Re: 9 DB Antennas
Any VHF antenna should be mounted high and in the clear. Think of the figure eight sideways and do that all the way around the antenna. Most marine antennas do not require a conterpoise like an automobile antenna, so mounting on a metalic surface is not usually required, but is acceptable if desired. You mentioned a pair of antennas and I am a bit confused of how they can be used(connected) together. As a radio engineer the only way two antennas could be used together is in a properly spaced(very critical) gain array which probably is not suitable for a marine installation. Putting two antennas which are connected together in the same field would cause the signals to combine and subtract in a unpredictable manner. I will not go into phase angles and other complicated matters, but I hope that you understand that two antennas connected together would perform such that communications in some directions would be impossible. This is due to interaction of the two signals being received (transmitted) by seperated antennas located at different points in space. Sounds complicated but think of how some waves on the ocean combine and others cancel out. A Decibel is a logrithmic expression which needs to be used and understood when we talk about antennas. Start with a 3 dB antenna which multiplies the strength of the signal on the horizon by a factor of two over a theortical explosion of that signal in all directions. You know like a fireworks burst in the sky. So a 3 dB antenna just doubles the signal strength in a given direction (along the horizon). After all, we don't need to send the signal from the boat straignt up into the sky or straight down into the ocean so we just take that part of the signal that would be wasted and direct it out on the horizon where we would like it to go. Then we use decibels to describe performance: 4 X for 6dB, and about 8X for a 9dB gain, to describe an antennas effectiveness at redirecting the signal from our undesireable fireworks blast. If a second antenna of 9 db is added to another 9 dB gain antenna this could only produce a theortical gain of another 3 dB or doubling of the signal(not 18 dB). But unfortuantely when we connect two antennas together we get some very undesireable results. Nature is complex and those signals do add sometimes but they also can subtract sometimes depending on direction and spacing of the two connected antennas. Rememeber we are only trying to get another doubling of the signal on the horizon with the second antenna (3dB) and in practice we end up doing a lot more damage in our attempt. Deep nulls can result that mean communication in a given direction is actually cancelled out. My suggestion is to mount the one 9 dB gain antenna as high as possible and try to place it above any metalic hull objects which can interfere with the antenna signal going out to the horizon. Fiberglass objects don't provide any attenuation and can be ignored. Radomes for radar several feet away don't cause any big problems as they present a small profile. Metalic masts several feet away, say 6 feet or so don't provide any real problems either, as they only slightly alter the pattern. I have not forgotten about the second antenna and it too can be used very effectively in a proper installation. Just don't connect it to the other antenna. It should also be mounted in a good location on the boat using the same guidelines as the other antenna. Then a switch (coaxial) could be used to switch the VHF radio to either of the antennas, one at a time, to take advantage of their unique location. An antenna switch with these postitions would provide a good setup: Antenna one, Antenna two. Now you have something very useful. Each antenna has a slightly different pattern with an advantage, due to mounting location. that could be recognized, noticed and used to the Captains' advantage. And best of all this arrangement produces no dead spots like the two array system. And finally the answer to your orignal question. This is the best utilization of a fine Christmas present. <br /> Other helpful info: A good coaxial switch is not expensive and even a marine quality CB one from your marine supplier or radio shack will provide good peformance. Silver plated is best but not mandantory, new or used, doesn't matter. I use a good quality "Bird" Watt meter to check antennas when I am through with an installation but have found CB meters are good enough to find VHF antenna installation problems. I always check SWR or standing wave ratio before pronouncing a job finished. You can borrow a CB SWR bridge from a friend if you don't want to buy one at Radio Shack. Remember to identify youself when you go key down on the mike and always use an unused non-emergency channel for your test. Good Luck
