be able to just barely receive a signal at about -120 dBm, and should give suitable performance at about -100 dBm. This means that at 1 km, to have acceptable performance, you'll need to transmit a signal of -29 dBm. That's not even two microwatts. (In practice, you wouldn't get that far on two microwatts, because your HT's antenna probably sucks. More on that below.) Even at the 80 kilometers long-range case above, the path loss is only -115 dB; a signal of 15 dBm, or about 30 milliwatts, would be sufficient. Even with antenna losses, that's possibly within the typical 5 watt (37 dBm) capabilities of your average HT.
However, this discussion ignores one critical factor: the poor antennas that most HTs have. In practice the antenna of a handheld radio has negative gain in most operating environments, and you can expect to lose anywhere from 2 to 5 dB due to this issue alone. Obstacles (other than terrain) can also wreak havoc with the signal; if you're in a car, for example, expect to lose as much as 20 dB due to absorption from the car; other things that can reduce range include trees and buildings. Also, when transmitting within one wavelength of the ground (which for an HT is almost always) a significant portion of the signal is lost into the ground; this can account for up to 10 dB of loss.
However, it remains the case that in almost all cases, radio horizon, and not power, is the primary limitation on VHF range. So, in ordinary conditions, your 2m HT range is going to be about 8 km HT-to-HT simplex and about 25 to 40 km HT-to-repeater (depending mainly on the height of the repeater's antenna).
P.S. All numbers in this article may be wrong. I've tried to get them right but it's late and I may have made mistakes. If you do spot anything wrong, please feel free to hit me.
The answer, of course, is "it varies". The amateur radio two-meter band is a VHF band, and as such is almost entirely line of sight. (Sporadic E skip, tropospheric ducting, meteor scatter, auroral skip, and earth-moon-earth are all quite difficult with an HT, so I won't dwell on them here.) So, basically, the range of any 2-meter radio is going to be limited by the horizon. However, because the atmosphere refracts radio waves considerably more than it does light, the effective radio horizon is about 15% further than visual horizon. So that establishes one of the limits on range: the receiver at the other end must be above the effective horizon. This is a function of four things: the altitude above ground of the transmitter, the altitude above ground of the receiver, the distance between the two, and the terrain between the two. The effective radio horizon for a given location, assuming flat terrain, is about 4.11 kilometers times the square root of the height of the antenna (in meters) above ground. So two stations with their antennas each one meter above ground will be in each others' horizons if they are closer than about 8 kilometers (a typical handheld-to-handheld case). If one of the stations is, instead, a repeater with its antenna 60 meters above ground, the range is nearly 36 kilometers. If one of you is atop the John Hancock Center in Chicago (344 meters), the range would be a whopping 80 kilometers. Of course, all of these are assuming flat terrain; if one of you in on top of a mountain, then that will also increase range, and if there's a mountain between you then you will be out of luck.
This isn't the end of the discussion, though. Not only do you have to have to be within each other's radio horizons, you also have to have enough power to survive path loss. Path loss represents the reduction in strength of the radio wave as it travels through space. Much of this is simply due to the inverse square law: as the wavefront grows in size, it occupies more space without having any more power, and thus has a lower power density. The receiving antenna's size doesn't change, so if it's moved further away it will receive less of the transmitted field. This is "freespace path loss", and if this is all you were facing loss would simply double twice with each doubling in distance (a loss of 6 dB for each doubling in distance, or octave). However, in real situations, other factors also contribute to loss, and in average conditions the effective loss in VHF is closer to 7 dB per octave, with a base at 1 km of about -71 dB. Most HTs will be able to just barely receive a signal at about -120 dBm, and should give suitable performance at about -100 dBm. This means that at 1 km, to have acceptable performance, you'll need to transmit a signal of -29 dBm. That's not even two microwatts. (In practice, you wouldn't get that far on two microwatts, because your HT's antenna probably sucks. More on that below.) Even at the 80 kilometers long-range case above, the path loss is only -115 dB; a signal of 15 dBm, or about 30 milliwatts, would be sufficient. Even with antenna losses, that's possibly within the typical 5 watt (37 dBm) capabilities of your average HT.
However, this discussion ignores one critical factor: the poor antennas that most HTs have. In practice the antenna of a handheld radio has negative gain in most operating environments, and you can expect to lose anywhere from 2 to 5 dB due to this issue alone. Obstacles (other than terrain) can also wreak havoc with the signal; if you're in a car, for example, expect to lose as much as 20 dB due to absorption from the car; other things that can reduce range include trees and buildings. Also, when transmitting within one wavelength of the ground (which for an HT is almost always) a significant portion of the signal is lost into the ground; this can account for up to 10 dB of loss.
However, it remains the case that in almost all cases, radio horizon, and not power, is the primary limitation on VHF range. So, in ordinary conditions, your 2m HT range is going to be about 8 km HT-to-HT simplex and about 25 to 40 km HT-to-repeater (depending mainly on the height of the repeater's antenna).
P.S. All numbers in this article may be wrong. I've tried to get them right but it's late and I may have made mistakes. If you do spot anything wrong, please feel free to hit me.
I've found that 3/8 wave antennas are quite good for 2m HTs wrt size & performance.
ReplyDeleteOkay,
ReplyDeletePlease. Plain english for us newbies.
2 Meter, handheld in a downtown, urban environment. Obviously, there are variables, but what are we talking? A few blocks? A few miles? Better off going with an off the shelf, 5 watt 2 way radio and a GMRS licence?
To put it this way:
I'm in downtown San Francisco, about 2 miles away from you. You need a battery powered, handheld radio to get a hold of me while I'm on the bottom floor of a building. What would you suggest we use to chat?
THANKS!!!
thanks for being the first person to EVER answer this question with anything other than an obtuse "it varies". I just got my call sign and a 2 meter HT and have been trying to find out a rough idea of the kind of area it can cover. For example, I know of one repeater that I Rx VERY well, and I was wondering if it was hopeless to hit it. It is pretty far away, but 2700 feet above my elevation. It seems, acording to your explanation, it should be right on the edge, but worth a try.
ReplyDeleteyou have given me a lot to think about, thanks.
@Anonymous:
ReplyDeleteYou're welcome. I should update this post, as I've learned quite a bit about VHF propagation since I wrote it, but nothing in it is actually wrong. The main thing I left out is that VHF actually works over the horizon for quite a ways, thanks to diffraction. Predicting diffraction, however, is difficult.
On the specific question of working a repeater you can hear: Say you are hearing them at S9, and you know they are putting out 100 watts. Your transmitter is only 5 watts. Path loss is almost always symmetric, so that means if you're getting S9 at 100 watts, they'll hear you at S6 (because your 5 watts is ~12 dB down from 100 watts, and 12 dB is three S-units). Most repeaters will open for S6, so you'll probably get in. If you're hearing them at S6, they probably won't hear you (few repeaters will open for an S3 signal).
Of course, for this approach to work you need to know the repeater's output power as well as its receive sensitivity; it's unlikely you will get straight answers on either of these. However, what you can do is learn over time what the weakest receive signal strength from that repeater that represents a workable transmit position. This is just a matter of paying attention and being willing to make "test" transmissions from time to time.
Don't be afraid to experiment. Your license gives you permission to transmit. Do so; you won't learn if you don't try. RF propagation is complicated and while there are rules of thumb in practice the behavior in the fringe is really complicated and difficult to predict.
This is a very useful discussion.
ReplyDeleteI just passed the Technician exam and am considering my first radio. My primary area of use will be a coastal area with a collection of repeaters inland, at least one located on a mountain that is just beyond view, mostly due to the height of the trees. I can pick up NOAA broadcasts on a NOAA weather radio from a transmitter located near the repeater, and I'm wondering how much power I will need to reach the amateur repeater.
As a frame of reference, aircraft communications systems (VHF / UHF) commonly put out between 5 & 10 watts and have considerably more range than I need.
5 watts should do it, however the only way to know is to actually try it.
Thanks for the essay.
Jim Hudspeth
Call pending
I found 2 meters alot of fun especially while living in Honolulu, during rare tropo conditions I worked the West Coast(both repeaters and simplex) Santa Barbara to San Diego running just 2 watts from my handheld while standing on Waikiki Beach ! And while living in Los Angeles on occasion I worked Washington State on 2 meter SSB running just 25 watts and a three element beam ! Here in Arizona I have worked 2 meter mobile stations over 15 miles away running less than a half a watt thru a rubber duck on my qrp 2 meter handheld !To work 2 meter dx go to a high elevation either a tall building or a hill or moutain, Southern California take the Palm Springs tram to the top of the montain, great dx from up there, 1 watt I've worked repeaters in Las Vegas and Kingman Arizona !
ReplyDeleteBro... Thanks. The search that got me here was exactly as you posted. Thanks for using unit examples- ex: dbm to watt values. Seriously- very informative post.
ReplyDelete