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VHF antenna

Old 12-06-2002, 04:49 PM
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I boat on the L.I.Sound, and for years I had a 36"
stainless steel VHF antenna. I never had any problems with reception and could always talk cross-sound to CT. Most of the boats seem to have
the 8' fiberglass antennas. I'm getting a new boat this year, and need to know if there are any advantages to the large antenna. (I do not go out in the ocean) .................TOMMY
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Old 12-06-2002, 08:45 PM
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Take a look at http://www.digitalantenna.com/ and http://www.boatantenna.com/ They have a 4' Antenna... It's a matter of db. Alot of information here. I once had a 3' whip, ss on my Micrologic Loran. Never used on on a VHF. Some one else may know. [img]/infopop/emoticons/icon_wink.gif[/img]
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Old 12-07-2002, 04:38 AM
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Thanks, those sites were good. one question though.They talk about power from 100 up to 350 watts. I may be wrong, but I think my radio only puts out 10 watts max.? [img]/infopop/emoticons/icon_confused.gif[/img]TOMMY
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Old 12-07-2002, 05:28 AM
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If your radio is fixed mouted, it probably puts out 25 Watts. If it's handheld, probably 5-6 Watts, max. The 100w, let alone 300w, is just the maximum watts that can be used. In general, as you have probably read, VHF is a line of sight thing...the higher the tip of the antenna the farther out you'll be heard. Also, in general, the higher the db the farther away you'll be heard. I'll leave it like that. Finally, again in general, the higher the wattage the more 'punch' you'll have to get through. So the trick is to maximize watts, height, and dbs...bigger is better! [img]/infopop/emoticons/icon_eek.gif[/img] [img]/infopop/emoticons/icon_wink.gif[/img] I like to carry a handheld (waterproof) as a backup. [img]/infopop/emoticons/icon_rolleyes.gif[/img]

[This message was edited by Mumblerone on 12-07-02 at 10:02 AM.]
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Old 12-07-2002, 05:40 AM
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That was easy to understand when written in plain english ! Thanks a lot.........TOMMY

BTW, where is Skaneateles N.Y.?
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Old 12-07-2002, 06:06 AM
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Tommy,

Here's the long version. I can do this because its early in the day and my fingers haven't got real stiff yet.

I'm gonna start with the basics and then lead up to why you want to consider a longer antenna.

First off, your radio, no matter what brand or model it is, puts out a rated 25 watts when its set on high power. I say rated because that's exactly what it is, the actual power it puts out is likely a bit less than that. There are a couple of channels you can set it on that would cause it to automatically drop down to 1 watt, but don't worry about them.

Every one of the channels that is preset in your radio corresponds to a specific frequency within the Very High Frequency (VHF) band. Keep this in mind for later. Those channels have frequencys that range from Channel 1 at 156.050 MHz all the way up to Channel 88 at 157.425 MHz. It also receives on the weather channels, which are between 162.550 and 163.275 MHz. So, as you can see, the total range of frequencys you're radio operates on is about 7 megahertz. Not a very wide range really.

Now a bit about antennas. The most basic antenna would be a simple point in space. No such antenna exists of course but we want to think about one like that for a few moments. If you had an antenna that was a single point (called unity) it would have a radiation pattern that, if we could see it, would look like a perfect sphere (ball). All of the power that radiated from it would exit from it in every direction with the same energy and at the same speed. Just as much power would go straight up as went straight down as went forward, as went behind, as went to the left, as went to the right. No difference. Such an antenna, if mounted on a boat would be waisting a lot of energy. I say that because when you're on your boat you almost never find yourself trying to contact airplanes (the power being radiated directly up) or to fish (the power being radiated down). So, it would be very nice if we could take some of that power that would be radiated up or down by our single point antenna and focus it out towards the horizon, where our intended target listener is located.

We can do that, and in fact in the real world we have no choice but to do that. We can do that because in the real world there is no such thing as a single point, unity, antenna. Here on the earth where we live anetennas are almost always made of a length of wire and that wire will cause the signal that is broadcase from it to be directional to some extent or another. That's because when you use a wire as an antenna the power is radiated out from its sides with almost no power shooting out the end of the wire. So if you have a wire sticking up into the air the radiation pattern will look something like this OO

Here's a bit of information for you, every transmitting radio must have an antenna that is of a particular length, and that length is determined by the frequency of the signal it will transmit. As the frequency of the transmitted signal gets higher the length necessary to transmit that signal gets shorter. This type of relationship is called inverse. Higher frequency = shorter antenna, lower frequency = longer antenna. Pretty simple really. Generally the length of the antenna has to come pretty close to equaling the length of the radio wave itself, or some multiple of it. When you see folks talking about Standing Wave Ratio (SWR) what you can think of it as is the ratio between the wave length of the signal at some particular frequency and the length of the antenna you are using. For instance if the wave length is 6 feet and you have an antenna that is 4 feet long the SWR would be 1.5, with the wave being 1.5 times as long as the antenna you are trying to get it to emit from. When determining the required length of an antenn you simply plug the frequency you intend to use into a formula and out pops the necessary length. As it turns out most antennas would have to be pretty long for most radios in general use. For our VHF radios the actual length would be somewhere around 6 feet if we were wanting to use an antenna wire that is one full wave length long. You might have noticed above that I said that the antenna length could actuall be some multiple of the wave length, well not only could it be a whole number multiple of that length but it can be a fractional multiple. So, our antenna, which must be about 6 feet long could just as easily be 3 feet long (1/2 as the multiplier) or it could be about a foot and a half long (1/4 being the multiplier). Almost all of our VHF antennas are actually 1/4 of a wavelength long as it works out in the real world, and means that the radiating element of your antenna is about 18\" long and that is encased inside (and usually going all the way to the tip of) that fiberglass tube, that could be of any length..

Now, I said that the antenna is a wire, but what you've got is a pole. That doesn't mean that I'm clueless here, it means that you can't see the wire. The wire is inside the pole, unless you have a metal antenna, in which case the pole is the wire, just thicker. There are actually two parts to the antenna inside that pole, one is the radiating element and the other provides what is called the ground plane. I am not going to go into why that is, just trust me on it.

Now think about that wire as compared to our single point antenna and at the same time we'll think about the effect of the multiple. Because the antenna is a wire the radiation pattern emitted from it will not be a big round ball. In fact it will be shapped a lot more like a pair of funnels back to back, something like this >!< with the antenna in the middle. So, what happened to that power that would otherwise have been shooting down or up? Easy, it got compressed and shot out to the sides of the wire. Because half of the power that would have been wasted is now going in a useful direction we can say that we have increased the effective power we are putting out. Remember, your radio only puts out 25 watts. But now, because half of the power that would otherwise have been wasted is being sent in a useful direction we can say that the effective power output comming off of our antenna is 50 watts, because its putting out the same power at the horizon that a radio twice its strength would have, if the twice-radio had been using a single point antenna. In order to compress the pattern going out of the antenna so that the effective power doubles you simply cut the length of the internal wire in half. If you want it to double again cut it in half again. Pretty neat, don't you think? But, remember, I said that our antennas are actually not compressed by 1/2, but that most of them are compressed by 1/4. And that means that even less of our output power is being shot out into space, and it means that our radiation pattern is being compressed even more and that much more of our power is being sent directly towards the horizon and not up into space or down into the water. Now a quick note about decibels, all they are is a way to measure things relative to one another. A decibel is not an absolute measure of anything, its a measure in comparison to something else, and all you really need to know is that when someone referes to them 3 is the magic number. Anything that has 3 decibels is twice as powerfule as anyting that has zero, and anything that has 6 decibles is twice as powerful as anything that has 3, and so on. So, a 6 dB antenna is one that focuses its power in such a way that it is 4 times (twice times twice) as powerful as our mythical single point antenna. It does this because as you'll recall its wire inside is 1/4th as long as its intended wavelength. Now please don't get me wrong, I have simplified this quite a bit, but in fact if you understand what I've just told you then you know more about it that 99% of the people on earth.

Now let's move on. Radios that operate on different frequencys and those that use different sorts of modulation (either Amplitude Modulation (AM) or Frequency Modulation (FM) have different characteristics - there's a surprise for you, different things act different ways). The most important characteristic of our VHF radios, which happen to be FM by the way, is that they operate by what is called Line-of-sight. That means that the radio wave itself travels in a straight line and does not bend to follow the shape of the earth. That has very definite implications for us. It means that we can not communicate with a target that has its receiving antenna over the horizon . It also means that taller antennas are better antennas for our purposes. That's because the taller our antennas are the farther out the horizon is for them. If you stand on a mountain top you can see a lot farther than you can from down in a valley. Its that simple. Now the actual distance to the horizon is very easy to calculate. You just measure the height of the antenna in feet, get the square root of that number, multiply that square root times 1.42 and the number you get is your half of the potential range of that antenna. I say your half because you have to do the same thing for the receiving antenna as well.

When you do the calculations for potential transmission distances for various lengths of antenna on both the transmitting and receiving antennas one of the things you come to realize very quickly is that our radios have a very limited range. Now, I want you to understand that there are some circumstances under which you can communicate at distances greater than the calculation would indicate, but that you can not reasonably expect that to happen very often and you sure can't depend on it to be happening when you most need it, in an emergency.

So, the bottom line is pretty straight forward, taller antennas are better for you on a boat than shorter antennas are. There are two ways to get a taller antenna and there's no mystery at all what they are. The first is to mount the antenna up high, making it taller, and the other is to use a longer antenna in the first place. If there are any other ways no one has explained them to me. By the way, I want you to notice that I did not mention that the power your radio puts out has anything to do with how far it will transmit. I'm gonna leave that alone for now but think about it.

And that's really about it. I'm going to give you a bit of practicle shit here for a moment though. Here it comes:

You have a small boat and I'm going to say that your current 3-foot antenna is mounted 4 feet above the water. That puts your current antenna height at 7 feet. We are not going to worry about the fact that the actual radiating element of your antenna is really about a foot lower than that, although some nit-picker is likely to tell us all about it. OK, you're is 7 feet up. You have a buddy with a boat that is just a bit bigger than yours is and he uses the same antenna you do. His total height is 8 feet because his is mounted a foot higher up than your is. OK, lets do the math. The square root of your 7 foot tall mast is 2.65, and we multiply that times our constant of 1.42 and we get your side of the distance equation of 3.76 miles. Now get the same thing for the other radio; the square root of 8 is 2.81 and that multiplied by 1.42 is 4.0 miles (notice that the additional foot in mounthing height added about a quarter mile to the communications range). Now add the two together, you find that you could talk to your buddy when the two of you are 7 and three quarters (3.76+4.0) miles apart. Of course that would be on dead calm flat water. You start bounching those two antennas around and the distance gets shorter, never longer.

So, with all that in mind what do you now know about how high you want your antenna? Easy you want it as long as practicle. For most small boats that means an 8' antenna and its why they are so very popular. It also means that you want to mount it as high up as you reasonably can. That should be obvious. You might have also noticed that most of the antennas that you see for sale are rated at 6 dB (decibles). You now know that is because its using a piece of wire inside that is 1/4th of a wavelength long and that happens to be about 18\".

Of course antennas have other characteristics as well. We're not gong to go into them other than to say that all antennas are not created equal. Its really mostly a matter of construction techniquest though and in a very real sense you get exactly what you pay for in an antenna. Without mincing any words at all I'm just going to tell you that the two best antennas commonly available for small boats are the Digital Antennas model 529VW and the Shakespeare model 5225 (in one of its several versions). There are other and better antennas to be sure, but if you pick from one of those two you will have pretty much maximized your quality per dollar.

There, did that help at all? If not shoot out your questions.

Thom

\"Just because you're on their side doesn't mean they're on your side.\"
(--Teresa Nielsen Hayden)

[This message was edited by Thom on 12-07-02 at 04:00 PM.]
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Old 12-07-2002, 06:30 AM
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Thanks Thom,
I hope you didn't get to tired! My old
Standard Horizon is a 1995 model in as new condition.It is always stored in my house. Have radios improved that much over the years that I should break down and buy everything new? If it were to make a big difference I would do that. All opinions are welcome. Any L.I.sound boaters out there? ............................TOMMY
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Old 12-07-2002, 06:55 AM
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Whew...Thom, couldn't have said it better. You filled in the blanks. Skaneateles is 22 miles SW of Syracuse, NY and is one of the Finger Lakes (there are actually six Finger Lakes, I don't know whos hand they were using. Thom, how many fingers do you have on your hand? [img]/infopop/emoticons/icon_biggrin.gif[/img]). [img]/infopop/emoticons/icon_razz.gif[/img] [img]/infopop/emoticons/icon_wink.gif[/img] Keep the existing radio.
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Old 12-07-2002, 07:21 AM
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I haven't been up that way in about 20 years!
I have camped in the area several times, can't remember the names of the lakes. But it sure is beautiful country up there!!! .....TOMMY
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Old 12-07-2002, 09:53 AM
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Thanks for the crash course- Radio 101. Very informative, and most appreciated.
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Old 12-07-2002, 09:58 PM
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Thom
That was a super explanation - thanks.
It begs the question, however, that on small boats that are bouncing around in medium to worse conditions, that it's transmissions on a 6db antenna will sound broken to recipients because the flatter shaped output is at times shooting into the sky and down into the water? And if so wouldn't a 3db on a decent (perhaps 6') mast be a better choice because it has a more forgiving pattern of broadcast? Not quite the range but unbroken transmissions to the boats that can actually hear him?

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Old 12-08-2002, 05:52 AM
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Chaps...theory is one thing; which is correct. I had a 9 db antenna on a 18' Whaler for ten years until I recently sold it. I primarily fished Lake Ontario and the Marthas Vineyard area. I never 'noticed' or was told my signal was breaking up, even in 4'-6' seas. I believe you will give up distance if you go to a 3db, etc. Orginally, and I'll stand corrected here, the 3db antenna was basically for sail boats, that were heeled over for extended periods of time (hours). Their signal would (with 6 or 8 db antennas) be in the water or into the sky, if you see where I'm going. I, at one time, tried to put my 8', 9db antenna on a 4' extention...worked great till the antenna broke...too much stress (whipping when running in seas). Let's face it, most of the time you can get away with a handheld...but if your building a fixed mounted VHF, let's maximize it. I currently have a 8' antenna, on the T-top. Of course, if you really have concern here, you can install two antennas and a switch box for calm or rough weather use. [img]/infopop/emoticons/icon_razz.gif[/img] I believe this would be unnecessary. [img]/infopop/emoticons/icon_wink.gif[/img]
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Old 12-08-2002, 02:22 PM
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shakespere has a great web site,even if you dont buy there brand.it is very helpful and spoke to tech people on web site,very impressed.Doug
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