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Old 03-14-2015, 09:52 PM   #1
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VHF Antenna

I'm installing a new mast and spreader bar on my 37 foot C&L Trawler. I'm thinking about installing a 4 foot 3db antenna on the spreader bar. This will put the tip of antenna about 8 feet higher than the 8 foot 6db antenna already installed on the side of the flybridge. Can anyone tell me if I will lose or gain by using the 3db antenna instead of the 6db? I also receive AIS info through the antenna.

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Old 03-14-2015, 10:17 PM   #2
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Old 03-15-2015, 06:51 AM   #3
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Higher the gain the farther the reach to a point....

The 6 dB are the all around antenna...the 3 is an OK backup and a good one to use on a radio that you mostly do low power like ship to ship or call I g a bridge and is fine for AIS unless you want to receive at the max range all the time.
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Old 03-15-2015, 07:15 AM   #4
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It will be fine. I have a 4ft Shakespere 4400 antenna on a small center console boat. The tip might be 7 feet off the water. No trouble talking to the USCG antenna 10 miles away or hearing boats calling bridges 6 miles away. I rountinely talk to a friend with one in Atlantic City NJ from my other boat in Cape May. Thats well over 30 miles. Some days are better than others.
With your additional antenna height, 10 miles or more should be no problem.
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Old 03-15-2015, 08:06 AM   #5
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From your post, I assume that you receive AIS but do not transmit AIS. If that is the case all should be fine. If you are transmitting the AIS antenna should be located as far as possible from any other antenni, including FM, sat radio, and SSB.
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Old 03-15-2015, 10:39 AM   #6
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I have a 4' Digital VHF/AIS antennae on my Nordic Tug 32 and it is great here in the PNW. However, if I were to head into remote areas where range is the priority, I'd replace it with the 8' Digital.
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Old 03-15-2015, 11:12 AM   #7
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Remember that VHF radio is line of sight. Because of that a higher antenna is always better than a lower one for range. Antenna gain helps, but primarily in terms of making the transmission clearer and letting you pick up weaker signals. All the gain in the world won't help much if your antenna is low though. Range is proportional to the square root of the antenna height. So doubling your antenna height will increase range by about 40% The range for communication is determined by the heights of both antennas involved. Coast Guard antennas tend to be relatively high, which is why you can talk to them at considerable ranges.
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Old 03-15-2015, 12:21 PM   #8
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I've run a 4 foot Metz antenna next to a Digital 8 foot antenna with the Metz on a 4 foot extension so the total heights were the same. I didn't notice a perceptible difference on the receive side, but could notice the difference while transmitting. Transmitting was always better on the 6dB gain antenna.


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