Re: VHF ant info
Let me see if I can give you some sort of understandable explaination of gain and height and construction methods to help you out. Let's start with the hard parts and move to the easy ones.<br /><br />Gain. Gain, in antennas (it is a word that means different things in different context) simply referes to the focusing of the radiation pattern comming from the antenna so that more of the power is sent out in one plane than in any other. <br /><br />Let's begin this part by simply saying that you have the perfect VHF Marine radio and that it puts out 25 watts of power at all times when it is transmitting. The power which is produced in the radio is sent to the antenna via a 2-part cable. The cable (coax) has a center conductor that is insulated from an outside braid which is further insulated from the outside world. While this isn't true I want you to think of the center wire in the coax as being Positive (+) and the shielding as Negative (-). The coax connects the radio to the antenna and the power is emitted somewhere from the antenna. Next I want you to think of the place on the antenna where the power is emitted as being a single little dot, a point in space. All of the power that is emitted from that single little dot goes out in ever direction with equal force (25 watts) and so if you could see the power's radiation pattern it would be a big ball, perfectly round. That sort of antenna would be described as "Unity" and because there is no focusing of the power comming from that little point in space we would say it has a gain of Zero (0). That unity antenna with its zero gain is the reference point from which all other antennas are measured. Remember, no focusing of power and a radiation pattern that is as round as a basketball. No such antenna actually exists, its just a theoretical reference point.<br /><br />With our unity antenna just as much power will to straight up as goes straight down and if its on your boat about a third of the power from your radio will be wasted because its shooting off into space and about a third will be wasted because it is shooting down into the water (from which it will 'bounce' and be reflected to some extent back into space.<br /><br />It is possible, by way of the design of the antenna itself, to focus the power from an antenna. When this happens the radiation pattern will be flattened so that the power is focused. Instead of it looking like this 0 it will look like this ( ). Now the briefest explaination of what is meant by Decibel in this context. All you need to know is that for every 3 dB effective power is doubled, I'll explain in a few moments. If the antenna is constructed such that half of the power that used to shot off into useless directions is now focused into one direction it can be said that the effective power of that antenna has been doubled. There is still just exactly as much power as you began with, its just that now it is all being directed in a useful direction rather than half of it being lost. So we would say of such an antenna that it has a gain of 3 dB. The effect of this is that if you were at a receiving station at some distance away and were able to measure the power from a transmitting station it would seem to be comming from a radio that had 50 watts of power, even though our little VHF is only putting out 25 watts. So that is the effect of gain, it refers to the focusing of power, resulting from the design of the antenna, and its effect is to artificially increase the effective power output from a radio. If you were to further compress the outgoing signal from our 3 dB antenna you could focus more power and direct it at the horizon, in our case, and narrow the radiation pattern in half again and have the effect of once again effectively doubling the outgoing power, so that our 25 radio would seem to be a 100 watt radio at the receiving station. Oh, and if we did that second doubling of the radiation pattern the dB rating of the antenna would increase by another 3 dB, or be the typical 6 dB antenna that is sold all over the place.<br /><br />From that it would appear that it might be desirable to buy the highest gain antenna you can find. After all, if the effective power doubles with each 3 dB increase in gain then the more doubling the better, right? Well, no. The problem with continously increasing the gain isn't in the antenna or radio, its in the nature of boats. Boat's bounce, move side to side, and move up and down fore and aft. An antenna mounted to a boat does too. The power that is emitted from your boat's antenna comes off of the stick at a 90 degree angle to the direction the antenna is pointed. So if the antenna is pointed straight up, as it should be, then the power is directed out generally parallel to the water, towards the horizon. This is good. If your boat were able to stay level all the time and your antenna were to point straight up in the air all the time you would want all the gain you could get. Unfortunately it doesn't work like that.<br /><br />If your boat rolls to the side, lets say that 4 people all go to one side to look at a fish at once, and you pick up the mike to call your buddy to tell him about the fish all at the same time, what happens to the radio signal? Well, les't say that the boat leans way over as everyone goes to one side. Now the power being radiated from that antenna is being directed in some strange directions. The radiation pattern still has the power emitted at a 90 degree angle to the stick, but now that will be focusing a lot of the power down into the water on one side of the boat and up into space on the other side. The greater the gain, which is to say the more the outgoing power is focused, the worse this effect is. That is why you don't see extremely high gain antennas being used on small boat. The smaller the boat the more it rocks and the more it rocks the more power will be lost because it is directed into a useless direction.<br /><br />For practicle purposes most small boats are best served by using an antenna of about 6 dB gain. It is a great compromise that balances the desire for higher effective power with the reality of instability in small boats.<br /><br />As long as we're on this subject let me give you a gross over simplificaton of how this gain increase is accompished. You may have heard the term wavelength used from time to time when refering to radio signals. If you think about signals being waves then the distance between the crests of successive waves is the wave length. It is one of the characteristics of radio waves that the higher the frequency of a signal the shorter the wave length. In the case of our VERY HIGH frequency (VHF) radios the length of the wave we send out is about 6 feet. In making an antenna the radiating element should be matched to the wavelength, but it can also be some fractional multiple of the wave length. So if you had a marine VHF antenna that had radiating element that was about 3 feet long it would be described as a half wave length antenna because the length of the radiating element would be one half the length of the wave of energy that is being emitted. Guess what the gain of such an antenna would be? 3 dB. Have you noticed that most of the antennas in the catalogs are quarter wave antennas? The radiating element in those antennas is about a foot and a half long, one fourth of a wavelength. What do you suppose happens to the radiating pattern when you shorten the antenna from 3 feet down to half that, a foot and a half? Easy, you get a 6 dB antenna out of it. Pretty amazing how those two things are related, isn't it?<br /><br />Now I want to tell you something that I expect and hope you willnever have to use. If you were caught on a terrible storm and your antenna broke off and you were taking on water but couldn't call for help because of he loss of the antenna what could you do? Well, if you took the coax from the radio and skinned back the insulation from about a foot and a half of the center conductor and then another foot and a half of the braided shielding you could use it for an antenna. You would have to make sure they were not touching, the center conductor and the shielding, but you could tape the center to the tip of a fishing pole and then pull down the shielding in the opposite direction so it was center wire pointing straight up and braid pointing straight down, and you could transmit and receive with it. Some of the cheap antennas made by Shakespeare and other manufacturs aren't much more than that internally, just the coax stripped back, center up and shield down, and held in place with little foam blocks and silicon sealer. Really. The much better made antennas use metal radiating elements, usually brass rods, and other devices to improve transmisson and reception.<br /><br />Height. Higher is better. Very High Frequency radio waves transmitting frequency modulated (FM) signals do not bend to follow the coutour of the earth. They will bounce off of some things, but the upper levels of the atmosphere is not one of those things. So there is no 'skywave' bounce from them and the signals are often refered to as being "Line of Sight". That means that your signal can't drop below the horizon. The greatest limiting factor the the range of our VHF radios is antenna height. Power almost never has anything at all to do with the range we have available to us. If you could increase the power of your radio from its current 25 watts to 100 watts, or even a thousand watts, it would not increase your maximum range by a single inch. You are limited by the line of sight. The higher the tip of your antenna the farther out the horizon it can 'see'. On most small boats the only way to get the antenna higher is to buy a longer antenna. Most of us don't have a standing superstrucure to mount antennas to to get them higher. For most small boats the largest practicle size antenna is about 8 feet. For some folks shorter antennas are required. In any event the higher the antenna the greater your range will be. It is that simple.<br /><br />You can determine the maximum range you can expect to see from your radio if you know the height above the water for both your antenna dn the heigh of the station you are trying to communicate with. It works like this, you find the height of your antenna (in feet) and then take the square root of that number and multiply it times 1.54. Then do the same for the other station's antenna. Add the two numbers together and it will give you the maximum range, in statute miles. at which those two stations could communicate. It is a lot shorter distance than many people will tell you.<br /><br />I want to comment on that and it is something that just makes me furious. If you read these internet boating pages for any length of time at all you will find all manner of foks who tell you that they routinely communicate with other boats that are 50 miles away, or 100 miles away, or sometimes even more outragous claims are made. I get mad as hell because I think this BS makes some new boaters believe they can safely goes miles and miles off shore and away from other boats because they have great range in which they can call for help. It is simply not at all true. You will be lucky under normal circumstances to talke to people more than about 15 miles away and if you both happen to be on 20-something foot boat that distance will be more like 10 miles. Now there are some unusual atmospheric conditions (called atomspheric ducting) that will increase your range, but you can not count on them, cerntainly not rely on them with your life. So, every time you see a message by some guy that says he regularly talks to someone else 50 iles away you might just want to skip on to the next message, because you are being fed a line of pure BS by a guy whos advice just might get you into the last serious trouble you will ever see.<br /><br />Construction methods. Its a fiberglass tube with some wire in it. On the bottom its got some sort of fitting that allows it to be mounted. That is all our antennas are. How much better or worse is one fiberglass tube than another one? If find it interesting that if you go to the Shakespeare main home site one of the first things that jumps out at you is that they make a lot more than antennas. What they make is fiberglass tubes, for every purpose under the sun. Antennas are just one of the uses. So, the tube really doesn't matter much, after all, how much difference is there between the best made one one earth and an average one? Nothing you or I would particularly notice. Maybe better materials, maybe a little thicker or thinner, but not much. How about the furrell, the thing on the end where you screw it into a mount? Well, you get two choices, plastic or stainless steel. I'll leave it up to you to decide which is stronger. Then there is the internal stuff. Remember the stripped back wire on a fishing pole? That is what a $39 antenna from anyone is like inside. You know those somewhat expensive antennas by Shakespeare, the Gallaxy line? They all use stainless furrels, they all use some sort of spiral wrapped glass in the tube (better I suppose), they have a nice finish, but more imporantly, they use brass radiating elements that are soldered at all connections. The only complaint I have about Shakespeare antennas is that the elements are usually not very sell supported within the tube. Grab most any Shakespeare antenna and give it a good hard shake. Hear anything ratteling inside? You will never hear that with an antenna made by Digital Antennas (my favorites, a little more money but worth it).<br /><br />And that is about all that comes immediately to mind. Did I give you enough to ask some more quesitons?<br /><br />Thom
