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VHF Marine Radio Range
|Author||Topic: VHF Marine Radio Range|
posted 01-01-2005 02:09 PM ET (US)
I am a new classic Whaler owner studying to purchase my first VHF marine radio.
The Shakespeare antenna website says VHF antenna range is calculated as follows:
I can't find any hard data like the above antenna formula for radio range. It seems that radio [power] output has very little to do with range. So a handheld radio at 5-watt may broadcast as well as a mounted 25-watt model, if they use the same antenna.
Does anyone know of a reason not get a handheld 5-watt radio instead of a mounted 25 watt radio?
posted 01-01-2005 04:49 PM ET (US)
The formula you quote is for line of sight distance to the horizon. For example, the center of my 4' antenna, mounted 5' above the waterline is SQRT(7)*1.42= about 3.75 miles.
So if I'm talking to a similarly equipped boat, our maximum range is 7.5 miles before the curvature of the earth interrupts the line of sight between us.
However, if I'm talking to a base station with a 8' antenna on top of a 60' tower, on land 6' above my water level, his distance to the horizon is SQRT(70)*1.42= about 12 miles, added to my 3.75 to the horizon = about 15.75 miles total distance, before the curvature of the earth interrupts the line of sight between us.
Radio signal strength increases with the square root of the power increase (i.e. about 2.25 times stronger with a 5 times stronger 25W than a 5W), at a given distance. It also decreases with the square of the increase in distance. In the examples above, the tower is 2.25 times the distance from my boat as from my boat to the other boat, and would require 5 times the power to put the same received signal strength at the tower 15.75 miles away as to the other boat 7 miles away. Many towers are even taller, with greater line of sight distances possible.
That's why I have a 25W fixed mount Icom on our Whaler. I want that USCG base station to get a good, strong signal rather than have to depend on another boat to receive and relay my call.
posted 01-01-2005 07:42 PM ET (US)
"For example, the center of my 4' antenna, mounted 5' above the waterline is SQRT(7)*1.42= about 3.75 miles."
Boy, on reading it again, I sure worded that poorly. I should've said,
"For example, with the base of my 4' antenna mounted 5' above the waterline, the antenna center is 7' high, so my line of sight distance to the horizon is SQRT(7)*1.42= about 3.75 miles."
posted 01-03-2005 01:55 AM ET (US)
At the frequencies used by VHF Marine band radios, the propagation of radio waves tends to follow line-of-sight behavior, and there is little bending over the horizon. This accounts for the formula you have cited above as being the principal determinant of the range of a VHF Marine radio.
You must also compute the range of the other station in the circuit, based on their antenna height. The sum of the two is the approximate range of communication.
In general the path loss on a 2-3 mile circuit typically found in VHF marine use will not be so great that there will be a fabulous difference between a 5-watt radio and a 25-watt radio. However, if the circuit is marginal, the extra power of the 25-watt radio can make a significant difference.
I would never recommend a portable radio as a replacement for a fixed-mount 25-watt radio for these reasons;
--poor battery life in portable radio
By the time you engineer a solution to all the problems of a portable radio, such as powering it from the boat's battery and using an external antenna and loudspeaker, you will be much farther ahead with a fixed-mount radio.
Most fixed mount radios are less expensive for a radio of similar quality than a handheld radio. A good commercial grade handheld radio is about $800. A very decent VHF Marine fixed-mount radio is about $150.
posted 01-03-2005 07:52 AM ET (US)
As usual, Jim and Moe have aptly "nailed" the technical information that answers your question. I will come at this more from a layman's stance. I could understand someone perhaps not wanting to deal with cutting holes in their console, running coax cables, mounting antennas and the like. They might reason instead that for their particular boating habits, a hand-held is better even though it might be a bit more expensive. After all, just put batteries in it, stow it below, and its there when and if you would need it in an emergency. Where you boat and how far from shore you venture plays another important part in your decision. If I were back home on Kentucky Lake and that's where I did all my boating, I'd probably go for a handheld to tell you the truth. Navigating on the ocean is a different story altogether. In my case, I live on a tiny island in the South Pacific (Guam) and go off-shore out to about 12 miles on at least a weekly basis. In this situation, not having a good fixed-mount VHF with a good antenna puts you in a "world of hurt" if anything goes wrong. You can quickly drift out to sea and out of VHF range in a short period of time and there's very little land and miles and miles of ocean in this region.
Bottom line: If you go off-shore, a fixed-mount is the only way to go. If you only boat in lakes and rivers (EXCLUDING THE GREAT LAKES), perhaps the handheld is ok for you.
posted 01-03-2005 08:15 AM ET (US)
All the math aside, I had an experience a while back where
a buddy of mine with hand held was out of gas (with his
then ex-wife on board ;-) and in very marginal communication with
the CG with a hand held. I was actually farther away from
the CG antenna, and had no trouble relay communications for
him with my 25 watt console mount with 8' antenna.
I carry both the console mount radion AND a handheld.
posted 01-03-2005 01:11 PM ET (US)
Even though we're seldom more than 5 miles from an island, and rarely that far from another boat, we're often 15-20 miles out from the USCG station, and towing services, on the Lake Erie mainland. I just want to be able to make a direct distress call to them, and have gotten a good radio check from them that far out.
We've also wanted a handheld as a backup, as Chuck has. This year, we'll be mooring our new sailboat on a small, few thousand acre lake, and I just can't see buying a fixed radio for that, so we'll get that handheld. I'd like to have a fixed mount for it if we ever get into towing it up to Lake Erie, but I'm concerned about the lightning rod effect with an antenna on the mast top.
posted 01-04-2005 12:04 AM ET (US)
My dad is a communications engineer and also maintains his radio licenses. I asked him to look at another thread here (about cell phone boosters) but some of his reply focused on VHF radios. With his permission, I am posting his comments here. Paragraphs 4 and 5 pertain to radios:
"I had a long dissertation on the economics of engineering cellular coverage but I scratched it. The point really is that a handheld cellular telephone has about 0.5 watts power input to the antenna. That is very little power so you should not expect great range.
One of the threads mentioned people using the 'bag' type telephone and they do have much higher power; about 3 watts.
If that is an issue look for a used bag phone on e-bay and you might get greater range.
On the other hand you have a radio designed for fairly long distance transmission. I would concentrate on keeping it and the entire antenna circuit in good condition. Corrosion will kill a radio signal tying to get through a connector. Also it might pay to have a radio technician check the installation to be sure the antenna is installed properly andthat the antenna is matched to the transceiver.
There are several tests they can make. One of them is SWR; standing wave ratio; to check the match. The better the match the better the power is used in getting your signal out.
Save your money on a cellular antenna booster."
posted 01-04-2005 11:48 AM ET (US)
How about this for stupid?
I keep whacking my 8 foot antenna, and I'm getting tired of buying them for $50+ dollars. So I hacksawed the buggar off around 3 feet and bent and stuffed the wire in the larger hole. The radio still works, but I haven't tried her out at sea yet. Will this seriously impede my signal strength? Would cutting the copper wire affect reception and transmitting power?
posted 01-04-2005 02:30 PM ET (US)
yes, yes, and yes
posted 01-04-2005 02:33 PM ET (US)
I'll close my mouth now.
posted 01-04-2005 02:35 PM ET (US)
Something that Sheila's dad brought up inspires a quick question about the advisability of cutting the Coaxial cable at the antenna base, adding a connector so that the antenna can be removed without removing the antenna cable.
Does that make sense, and what precautions should be taken to protect signal integrity if such an action is taken?
posted 01-04-2005 02:37 PM ET (US)
I just hope that before you TRANSMIT you realize the damage that the VSWR (reflected vs incident power) can do to the output transistors of your radio.
Not a really good thing to do. Your antenna should be cut at specific mulitiples (fractions of multiples e.g. 1/2 WL, 1/4 WL, 1/16 WL) for least damaging VSWR.
posted 01-04-2005 02:39 PM ET (US)
Sorry, Buckda posted while I was composing my answer.
Also, cables should also be cut at wavelengths as well.
posted 01-04-2005 05:31 PM ET (US)
The key word in the statement by Sheila's dad is "Corrosion". Many antennas have a connector at the base of the antenna rather than a length of coax running to a connector. The main thing is making sure the connection is properly water-proofed so there is no chance of corrosion.
posted 01-04-2005 08:11 PM ET (US)
I haven't actually cut the wire yet, but the wire likely touches itself as I stuffed it down in there with needle nose pliers. After cracking it a bit more, I cut it again and stuffed it more carefully. I heard an emergency call when out in SF bay recently testing the motors(I keep my handheld cliped to my chest. The weather still comes in good. My fishing buddy lives about 2 miles from my house and we're loud and clear on the radios. I guess I need a rubber whip ammune to my bull in a china closet ways!
posted 01-04-2005 08:20 PM ET (US)
If I do the math right, the wavelength of Ch 16 is 6.2 feet.
VHF cables are going to be LONGER than that, not shorter.
And everything I've read says VSWR is not a problem in VHF
like it is in CB.
posted 01-04-2005 10:25 PM ET (US)
I shoulda been more clear. The antenna (radiator) is usually a fraction of a wavelength. The lead wire is usually a multiple of the wavelength.
And 17bodega, ANY length of wire will work on the receive side -- its the transmit side where VSWR becomes a consideration (I would still say concern, but I'm an EE).
PS I worked on a Navy system in the '80s that had a wavelength of about 2500 miles. Our transmit antennas were in Michigan and Wisconsin and were about 53 miles in length. With about 10 MW pumped in, our effective radiated power was about 3 W. (I know this has to be unclassified by now because I read about it in a Tom Clancy book)
posted 01-04-2005 10:34 PM ET (US)
The black helocopters should be landing in your backyard soon!
posted 01-04-2005 11:30 PM ET (US)
If I do the math right, that Navy system would be 74.4 Hz,
not KHz, not MHz, Hz. And I'll bet about 15 bits per second.
And punched through saltwater a bit.
posted 01-04-2005 11:48 PM ET (US)
you guys went over my head a while back. ;-)
At this point I'm wondering how much shoo goo to slop in the hole. I'm the mouse in the cat in the hat waiting to cut the phone wire. Not funny when you need a radio at sea..
Thanks for all the feedback.
posted 01-04-2005 11:50 PM ET (US)
Ahem, are you guys saying there are two sets of wires? One for transmit and one for receive? Maybe I should read the shakespear website? Crash course?
posted 01-05-2005 12:35 AM ET (US)
In the past, before my long suppressed passion for boats took over, I was totally immersed in radio communications, both professionally (as a broadcast engineer) and as an avocation (amateur radio). I also was an associate editor of one of the more respected technical periodicals in amateur radio communications for several years.
Many people in the business of selling antennas (and some quite successful at it, too) can't make a good antenna, so I would not recommend that most boaters just start tinkering with their marine antennas. It is unlikely you will happen upon some arrangement that works substantially better than what you can buy for $35.
Many people in the business of writing about antennas (and, again, some quite successful at it, too) don't really understand them correctly, so I don't recommend taking as gospel everything you read about antennas or feed lines. There is a great deal of misinformation floating about.
Claims about antenna performance are particularly difficult to prove or disprove, so much so that the most prominent periodical in which many antennas are advertised (QST magazine) long ago adopted a policy of not publishing any claims about antenna gain or patterns, with the exception of the antenna VSWR plot.
If you are the sort of fellow who periodically breaks the top three feet off of an 8-foot antenna, I recommend you purchase less expense models and replace them as they break.
posted 01-05-2005 07:39 AM ET (US)
17, You may want to consider just continuing to replace those 8-footers as you break em off. I say that as sub-$100 for a good 8 footer is way better than we can get em for here. I just bought a Shakespear 5225-XT locally for $150 and that was at a discount. Such is the issue when living on an island and not having the cheap UPS ground transport mode that many of the way cheaper online stores use. I could have bought that same antenna at onlinemarine.com for about 85 bucks but the UPS Air would have been another $150 - OUCH! I asked if they would ship it good old first class U.S. Mail. They would for about $20 but said if it gets broken in shipment, I'm on my own since those long flimsy cannisters don't provide adequate protection from rough handling in standard mail system. I thought better of it, sucked it up, and payed nearly double what it costs back there. Just thought I'd give you an alternate perspective on those "expensive" $85 antennas:-)
|Knot at Work||
posted 01-05-2005 08:25 AM ET (US)
I subscribe to the KISS theory, Keep It Simple Stupid.
I can expound on all the theories of RADIO, RADAR Waves and square root formulas or max output and effects of environmentals on your super and sub refraction, etc... all that will do is make me feel important and will not really answer your question.
So keeping with KISS method, I recommend you just go buy a 3 ft Stainless Whip antenna, with ratchet mount, mount it on the stbd side of your center console and take satisfaction in your choice for diversity on the small Whaler, the ability to transmit and receive to a decent range and not have to break out a calculator to figure how big the human head ways.
|Knot at Work||
posted 01-05-2005 08:29 AM ET (US)
Correct please to "Weighs"
posted 01-05-2005 10:00 AM ET (US)
Good thread and very timely I note.......
Installing a T-top on my 22 and already have a 3 ft shakespeare SS wip antenna that I intend to install on the top of this bad boy. By my calculations, it should work just fine here on the Chesapeake, seeing that it will be mounted on a top that is 8 feet or more up from sea level.
I to have snapped/replaced enough of the 8 footers to realize that it could happen again especially out of site on top of the T-top.
Any comments on this setup....? I figure my range will be in the neighborhood of 3-4 miles line of sight, unless the the boat is pitching excessively in heavy seas.
Thanks in advance
posted 01-05-2005 11:14 AM ET (US)
Again thanks for the info. I'm surprised that more folks here don't go with the shorter whips. My particular problem with the "montauk" size boat and rails is the 8 footer extends beyond the rear rails when tied down for storage. When I reach in the boat to grab bait, gear or attaching the boat cover, then snap! The hard fiberglass shell is the problem. I'd pay $250 for a "flex" type that was stiff enough to stand up. That would be well worth it to eliminate the need to replace regularly.
I'll have a chat with our local electronics guru (Fred Fritz Electronics in Petaluma for you Bay Area homeys) and report back his thoughts on the matter.
posted 01-06-2005 07:29 AM ET (US)
17, in reference to the 8 footers extending beyond the rear rails, what about an install up on the forward rails (either side). Just ensure it is mounted at least 8' forward of the rearmost section of rail - thereby ensuring there will be a means of mounting a support mechanism. Of course I say this not really knowing what year of boat you have so maybe this won't work. I can say this works fine in my '05 170 though.
posted 01-06-2005 08:32 AM ET (US)
dagpaddle - where'd you go?? Has your question been answered?
I subscribe to the "more is better" and the "be prepared for the bigger things and the smaller things will take care of themselves" schools, I guess - I'll go with a bigger antenna for better transmit and receive capabilities every time. If and when the time comes that I really need help, I want all the cards stacked in my favor...
We on our Montauk, as well as a number of others here on the forum, use an 8' antenna on a rail ratchet mount afixed to the aftmost side rail stanchion on the starboard side. The antenna can be laid down and velcroed to the stanchion stand-offs when not in use, and that way you don't have the opportunity to crush it by stepping on the gunwale.
posted 01-06-2005 08:34 AM ET (US)
Oops - that should have started with, "dawgpaddle" - sorry...
|Knot at Work||
posted 01-06-2005 08:55 AM ET (US)
My Montauk uses a 3 ft SS whip in the Gulf Of Mexico.
No Problems with reception or transmission.
Knot at Work, OUT
posted 01-06-2005 01:13 PM ET (US)
I've considered the forward rails and may go that route. I'm also seriously considering Knot's approach, but will first consult my local expert and discuss our local seas and terrain and find the best all around solution. My new Yaesu (Standard Horizon)radio is great though. West Marine is putting in a store a couple miles from my house! It's gonna be hard to stay away!
posted 01-07-2005 02:49 AM ET (US)
A high quality (expensive) 8 foot antenna is the best way to go. Ask any marine electronics installer. Yes, a 3 foot antenna will work, but for $50 more why not get the best on the market?
Here's a picture of the 8 foot antenna that is in the lowered position.
posted 01-08-2005 12:09 AM ET (US)
alot of good info, thanks! this week i've been to Venice LA fishing in the marsh around the mouth of the Mississippi River.
we usually fish in pairs of boats, kinda watching out for each other. i hope to get a 25 watt radio before i get too far from the marina in my 36 year old 16'7" boat, with a 7 year old 70 hp motor.)
i'm still reluctant to cut a hole and mount a radio in my newly remodeled wood console. i may start with a handheld radio and a good 8' antenna, and later use the handheld as a backup when i upgrade to a 25 watt.
posted 01-08-2005 01:34 PM ET (US)
For more discussion of VHF Marine Radio communications see:
Marine VHF Radio Communications
posted 01-08-2005 02:57 PM ET (US)
Another well-written, quality article! Thanks for taking the time to put it together, Jim.
posted 01-09-2005 06:53 AM ET (US)
I use a rail mounted 8 ft fiberglass antenna for my DSC Standard Horizon set on my 36 y.o. Nauset. I boat at the mouth of the Chesapeake Bay so I'm about 12-15 miles from the nearest Coast Guard stations. I do patrol work with the CG Auxiliary and must be in contact with a station every hr. so good comms is a must. I found that the standard 15 ft long cable that comes with the 8 ft antennas won't reach my console mounted VHF. I run the cable through flex tubing along the gunnel and then through the hull tunnel so I had to add a cble extension to make it reach. Because the coupling is inside the tunnel it gets wet every time I am out. I keep a check on it for corrosion and will be replacing it this season. I carry two handheld VHF radios as a backup but they don't have the range to reach the towers consistantly. Something to keep in mind is that boat antennas will deteriate over time if exposed to the elements so they will need replacing from time to time wven if not "broken". an antenna is like a car tire, the time to replace them is BEFORE they go flat due to age or wear, not when the problem occures... sepecially when you are 15 miles from land.
posted 01-09-2005 11:03 AM ET (US)
The Shakespere Galaxy VHF antennas have 20' cables. On my
Montauk, that's long enough to get from the antenna mounted
on the aft end of the starboard rail to the radio on the
console. 15' wasn't enough, though the end of the 15' cable
did make it out of the tunnel and into the console.
While the fibreglass of the antenna does break down over time
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