Have noted my VHF radio performance has degraded since I installed a Garmin AIS system. I did some checking before I installed it last summer and Garmin says the loss is about 0.5 dB. Well our first cruise this year, our VHF hardly worked at all. Remembered that last fall, I did some wiring changes on the AIS at it was not shutting off automatically when I shut down the Garmin plotter. Well this is what I found today. I used a VHF radio meter to measure power and VSWR at the radio. When the antenna is hooked up directly to my VHF antenna, the radio measures a clean 25 watts out and a good 1.1 to 1 VSWR. When I run the antenna through the AIS with the AIS off, my radio test showed only 10 watts out and a VSWR of 3.0 to 1. When I turned on the AIS the radio improved somewhat to a power out of 20 watts and a VSWR of 1.5 to 1. Well I am for sure going to add a separate VHF antenna for the AIS. I am now looking for the shortest smallest VHF antenna I can find, a foot overall length would be excellent. I do not need to see boats over a few miles away, certainly not more than 5 miles to clutter up the plotter when I change scales. Any ideas for a very short VHF antenna?
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Started boating 1965
Bayliners owned: 26 Victoria, 28 Bounty, 32, 38, and 47 since 1996 -
Most of the dedicated AIS antenna are 4 foot.
Shakespeare has some 3 foot in their older series of antenna.
Jim McNeely
New Hope a 2004 Bayliner 305 Sunbridge Express Cruiser
Twin 5.7s with Bravo2 drives
Brighton, Michigan USA
MMSI # 367393410 -
The Shakespeare emergency VHF antenna typically carried by sailboats in case of dismasting is about a foot long. It comes with a suction cup mount but would be easily adaptable.
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You need to have about 18" (1/4 wave) if you have the antenna mounted on a metal counterpoise, such as a metal arch. Otherwise, you need at least an electrical 36" (1/2 wave) to have any kind of coverage without a counterpoise.
Otherwise, you are just hanging out a piece of wire and you take what you get. Odd lengths other than mentioned may even work better when they aren't vertical, at least in some directions.
I suggest the 1/2 wave antenna from Metz. Virtually indestructable and low profile from a visual aspect. I have them up in both VHF and UHF as backups and I'm very pleased with the performance. Here's a link to the VHF Marine model: http://www.metzcommunication.com/manta6.htm
Splitters of any kind can be nasty. You could probably solve some of the problem by adjusting the coax length between the radios and the splitter, but, that's a real pain to get right.
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I added a dedicated antenna to mine as well. Didn't notice any degradation really, but wanted to be sure there was no loss.
Love the AIS 600 btw.
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JohnWms wrote:
The Shakespeare emergency VHF antenna typically carried by sailboats in case of dismasting is about a foot long. It comes with a suction cup mount but would be easily adaptable.Man, we have some really good technical people on BOC who only drop in once in a while. Seriously, you guys should come out to play more often. :worthrodell wrote:
You need to have about 18" (1/4 wave) if you have the antenna mounted on a metal counterpoise, such as a metal arch. Otherwise, you need at least an electrical 36" (1/2 wave) to have any kind of coverage without a counterpoise.
Otherwise, you are just hanging out a piece of wire and you take what you get. Odd lengths other than mentioned may even work better when they aren't vertical, at least in some directions.
I suggest the 1/2 wave antenna from Metz. Virtually indestructable and low profile from a visual aspect. I have them up in both VHF and UHF as backups and I'm very pleased with the performance. Here's a link to the VHF Marine model: http://www.metzcommunication.com/manta6.htm
Splitters of any kind can be nasty. You could probably solve some of the problem by adjusting the coax length between the radios and the splitter, but, that's a real pain to get right.
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I put a second 8' antenna up for the AIS. The reason is twofold; AIS Class B transceivers only put out 2W so a short 3db emergency antenna was tempting (visually) but the full 6db gained from the Shakespeare 5225 was worth it. Second reason, if I ever lose the primary VHF antenna, I'll have a spare ready to connect in. Ya never know when a connector is gonna rot away or a coax gets pulled apart. Having two gives me redundancy and a much greater range for both receive and transmit.
I get signals very consistently out at around 15-17 miles even when under our boat shed.
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