WiFi Antennas and Tidal Fluctiation

AntennaPatternsMost WiFi installations are designed for short distances and fixed and stable environments. Most cruisers are interested in longer ranges and operate from moving (even if anchored, moored, or slipped) and unstable platforms (pitch and roll). When setting up a WiFi antenna/booster for this reality, it is useful to know when it comes to antenna gain,  more can result in nothing.

RangeAntennas are rated by decibels (dBi) — power gain — usually referenced to an omnidirectional* antenna. When increasing antenna dBi from “2” at constant transmit power, this loosely translates to range improvements shown. This range comes at the expense of reducing the vertical width of the central beam of energy coming from the antenna. See top graphic.

Where tides are concerned, let’s assume the shore-side provider wanted things to stay dry, and the antenna to “see” as far as possible. In our case, at high tide, the shore WiFi antennas (yellow dot below) for our dock will be about ten feet higher than our boat antenna. Given the picture above, the half angles of 12.5°, 15°, and 20° are worth considering, if one’s boat is close to the shore antenna, it is possible to be below these angles as the tide drops. In our hurricane hole marina, the angles and antenna geometry create the picture below:


High Tide Antenna Beams (Red arrow is tide range.)

Low Tide Beams

Low Tide Beams

Three things then happen.

  1. Connection via the main beam weakens due to edge attenuation (For us, when this happens, we get an 83% signal strength reduction that lasts about three hours).
  2. Reflections off masts, and rigging and hulls more effectively compete with the main signal for the connection,
  3. Connections through side-lobes† do the same thing — In sum, WiFi services slows, fades in and out, and some folks start firing up their 3&4G hotspots to stay online.

If the marina is using a higher gain antenna than 5 dBi:

  1. Avoid slips too close to the shore antenna — 120 feet out and beyond should work, however being farther out means more mast and rigging interference.
  2. Connect to a dock antenna farther away (on a different dock) for the same reason. Also possibly better from a mast and rigging standpoint.
  3. Keep a low gain antenna (2-3 db) on hand for situations where a close in slip is unavoidable or another shore antenna is unavailable.
  4. Hoist an antenna in the rigging to approximate the height of the shore antenna

* WiFi antennae can be referenced to dipoles, but those are rare in this application.

Well made 8db WiFi antenna pattern (vertical plane)

Well made 8db WiFi antenna pattern (vertical plane)

†Side lobes (teal color) are small beams of energy caused by the interaction of wave patterns coming from the antenna. More sophisticated antennas (e.g., radar) have very complex designs and management algorithms to eliminate these and their effects.

Posted via my Android Phone because lighting fried one of the marina’s routers and the overload on the survivor coupled with low tide made it the only working connection.

3 responses to “WiFi Antennas and Tidal Fluctiation

  1. Alan V. Cecil

    Thank you!

    Alan V. Cecil

    Hampton Roads

  2. Great posting! I have forwarded it to a few friends that will be interested. We got our wifi gear yesterday! However we will probably wait for the rain and lighting to stop before installing it on the arch. ha!

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