Mounting Two VHF Marine Band Antennas on a Small Boat

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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Mounting Two VHF Marine Band Antennas on a Small Boat

Postby jimh » Fri Feb 15, 2019 3:13 pm

When mounting two VHF Marine Band (156 to 162-MHz) antennas that will be in close proximity, as will necessarily occur on small boats, the two antennas should be located so as to provide the greatest electrical isolation between them, that is, installed so that there is the least signal coupling between them. The reason for the least coupling to be desirable is to permit each antenna to work separately and without interfering with or being interfered by the other antenna. There are two benefits from good isolation:
  • the second antenna will not tend to detune or alter the radiation pattern of the first
  • with enough isolation and with enough separation between frequency of operation, simultaneous transmission on one antenna while still receiving on the other will be possible.

With regard to vertical monopole antennas as used on boats for the VHF Marine Band, there are three possible arrangements or orientations of the antennas:
  • mounted in-line vertically, with separation, called collinear
  • mounted on the same horizontal plane with separated, called broadside
  • mounted on different horizontal planes and not in-line vertically with separation in both planes, called echelon.

threeAntennaMountings.png
Fi. 1. Three methods for arranging two antennas.
threeAntennaMountings.png (4.64 KiB) Viewed 120 times


The arrangement that gives the best isolation is collinear. The radiation pattern of a vertical antenna will have very deep nulls off the upper and lower end of the antenna radiator. If you mount two vertical antennas in a collinear array, each antenna will be in a deep null of the other antenna. Since these nulls may be as deep as -40 dB, the result will be as much as -80 dB of isolation between antennas. The greater the vertical separation, the better. A typical collinear mounting for two verticals will be to attach them to a tall mast, with one antenna at the top of the mast and the second lower and in-line with the top antenna. This is a feasible installation on a sailboat mast, but on small power boats a tall mast is unlikely to be found.

The arrangement that gives the least isolation is to mount the two antennas on the same horizontal place and separate them by the greatest distance possible. On a small boat this distance will usually be limited to not more than perhaps eight feet. In an arrangement like this, each antenna is positioned in the main lobe of the pattern of the other antenna. There is no isolation provided by the antenna pattern. In fact, the opposite occurs because each antenna will have some gain in its main lobe. This mounting causes the least isolation and the most interaction between the antennas. It should be avoided.

The echelon arrangement is a mix of vertical and horizontal separation. This is a workable arrangement on a small boat by mounting the first antenna on an insulated extension last, and the second antenna on the horizontal plane of the mast base, but offsent only slightly from the insulated extension mast. In this way the two antennas will remain mostly in the deep nulls in their radiation patterns. The farther away from an in-line vertical alignment, the less isolation. Use of an insulated extension mast to raise the upper antenna is mandatory, as without an insulated extension mast, the lower antenna will be affected by the close proximity to a conducting mast.