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 separation, called broadside
  • mounted on different horizontal planes and not in-line vertically, called echelon.

threeAntennaMountings.png
Fi. 1. Three methods for arranging two antennas.
threeAntennaMountings.png (4.64 KiB) Viewed 7748 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 mast, 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.

jimh
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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Mounting Two VHF Marine Band Antennas on a Small Boat

Postby jimh » Sun Dec 22, 2019 9:35 pm

With the Friis formula for path loss, we can calculate the path loss between two antennas at 156-MHz (the VHF Marine Band) that are separated by 10-feet. Two arrangements of the two antennas will be investigated: broadside and collinear.

First, where the antennas are broadside to each other and are mutually located in the main lobe of the other. For this case we assume the antennas each have a gain of 3 dB. For this situation, the calculated path loss between the antennas is a very modest -20 dB.

For the second case, we we consider the two antennas to be collinear, and to be mutually located in a null in their patterns. For this case we assume each antenna has a gain (loss) of -20 dB. For this situation, the calculated path loss between the antennas is -67 dB, a substantial improvement of 46 dB more isolation between the antennas.

Next, we consider the signal that will occur in the receiver of a radio connected to one of the antennas when a transmitter is sending 20 Watts of power into the other antenna. A power at the antenna of 20 Watts is +43 dBm. (We use dBm because it is a convenient measurement of power levels for receiver inputs.) We then calculate the power into the receiver in the two cases of antenna placement described above.

For the antennas broadside at 10-feet separation, the level at the receiver will be +23 dBm. This is a very strong signal, and many VHF Marine Band receivers will not be able to tolerate this much signal at their input. The receiver will be overloaded, and reception of other signals will be degraded.

For the antennas collinear at 10-feet separation, the level at the receiver will be -23 dBM. While -23 dBm is a strong signal, most VHF Marine Band receivers will not be severely affected by a -23 dBm input, and they should be able to receive signals on other channels without too much degradation.

In the case of the transmitter being an AIS transmitter, the power level will be reduced to 2 Watts, a reduction of 10 dB from our earlier calculation. Also the AIS signal will be at at the very high end of the band, at 162-MHz or about 5-MHz away from the frequencies the receiver using the other antenna will be tuned to. The receiver input is therefore reduced by 10 dB in either case, or +13 dBm for broadside configuration and -33 dBm for collinear configuration.

On the basis of these calculations, if two antennas are arranged in a collinear configuration with 10-feet of separation at 156-MHz, reception on the VHF Marine Band ship station simplex channels on one of the antennas should not be affected when a 2-Watt AIS transmitter operates on the other antenna.

For a nice on-line calculator of Friis loss, see https://www.pasternack.com/t-calculator-friis.aspx.