Re: Now, on to Marine Band radios
This is such an easy question to give a short answer to and such a great question to ramble on and on about .... and you know I'm gonna do it.
<br /><br />OK, the short answer is to just make a list of the current models by ICOM and put them in order by price. Pick the one that suits your pocketbook. You absolutly can not go wrong by choosing one that way.<br /><br />Now what not to do, do not pick a VHF to match your other electronics. Do not buy based on price - because this is without a question the most important piece of safety gear on the boat, bar none (the PFD is your's, not the boat's).<br /><br />Now I'll make it even easier, get the 502A. Not the plain 502, but the very recently released revision to the 502 that has DSC output as well as input. If you don't know why you want that it's OK we'll get to that when you come back and ask how to connect it to your chartplotter and what DSC is all about.<br /><br />Now on to the antenna. Everyone has antennas by Shakespeare, and that is just fine. A few foklks have antennas by a company in Florida called Digital Antennas (brand name, has nothing at all to do with digital signals). Digital's antennas cost just a bit more than the top of the line models by Shakespeare and they are better made. They were well worth every dime of the price.<br /><br />Now about antenna heights, or lengths, and range, and gain, and all that stuff.<br /><br />No matter what anyone tells you, and there are a lot of myths out there about radio range, the maximum range of our VHF marine radions is a function of the curvature of the earth, not the radio's power. Because the earth is close to round the maximum range can be calculated, and calculated quite easily actually. <br /><br />Here is how you find the maximum range of 2-meter FM signals, which is what the marine band is part of. First you find the height of each antenna involved, both the sending and the receiving. Take each height, measured in feet, and find its square root. Mulitiply that number by 1.53. Do the same for both antennas and then add the sums. The total you come to is the maximum range that mix can communicate out there on the water. The range is far shorter than most folks will claim.<br /><br />Here is a link to a chart that I made up that shows you the range possible with various heights of antennas. Please note that the first column should have had 4 feet at its top label but somehow that got moved over. Here it is:
Matrix of Antenna Ranges <br /><br />What you get to see quite quickly when you do the calculations is that antenna height is everything when you want to communicate. That's why far more 8' antennas are sold than 4' versions.<br /><br />So what about power? Well, for one thing its fixed by regulation issued by the FCC (which in turn is in accordance with international agreements for all marine radio worldwide) to 25 watts output on most channels and only 1 watt on some others. So they all put out the same power .... sort of.<br /><br />Cheap radios have one thing in common, that is that their power will fade out if you hold the mike keyed for an extended period. Sometimes that can be as little as a half a minute. I also know of one very well known brand that a friend who own's an electronics shop tested right out of the box (14 units tested for power output) and not one of them put out over 12 watts even though they were rated at 25. Those were NOT ICOM radios.<br /><br />OK, gain. Gain in the sense used here is a multiplyer and its a relative strength where each increase in gain of 3 doubles the effective power output. Gain is achieved by artificially flattening out the signal, which otherwise would be emitted in a perfect hemisphere from the radiating point. By compressing that ball of power in half the effective power is doubled - its like focusing the beam of a flash light, half the area covered means that the smaller circle of light is twice as bright. So a 3 dB gain antenna would make a 25 watt radio act like it was putting out 50 watts, and a 6 dB antenna (the most common designation) would double that again, so our 25 watt radio would act like it was putting out 100 watts.<br /><br />So, you gotta ask, if the range is a function of antenna height what do you care what the gain is? Well, here's something you may not have noticed about your VHF radios. You never hear two guys talking at the same time. Its not like the AM radio in your car late at night where you can hear a dozen stations from all over the universe talking at the same time. With the boat's radio you hear the one that has the strongest remaining signal when it gets to your antenna, and that strong one blocks out the others. That is the up side to gain, it makes your radio act like it has more power than it does and that puts you on top. There is a down side though. As that signal is compressed further and further it becomes more and more directional. So in a rocking boat a lot of the time high gain antennas will have you shooting some of your energy out into space and some of it into the water. Now, this isn't really a problem with 6 dB antennas, but when you get up there to the 9 dB and above sticks it becomes important.<br /><br />Oh, and you should know this too. When you buy that 4' or 8' or even 3' antenna what you don't see is that the actual antenna is not really that long. It is made up of a piece of wire inside the white fiberglass tube that is about 18" long. Its held up in the tip end, usually by a foam plug and some adhesive (unless you get one by Digital, in which case its completely supported and it won't be a length of wire, it will be a brass rod).<br /><br />So, how's that for a start?<br /><br />Oh, if for some reason you don't like ICOM, and there is no good reason on earth for that to be the case, the only other company who you want to consider at all is Standard Horizon.<br /><br />That help any?<br /><br />Thom
