There is a gentleman on another board named Thom. He is not only extremely knowledgeable, but he has a way of writing that takes a difficult topic and makes it very easy to understand. Hopefully this post will help settle the discussion with your Dad. Yes, it's long, but well worth reading. Below is Thom's post:
"I'm gonna start with the basics and then lead up to why you might 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. 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 reset in your radio corresponds to a specific frequency within the Very High Frequency (VHF) band. Keep this in mind for later. These 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, 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 very most basic antenna would be a simple point in space. No such antenna exists but we want to think about one like that for a few moments. If you had an antenna that was a single point 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 it, as went to the left, as went to the right. No difference. Such an antenna, if mounted on a boat would be wasting 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 antenna. Here on the earth where we live antennas are almost always made of a length of wire and that wire will cause the signal that is broadcast from it to be directional to some extent or another.
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. The 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 determining the length 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.
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.
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 shaped 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 coming 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. 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 and all you need to know is that when someone refers to them 3 is the magic number. Anything that has 3 decibels is twice as powerful as anything that has zero, and anything that has 6 decibels 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 frequency's 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 is 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. 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.
And that's really about it. I'm going to give you a bit of practical **** 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. 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. 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 bouncing 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 practical. 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.
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. It's 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"
After reading that, I will purchase an 8 foot Digital Antennas model 529VW.
I checked prices and found:
1. Brokenlegdave: $103
2. Bethel-Marine: $108
3. Consumers: $110
Hope this helps!
BoatGuy
(This post edited by BoatGuy on 02/14/2003)
