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ContinuousWave: Small Boat Electrical
Shore to Boat VHF communication range
|Author||Topic: Shore to Boat VHF communication range|
posted 01-28-2007 11:13 AM ET (US)
Shore based VHF indoors with 8 foot antenna mounted on the roof (roof is 30 feet high). Boat based VHF with 8 foot antenna mounted on top of pilothouse roof (perhaps 15 feet high in total).
What would be the range in miles one could expect between these two VHF's for effective, sparing, saftey-oriented communication?
A smart man had this to say:
"The range is still determined primarily by the height of the antennas. The shore station will need a fairly tall antenna to be able to have decent range.
posted 01-28-2007 11:15 AM ET (US)
In working through the formula, how is it Coast Guard stations have what appears to be substantially more communication range?
posted 01-28-2007 11:20 AM ET (US)
I answered my question poised in my second post on this thread: the CG makes use of repeaters, etc.
posted 01-28-2007 11:56 AM ET (US)
Not only are repeaters used, but several CG VHF installations I have seen have been in the 100+ foot range.
posted 01-28-2007 12:43 PM ET (US)
Either there's something wrong with your formula or with the way you utilized it, I think - I have not gone thru your numbers *or* the formula itself, but I can tell you the following:
I understand (and your formula suggests) that VHF reception is based upon "line-of-site", so the curvature of the earth is what we're talking about here, at least theoretically.
The earth curves at a rate of about 8" every mile. So if on one end you have a 15' height to the top of your antenna, multiply 15' x 12 to get your height in inches, then divide the result by 8 because the earth curves 8" every mile: answer = 22.5 miles to the horizon.
Then on the other end you have a 30' height to the top of the antenna, so multiply 30 x 12 to get height in inches, then divide the result by 8 to get the distance in miles = 45 miles.
Add the two together and you get a total of 67.5 miles line of sight between the tops of the antennas, just grazing the horizon between them. I don't know where on a typical 8' whip antenna the waves are broadcast from, so that could easily be a significant issue. Let's say for the sake of comparison (worst case) two 8" whip antennas broadcast from their respective bases, and run the numbers.
We're then looking at 7' at one end and 22' on the other. 7' x 12 = 84 / 8 = 10.5 miles on one side. 22' x 12 = 264 / 8 = 33 miles on the other side of the horizon. Add the two together and you get 43.5 miles broadcasting from the *bases* of two 8' whip antennas, one the tip of which is at 15' and the other tip of which is at 30'.
There may be other issues here, such as the strength of the respective radios abilities to send or receive, I don't know. But strictly from line of site and the curvature of the earth, the above is what I understand.
Also, by way of practical example, I can offer this: This past summer, up in the North Channel, I was able to have a clear conversation sitting in my boat using the installed VHF radio, the top of the antenna for which is at about 15' high, on one end, with a friend who was a *measured* 20 miles away, standing on a rock with a hand-held VHF the top of the antenna for which *may* have been as much as 10 above the water on the other end, and there was a narrow but relatively high (40'-50') piece of land between us.
There must be more to the story than your formula suggests...
posted 01-28-2007 01:32 PM ET (US)
Now that I think about it, there was the narrow strip of high land (closer to me) between us *and* an even higher island (closer to the hand-held) between us...
posted 01-28-2007 02:31 PM ET (US)
Sal (et al) - Hold the presses!
I stand by my calculations above, insofar the curvature of the earth and line of sight are concerned, but the range of transmission/reception between two antennae apparently takes more into account when calculating than simply line of sight.
Shakespeare has an FAQ website in which they state that with 6db antennae, which typical 8' whip antennae would be, the correct formula *for each antenna* is as follows: Square root of the height of the antenna above the water multiplied by 1.15.
So let's do the numbers again, using this formula:
1. Square root of 38'(30' plus an 8' whip antenna) = 6.164 x 1.15 = 7.089 miles
2. Square root of 15' = 3.873 x 1.15 = 4.454
3. 6.164 + 4.454 = 10.168 miles
So Sal, your case of one antenna at 38' above the water and the other at 15' above the water should yield an effective communicating distance of 10.168 miles between them, according to the Shakespeare formula. I don't guess I know how we were communicating at 20 miles with one antenna at 15' and the other one at 10' - must have been sunspots or something...
posted 01-28-2007 02:29 PM ET (US)
The formula (that Sal quotes above) computes the "radio horizon". To see if two stations are within each other's radio horizon, compute the radio horizon for both and add them. If the two stations are separated by less than that distance, they will be in "sight" for radio.
The notion of a radio horizon suggests that radio waves do not carry beyond the horizon, but that is not true. Radio waves often do carry over the horizon, but the path loss is much greater on those paths. The mechanism of propagation varies. The radio waves may be bent by atmospheric conditions which arise due to weather. These phenomena occur sporadically in the e-layer of the ionosphere. Use "sporadic e-skip" as a search term to learn more.
Under conditions of unusual sunspot activity the f-layer of the ionosphere may support communications on the frequencies used for VHF marine, but this is also unusual. It is also subject to diurnal variation and tracks the sun spot cycle. See "sun spot cycle f-layer" as a search term.
Radio waves can also be bent by scattering or reflections from obstacles in the path. At frequencies close to VHF Marine Band, the moon has been used as a reflector, although the equipment needed to accomplish this is sophisticated and substantial.
Radio wave may bend and scatter from a variety of objects, including natural ones like a big rock--although not very often or efficiently.
Radio antennas for shore use are often mounted on tall towers, and the towers themselves advantageously sited. For example, in Georgian Bay there is a very tall hill just north of Wiarton. Atop this hill is a very tall tower. The antennas at the top of the tower have a radio horizon which probably approaches 50 miles as they are located probably about 1000 feet above the water level of Lake Huron. I believe there is a Coast Guard transmitter there. It gets out rather well.
In addition, modern radio systems use "trunked" systems in which many remote antenna locations are relayed to a central point. When you communicate with a trunked system radio station, the local antenna may be quite close to you, even though the person you talk to is many miles farther away.
posted 01-28-2007 03:46 PM ET (US)
The forumula John mentions and attriutes to Shakespeare antennas is bit more conservative in its range calculation. The one I mentioned and Sal refers to comes from the American Meteorological Society.
Their units are in meters and kilometers. If you convert you get the relationship I mentioned.
In actual practice your radio results will vary, and much depends on the intervening terrain. At sea, this is fairly predicable, but on land there can be obstacles.
For years we had a backup link to our network feed which consisted of getting the closest network affiliate off-air. We would use this in case every other source went dark. In actual fact, I don't we ever used it, but not too long ago someone was checking that off-air feed and found the signal had degraded. I was asked to investigate. A trip to the roof revealed the problem. A developer had built a new apartment building which was exactly in the line of the path to the remote station. We were effectively shadowed. I called the other station and got their precise LAT and LONG. I knew our LAT and LONG, and I calculated the azimuth from us to them. I went back to the roof with a hand bearing compass, and, sure enough, there was that big new high-rise apartment right in the path. No doubt about it.
You can use GOOGLE EARTH now to look at things like this.
In the old days radio engineers used a special chart paper called "four-thirds earth curvature" to plot radio paths. I guess the 4/3-factor (1.33) was to account for the "radio" nature of the horizon versus the optical.
posted 01-28-2007 04:12 PM ET (US)
Are VHF radios legal when used from something other than a boat? I question this as I mounted one in my motorhome sitting 10 feet from the surf a few years ago. It allowed my wife to call me when I was on the water in case of an emergency.
First time we used it the CG came on and told her to cease transmission. Told her she was breaking the law using the radio in something other than a boat.
posted 01-28-2007 04:16 PM ET (US)
I screwed up my calculations in my last post - I inadvertently neglected the 1.15 factor on the longer distance when adding the final numbers. The final numbers should have looked like this:
7.089 miles + 4.454 miles = 11.143 miles from 38' 6db antenna to 15' 6db antenna, using the Shakespeare formula.
posted 01-28-2007 04:19 PM ET (US)
You can use a VHF Marine Radio on shore, but you have to get a station license for it--and you probably can't get a station license unless you are a marina or some other commercial enterprise in business on the waterfront.
posted 01-28-2007 07:36 PM ET (US)
You can legally RECEIVE with marine band VHF radio on the shore without
a shore station licence, but it is not legal to transmit
without that licence.
posted 01-28-2007 07:57 PM ET (US)
I guess I am trying to achieve greater than 20 mile communication radius without relying on standard cell phones. I have some more research to do. Cell phone amplifiers and CB radio are two of the options mentioned to me. I should have just come and searched here first.
posted 01-29-2007 08:57 AM ET (US)
Please take note of the replies about shore stations needing a license. The FCC can and will go after little guys. And the fines are unbelievable. ($5000 I heard in one case).
posted 01-29-2007 11:14 AM ET (US)
Thanks for the clarification on that.
We moved the VHF radio to a little 12' Aluminum boat. Sat it on the beach and used that as our base station.
The USCG has, on a multiple of times come looking for it, knowing full well the transmission came from the camping area. With the boat sitting at the high tide line we continued using it in their presence. It usually gets a chuckle from them and us.
posted 01-30-2007 12:01 AM ET (US)
Another instance of the government searching for a "Phantom Menace". I find it ironic that they have chosen to search diligently for a VHF radio being operated from a point reasonably close to the sea shore. If you were using VHF-16 as a channel to have a general conversation, I could understand. Otherwise, it appears they are bored with too many toys. Get an inflatable 2 man raft and set it on the motorhome roof. The added height will help with range.
Back to the original question: http://www.paccrst.com/downloads/application_notes/AppNote_UHF_VHF_Calc. pdf
You will note after reviewing the above PDF that the radio horizon is merely one part of the equation. Then you start looking at the gain of your transmitting and receiving antennae and the loss in your antenna cable, and ultimately even the sensitivity of your receiver. In Florida, it is not uncommon for the USCG to use a 500' antenna mast. I'm all for the guys who would ultimately need to come rescue me having a VERY tall antenna!!
I find it odd that everyone is using a different constant in their equation. http://en.wikipedia.org/wiki/Radio_horizon
posted 01-30-2007 12:09 AM ET (US)
You don't need to leave CONTINUOUSWAVE to get a thorough explanation of how radio circuits can be calculated. Just read my article in the REFERENCE section:
Marine VHF Radio Communications
posted 01-30-2007 12:09 AM ET (US)
Shakespeare used a "constant" of 1.15 for 6db antennae and a different one for 9 db antennae. Wikipedia didn't seem to differentiate, using a single "constant" of 1.23.
posted 01-30-2007 12:17 AM ET (US)
If you read the Wikipedia article, you will see that different constants ARE NOT being used. The article first gives constants for the optical horizon, which assumes no bending of the radio waves, but radio waves do bend, and that is why the radio horizon is greater than the optical horizon. The Wikipedia article then goes on to cite a constant of 1.415, which is for all purposes the same as I mention, 1.42.
Further, articles in Wikipedia can be created and edited by anyone, so you cannot always assume they are completely authoritative. That article does seem well-written and cites reasonable references.
posted 01-30-2007 12:23 AM ET (US)
Let me repeat something from my article, which I think is the most important point:
"Antenna height will be found to be the primary determinant of communication range between two recreational vessels."
Now compare this to the other reference cited above wherein some fellow says:
"The easiest and cheapest ways of increasing the range in an RF data application come from careful selection and placement of the antennas."
I don't really see a great deal of divergence of opinion. If you want your radio to work well and have good range, the most important element is the antenna and its location."
posted 02-02-2007 03:57 AM ET (US)
The average range of the MANY over the counter radios/antennas Ive used on my (prior to cell phone) ship to shore VHF setup was 25 miles on high power with the squelch turned up.
posted 02-02-2007 08:44 AM ET (US)
And the average range of the many 25-watt marine VHF radios I've owned has been nowhere near 25 miles. I am on the "rocky" coast of Maine and a lot of line-of-sight to other stations is indeed blocked by land features.
You can't make a generic statement about range, unless you clearly state that it is over water with nothing in the way.
posted 02-02-2007 03:05 PM ET (US)
25 mile AVERAGE. Columbia River.
base station ant on top of a 2 story house adjacent to the River approx 350 ft elevation.
The mountains that border the twists and turns of the Columbia River both east and west of this location average 1000-3000 ft elevations...there are dead spots, certainly.
After 10 years of this, you can even diagnos if their radio has a power shortage or their antenna has problems based on the average location a well maintained radio & my base station could hear eachother Lima Charlie.
NOTE: on the safety aspect, the base station is only as good as the radio on the vessel trying to contact it.
BEWARE: do not transmit your base station on USCG safety channels without a FCC licence or extreme emergency. You can "get by" using working channels temporarily without a Ship to Shore Licence if you do not abuse it but FCC rules have changed and you probably should look into it if you plan to use ship to shore on a regular basis.
posted 02-02-2007 03:24 PM ET (US)
MORE ON TRANSMITTING FROM A UNLICENCED BASE STATION.
I operated my base station under one of my accounts FCC licence years ago. VHF 66. WYJ 7914.
2007: USCG is upgrading their antennas to be able to triangulate a vessel transmission on ch 16. I dont remember what the project is called right now but all new radios have a distress button and all you have to do is flip it, they will find you. I assume (absolutely no proof) logically that these antennas will be able to handle a bunch of VHF channels...and thus, will be easier to locate unlicenced landside base stations and BUST em.
Temporary solution. Dont abuse it. Imitate your base station as another vessel. Dont talk about anything to the contrary...unless it is an emergency.
Coincidentally, on occasions I could hear those guys 90 miles away. I didnt report them to the CG. Was some interesting conversations. hehe.
posted 02-02-2007 03:53 PM ET (US)
Rescue 21 USCG Upgrade
posted 02-02-2007 10:41 PM ET (US)
Anecdotal reports of radio communication between commercial shore stations and commercial boats may make interesting reading, but rest assured that that laws of physic are the same for those circuits as for recreational small boats.
posted 02-03-2007 03:09 AM ET (US)
exactly Jim, I think my disclaimers (to be a radio tech) are clear. Hell, I cant even figure out how to program my land line answering machine even with the directions in front of me. But Ive been using vhf Marine radios over 30 years.
sounds like you think Im full of "anecdotal" BS.
DID I EVER make a reference the tech numbers were not helpful.
wouldnt you say the combination of the two (bench math & field testing) should give Sal A the tools to make a decision whether or not to invest in a base station for SAFETY reasons?
The only thing commercial about the friggin post is my experiance.
posted 02-03-2007 09:07 AM ET (US)
Let's examine the anecdotal report:
Base station elevation = 350-feet
Ship elevation = 0 (sea level)
Radio range between the two = 34.7-miles (calculated)
The observed results are entirely consistent with the underlying mechanism of radio wave propagation that is required to produce the results. What's all the fuss about?
posted 02-03-2007 12:27 PM ET (US)
I felt you were disputing my generic numbers.
long day/night with Dad wasnt good.
I based my mileage on 2.5 hr ETAs going 10 knots.
posted 02-03-2007 04:38 PM ET (US)
Thank you all.
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