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Boat Forum / Electronics / July 2007AIS Transponder Antenna Placement
I will be pulling my mast and am planning on adding a second VHF antenna for a dedicated AIS receiver and eventually an AIS transponder. However, I'll end up with both of my VHF antennas next to one another. I suspect that this isn't a good idea. Any suggestions as to how to handle this? -- Geoff > I will be pulling my mast and am planning on adding a second VHF antenna > for a dedicated AIS receiver and eventually an AIS transponder. However, > I'll end up with both of my VHF antennas next to one another. I suspect > that this isn't a good idea. Any suggestions as to how to handle this? > > -- Geoff Mount them vertically, e.g. collinear. Perhaps one on the spreader. Bob >> I will be pulling my mast and am planning on adding a second VHF >> antenna for a dedicated AIS receiver and eventually an AIS [quoted text clipped - 7 lines] > > Bob I don't have spreaders as I have a carbon fiber mast. -- Geoff > I don't have spreaders as I have a carbon fiber mast. > > -- Geoff Keep the antennas away from that mast, which acts like a big RF resistor and absorbs, not reflects, RF signals.... The Metz Manta 6 requires no ground plane at all and will work just fine atop that mast and at least 12" away from it further down. Larry  SignatureWhile in Mexico, I didn't have to press 1 for Spanish. While in Iran, I didn't have to press 1 for Farsi, either. It just isn't fair. I use a little coiled ariel, about 6" long similar to the ones on a hand held VHF. It is about 20 feet up the mast and picks up pretty well everything within about 15 miles. Simply mounted on a right angled bracket riveted to the mast. Normal whip VHF ariel is on top of the mast. I believe a 2m (6ft) vertical separation is suggested as a minimum Phil >> I don't have spreaders as I have a carbon fiber mast. >> [quoted text clipped - 6 lines] > > Larry > I will be pulling my mast and am planning on adding a second VHF > antenna for a dedicated AIS receiver and eventually an AIS [quoted text clipped - 3 lines] > > -- Geoff The radiation pattern of the antennas is a donut out horizontal, perpendicular to the whip. Off the ends of the whip is a line of little to null signal. So, if you use VERTICAL separation, with the two antennas directly over/under one another, the coupling between them is negligible. There will be some interference, depending on how much vertical separation you can provide. The farther apart, the better. I'd like to suggest putting one antenna at the top of the mast, such as a Metz Manta 6 sticking up above the mast. Then another Metz Manta 6 hanging upside down from a shroud support about a foot from the metal mast. It won't foul the sails in line with the shrouds and will have "some" vertical separation away from the other VHF transceiver. If you have a radar mount on the mast, hang a Metz Manta 6 upside down from the bottom of it as far out from the mast as you can get it. This is also a good place for a second VHF 1/2 wave antenna. They will interfere with each other anywhere you put them but the signal on the receiving antenna will be as low as you can get it and certainly not dangerous to the receivers' front end amps. The interference comes from signal re-radiated by all the metal rigging up there, which forms parasitic antennas making your radiation pattern just awful and certainly not a donut...(c; Larry SignatureWhile in Mexico, I didn't have to press 1 for Spanish. While in Iran, I didn't have to press 1 for Farsi, either. It just isn't fair. >So, if you use VERTICAL separation, with the two antennas directly >over/under one another, the coupling between them is negligible. There [quoted text clipped - 6 lines] >mast. It won't foul the sails in line with the shrouds and will have >"some" vertical separation away from the other VHF transceiver. Will the donut shape suffer from the absorption by the shrouds and stays, or can that be neglected? >If you >have a radar mount on the mast, hang a Metz Manta 6 upside down from the >bottom of it as far out from the mast as you can get it. This is also a >good place for a second VHF 1/2 wave antenna. But be careful when hoisting large foresails - they can get stuck or, worse, pierced by the downpointing antenna. >parasitic antennas making your radiation pattern just awful and certainly >not a donut...(c; Picking up the crumbs... :) Cheers, U. > Will the donut shape suffer from the absorption by the shrouds and > stays, or can that be neglected? Any metal objects that are a significant portion of a wavelength will re- radiate all the RF that passes by them, inducing RF current in them. The Yagi beam antennas use this phenomenon to produce their directional pattern, as do many other antenna arrays with parasitic elements. The re-radiated RF from nearby metal objects, such as the sail rigging, has its own radiation patterns happening at different phases than the radiating element's output. The vector sum of all these various re-radiated patterns adds and subtracts from the main antenna's pattern, creating an effective pattern that looks just awful! There are huge nulls in the donut where the various patterns cancel each other in certain directions. In other directions, the combined pattern actually has more signal than the original. As the boat rotates in azimuth, so does this pattern, making the signals fade and get stronger as you turn. Larry SignatureWhile in Mexico, I didn't have to press 1 for Spanish. While in Iran, I didn't have to press 1 for Farsi, either. It just isn't fair. |
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