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Old 25-01-2010, 16:37   #1
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Weak VHF

I have a problem with my standard Horizon VHF Radio. When I bought the boat the antenna was missing, and since I didn't want to take the mast down to lead a new cable up, I mounted a antenna on my rail that runs around the boat. The antenna is very small, maby 50 cm (1.5 foot).

I then realiced that my handhelp VHF had a much stronger signal, both sending and recieving. I belive it's normaly the other way around...

I know it's hard to say, but do you think it's because the antenna is underdimensioned, or do you think it's the Radio it self that might be to old. It might also of course be the cables, I used screw on connectors instead of sodering...

I ask partly because I want better radio signal, but also because I'm thinking of getting an AIS system onboard and wonder if I have to get a new antenna, and if it then has to be up in the mast....

Here is a picture of the Radio from when I bought the boat;
Medvinden Navigationsbordet

The boat is now in Guatemala so I can't try anything out, but want to know what to do when I get back.

Best Regards

Oskar
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Old 25-01-2010, 16:45   #2
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My handheld has a wound coil antenna and a 5 mile range. My boat base unit, same brand as yours, has a coax up the mast and a 24" antenna with coil at the bottom. it has a 25-30 mile range. You'll have to put a real antenna on the boat. Also, remember that VHF is LOS unlike HF which bounces off the Ionosphere.
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Old 25-01-2010, 17:39   #3
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Like Charlie says VHF is line of sight ie the higher your antenna the better the operation. The fact that your hand held is working would be an indication of an antenna problem as the handheld is only 5 watts and the boat unit 25 watts. I would guess you have a bad connection or wire going to your antenna. You might want to get a professional to look at it as this is an important piece of safety equipment. If you can't get it checked I would check your antenna connections with a ohm meter. Also consider putting the antenna back on the mast head and use the best cable you can get.

Good Luck
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Old 25-01-2010, 20:21   #4
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Assuming the radio is ok (probably) it coulld be a high resistance in either power lead but if so, you would more than likely have other symptoms like the radio reverting to 16 when transmitting on another channel. If you don't have any other symptoms than weak transmit/receive, it's almost undoubtably a bad coax fitting or antenna connection. Check for moisture/salt/corrosion.
A bad antenna is unlikely, although possible.
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Old 25-01-2010, 20:38   #5
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The next question would be; Do you have a VHF antenna. CB's, SSB's, VHF .....actually all radios have a different antenna.
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Old 26-01-2010, 06:16   #6
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The antenna is this one SHAKESPEARE 5216 15" BLACK LIGHT WEIGHT SAILBOAT ANTENNA - Shakespeare Marine VHF Antennas - Shakespeare Marine VHF Antennas - Shakespeare Antennas

Mounted on my rail that runs around the boat. So I'll try to check the cable.

How important is it that the antenna is mounted on top of the mast? It seems like a big project to take the mast down to run the cable....
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Old 26-01-2010, 18:36   #7
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" Its 15" center-loaded design delivers Unity Gain, "
That antenna is a piece of ****. Get a real atenna that has some positive gain factor (3db, 6db) and mount it aloft. Yes, running a cable properly down the mast AND securing it properly AND terminating it properly are all pains, but if it isn't done right, you may as well save your money and just buy a 3db gain "emergency" antenna that screws onto the back of your VHF radio directly.
With antenna gain, more is not always better. As the gain goes up, the antenna gets more directional, and winds up having a shorter range when heeled hard over.
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Old 27-01-2010, 14:36   #8
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Ok, but It's a real hazzel to step the mast down.... So agian, how big difference will a better antenna make mounted in mast top or mounted on railing?

Also, how big does it have to be? I'm flying down to guatemala so if it can fit in a bag I would be happy. Othervise I could buy it down htere somewhere.
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Old 27-01-2010, 15:01   #9
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How big does an antenna have to be? Roughly one half meter or one meter in height. Either one is banned from carry-on luggage, since it can be used as a weapon. Although the TSA agents often ignore their own ban on sporting goods (including racquets) and similar items.

The difference in location is a straight difference in range. Your VHF range is roughyl line of sight. You take the range from xx height to horizon, once for your own antenna height, then again for the other party's antenna height, and add the two to get the combined range. From a six foot above sea level height, your range may be three miles. Up the mast, it may be 20. You can find the formulas on the web and run the actual numbers.
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Old 27-01-2010, 17:31   #10
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Quote:
Originally Posted by hellosailor View Post
The difference in location is a straight difference in range. Your VHF range is roughyl line of sight. You take the range from xx height to horizon, once for your own antenna height, then again for the other party's antenna height, and add the two to get the combined range. From a six foot above sea level height, your range may be three miles. Up the mast, it may be 20. You can find the formulas on the web and run the actual numbers.
Like so............
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Old 27-01-2010, 18:09   #11
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Good table--bad illustration they made though. Someone could mistake the vertical line in the sailboat for a mast, and then get confused over why the range is shown as the same, at the base, middle, and top of it.
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Old 29-01-2010, 05:45   #12
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So, what's said is that id I get a new, 3 or 6db antenna the reception will be as good or better that my hand held vhf.

If I put it in the mast it will reach alot longer that if I put it on the rail...

Is there anything special to think of if I want to use the antenna both for radio and AIS?
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Old 29-01-2010, 07:05   #13
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@Frelin: mount a 3dB stainless whip sailboat antenna at you masthead. Run a new RG-213 marine grade cable down to the VHF. First, have someone solder one plug on it and feed the other end down the mast so that this plug end up at the masthead. After feeding the cable to the radio have the other plug soldered on. Buy two extra plugs and a coupler for when you need to un-step the mast (you need to cut the cable then and splice it with these items.
If you are talking about an AIS receiver, you can buy a special little box (splitter for AIS receiver) that connects the AIS and VHF to the same antenna. If you have an AIS transponder, find a spot for a 2nd dedicated antenna (on a spreader, radar arch or post etc.)

You can get this stuff delivered in Guatemala and also find the help as there's a large cruisers community. Use Fedex to get the parts in, I know that Marine Warehouse can get and ship it for you (ask at miami@marinewarehouse.net). They can often give you a huge discount on Fedex costs (like 60%).

You current antenna: disable it (cut in two) and throw it in the garbage so that no-one else gets in the same trouble.

Check the radio power leads, make them as short as possible. Check with a meter if you feed it enough voltage... check that while transmitting.

You will get 30 nm range or better and be done with it. If you cut a corner like mounting the antenna on the railing, you will continue to have trouble.

cheers,
Nick.
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Old 29-01-2010, 12:12   #14
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Quote:
Originally Posted by hellosailor View Post
Good table--bad illustration they made though. Someone could mistake the vertical line in the sailboat for a mast, and then get confused over why the range is shown as the same, at the base, middle, and top of it.

A sailboat with 8' off the water? I guess, if the antenna were mounted on the rail.
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Old 29-01-2010, 14:26   #15
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Originally Posted by s/v Jedi View Post
If you are talking about an AIS receiver, you can buy a special little box (splitter for AIS receiver) that connects the AIS and VHF to the same antenna. If you have an AIS transponder, find a spot for a 2nd dedicated antenna (on a spreader, radar arch or post etc.)
There are also splitters available for use with AIS transponder's that allow use of a single antenna.

Eric
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