Antenna Installation...help

[divertodd]

Hi, I am new to this site and have a few questions. I am thinking about installing a VHF in the fiberglass runabout I am buying. My question is about the antenna. I am a Ham Radio operator and have done a lot of antenna work. I have however, never worked with a marine application. How does the antenna achieve a ground? If the boat were metal the ground would be no problem, but with fiberglass I am not surewhat the proper application would be. Can anyone give me some insight?
Thanks

 

[Gary H NC]

Re: Antenna Installation...help

Just checked both of my boats. They had the antennas already on them. No ground.That is a good question,maybe no ground is needed with VHF.
I know my CB and sideband radios in my truck needs one..

 

[johnson1989]

Re: Antenna Installation...help

You shouldnt need to ground the antenna itself, it should connect directly to the Radio.

 

[tashasdaddy]

Re: Antenna Installation...help

just lower the antenna during a lightning storm. saw a sailboat once with a hole blown thru the hull below the mast.

 

[bruceb58]

Re: Antenna Installation...help

Typically, marine VHF antennas incorporate the ground plane in the antenna.

 

[Rob454]

Re: Antenna Installation...help

the shielded cable acts like a ground. its either RJ11 or RJ 56 not really sure but its coaxial cable. when you poen it up the Coax cable has a center solid copper core, then a plastic sheath covering it then alluminum foli wrapped around core. then a mesh/weaved outer ( either copper or tinned coper or steel ) wrap then sometimes alluminum foil then finally the outer jacket.. I deal with electrical wire at work and I do a ton of RJ jacks. the shield ( outer wrave ) acts liek a ground. if you ever have a strand of the outer weave touch the inner core you wont get crap for a signal

Rob

 

[bruceb58]

Re: Antenna Installation...help

Of course the ground comes through the cable shield but the antenna itself provides the "ground plane".

There are certain antennas that need the ground plane and are not buit into the antenna.

 

[troypolla]

Re: Antenna Installation...help

yep these guys got it right. ground through the wire.

 

[Texasmark]

Re: Antenna Installation...help

I haven't had my "field theory" books out in a very long time so I am a bit rusty on this but as I recall the VHF antenna is cut to 1/4 wavelength and the water that the boat rests on acts as a reflector giving an imaginary additional 1/4 wavelength for a total of 1/2.

This allows for a convenient impedance at the exit/entry point where you connect your 50 or 75 ohm cable yet is adequate for reasonable omnidirectional radiation.

The ground circuit is usually made (as stated) from the cables shield, through the chassis of the radio to the battery's ground, to the engine and through the engine's drive unit to the water.

A full wavelength antenna would have more gain but has the problem if matching impedance to the cable and length would be prohibitive.

Mark

 

[ac439]

Re: Antenna Installation...help

Hi, I am new to this site and have a few questions. I am thinking about installing a VHF in the fiberglass runabout I am buying. My question is about the antenna. I am a Ham Radio operator and have done a lot of antenna work. I have however, never worked with a marine application. How does the antenna achieve a ground? If the boat were metal the ground would be no problem, but with fiberglass I am not surewhat the proper application would be. Can anyone give me some insight?
Thanks

I'm a HAM too and the antenna in marine is a 1/2 wave therefore you don't need a ground plane. However, there is a matching circuit (LC) at the antenna feeding point to match the typical low 50 ohms output impedence from the transmitter to the 4-5 kohms at the feeding point of the 1/2 wave element. A jpole is an example of antenna without the need of a ground plane. Hope this helps. - AC

 

[Boatist]

Re: Antenna Installation...help

Most VHF marine antennas are half wave and do not need a ground.

http://www.shakespeare-marine.com/faq2.asp?ReqNum=6

 

[jurgenscraft]

Re: Antenna Installation...help

Good afternoon all.
Thats the right answer a !/2 wave antenna is not ground plane sensative like
1/4 wave and 5/8 antennas. and will opperate with a low SWR at the resonant frequency, install the antenna as per manufacturer's instructions and check VSWR.once installed try the radio out and get reports on the air.
Regards William Wr4ight.

 

[divertodd]

Re: Antenna Installation...help

Thanks for the input everyone. I appreciate it greatly....Todd