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Old 23-04-2019, 04:19   #1
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Antenna

I want to take advantage of removing my mast to change out the coax for RG213U and other electrical faults by replacing the antenna for a 6dB whip. Can anyone recommend a good one that isn't 10ft long please?
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Old 23-04-2019, 04:28   #2
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Re: Antenna

You need to replace it with a 3dB antenna. The 6dB antennas are designed for installation much lower like on a motorboat. Also, their radiation pattern is less suited for sailboat installation at the masthead
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Old 23-04-2019, 04:39   #3
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Re: Antenna

Ha, I wondered why the 6dB antenna were so long? Thank you for your advice, I have been looking at Shakespeare.
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Old 23-04-2019, 04:55   #4
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Re: Antenna

This page explains why you don't want too much antenna gain on a sailboat. See, in particular, the illustration at the bottom of the page.


https://www.offshoreblue.com/communi...f-antennas.php
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Old 23-04-2019, 06:34   #5
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Re: Antenna

Well thanks for that the article was most informative. So now I'm looking for a 3ft SS whip. Cheers.
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Old 23-04-2019, 07:14   #6
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Re: Antenna

My three foot stainless steel whip works good at the masthead, and a four foot stainless steel whip AIS VHF antenna works well on the much lower davits.

I wondered how well the lower height AIS antenna would work crossing the Gulf Stream with ships all around. The lower location VHF AIS antenna did just fine following any ships that were a threat to our vessel. So on our boat a lower antenna for AIS works fine.
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Old 23-04-2019, 08:22   #7
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Re: Antenna

Quote:
Originally Posted by Whisper 2 View Post
I want to take advantage of removing my mast to change out the coax for RG213U and other electrical faults by replacing the antenna for a 6dB whip. Can anyone recommend a good one that isn't 10ft long please?


The reason you don’t want to go with a 6db antenna is that the radiation pattern is too directional. When sailboat he’ll too much of the athwartship radiation points up into the sky or down into the water.

Higher gain antennae are for power vessels that don’t heel consistently one way or the other.
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Old 23-04-2019, 10:09   #8
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Re: Antenna

Quote:
Originally Posted by maxingout View Post
I wondered how well the lower height AIS antenna would work crossing the Gulf Stream with ships all around.
For those interested in the math...


Say you have a three-foot whip mounted on the rear rail. The top of it is probably at least 8 ft. above the waterline. If another sailboat has a similarly mounted whip, line-of-sight between them will be 7 miles. That's plenty for collision avoidance. With a big ship, the AIS antenna is probably mounted at least 75-80 feet in the air. In that case, line-of-sight between you is more than 14 miles. Again, plenty of room.


Which explains why you found the low-mounted AIS antenna to be perfectly adequate.
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