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Marine Electronics Journal Blog

Antennas: Spacing and other basics

Antennas are a critical part of every vessel.  Often ignored and much maligned, they are the mariner’s link to shore.  We use antennas for all kinds of communications and “comms” matter when things go awry.  We depend on comms for more than just emergencies, since GPS, radar, sat radio, sat TV, AM/FM, Wi-Fi, cellular and other systems all depend on a working antenna to accomplish their tasks.

We’ve discussed VHF antennas in past articles. This time we will look at some of the other antennas found on boats and how they interact with each other. The spacing guidelines published in the NMEA 0400 Installation Standard show minimum spacing recommendations for various antenna types.  These guidelines are based on theory and practice and are guidelines only.  Let’s look at why these guidelines for antenna spacing exist.

 By John Barry

Radiation patterns matter

When we put a signal into an antenna, it resonates in a way the causes a radiation from the antenna element into free air.  This radiation pattern has characteristics that are controlled by the construction of the antenna itself and by the other conductive materials in the vicinity of the antenna. VHF antennas are called dipole antennas. We refer to them as vertically polarized, meaning that the radiation pattern is horizontal along the surface of the earth.  The radiation pattern from a vertically polarized dipole antenna is donut shaped, originating from the center of the antenna element.

Radar antennas and satellite antennas use waveguide or parabolic antennas.  The energy coming out of these antennas is cone shaped, originating from the center of the waveguide antenna.  The design of the antenna is optimized to resonate at the appropriate frequency and direct it in a specific radiation pattern.  This represents a concentration of the energy from the antenna in a certain direction, which we call antenna gainAntenna gain is not a free lunch; the radio outputs only so much power and we can direct it, and concentrate it in a certain direction, but then other directions are diminished.  High-gain antennas are very directional, which is good for radar (since direction matters) but can be bad for dipoles since boats rock and roll with the seas and a directional antenna may not be aimed close enough along the surface of the earth at all times.


TX vs RX

We have so far discussed antennas transmitting (TX).  Some of our marine systems are receive-only (RX) devices, such as GPS, sat TV, AM/FM, etc.  These antennas have the same characteristics of gain and directionality as all antennas.  Construction and placement have equivalent effects on both TX and RX antennas. Typically receive-only antennas are high gain and attached to a very sensitive receiver that is trying to receive weak signals from far away.  Since these sensitive devices are in close proximity to our transmitting antennas, they are susceptible to interference.  A good example of this is the GPS receive antenna, which must not be within the beam of a transmitting radar antenna.  

Generally, an antenna is happy in free air, outside of the transmission path of other antennas.  Putting an antenna on top of a sailboat mast accomplishes this—all other locations are a compromise! Radar antennas have a predictable path of transmission.  Sat comm antennas, like sat phones, VSAT (very small aperture terminals), Fleet Broadband, etc. have highly directional transmissions that stay locked on the satellite, so the transmission path is all over the place from straight up to a down-look angle. 

Both radar and sat comms are radiation hazards and require separation from crew spaces.  The NMEA 0400 Installation Standard states: “Traditional pulse style radar antennas shall be located so that the radar beam is above the spaces occupied by vessel crew and passengers.”


Minimizing interference

Sometimes the boat can accommodate the antennas using the guidelines from NMEA 0400.  Some boats have lots of options for mounting antennas with correct spacing; most boats, however, do not.  To say that a compromise is always necessary is true.  Understanding how antennas work and how interference problems manifest themselves is how these compromises are accomplished with minimum degradation of signals.  Some devices are more susceptible than others and some transmissions are more harmful than others.  The frequency and power matter here.

Antennas operating at the same frequency as each other are very susceptible to interference and must be widely separated.  NMEA's 0400 guidelines are for horizontal separation distances, including rules and exceptions. In crowded antenna installations it may be necessary to separate the antennas vertically.  Because dipole antennas transmit horizontally, they create very little energy above and below the antenna location. Vertical separation is often the key to solving interference problems between two antennas. For example, SOLAS (international regulations governing most large commercial vessels under the auspices of the Safety of Life at Sea convention) requires a 6 foot vertical separation between the AIS (Automatic Identification System) antenna and the VHF antenna. 

Any piece of metal will act as an antenna.  Rails, arches, masts, rigging, wiring and a plethora of other metal objects found on boats can interfere with an antenna’s radiation pattern.  Radio waves do not act like DC current or 60 Hz AC current.  RF (radio frequency) energy will reflect, bend and be absorbed by RF conductors.  If we could see radio waves, we would see that they are all in the same space with different strengths depending on resonance and conduction.  It is quite amazing that these devices work as well as they do.  Performance can be optimized by careful antenna placement. 

About the author

John Barry is a marine electronics dealer located in Pleasant Prairie, Wisconsin. His company, Technical Marine Support Inc., is on the western side of Lake Michigan. He also instructs NMEA’s technical training courses and writes a regular column for Marine Electronics Journal. He says there are four topics that seem to stick out in those training sessions—radars, autopilots, data and radios.


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