Marine Radio Antenna length

[drewpster]

I am still learning about marine band, so please correct me if any of my info is wrong. I am planning to install an in-dash marine radio. I use a handheld as we are river boaters, but our yearly big trip takes us out to ten miles from the nearest marina. So a good in-dash is needed. I was planning to mount the antenna on top of the canopy hardtop. My problem is that I do not want the normal eight foot stick I see around here on most boats. I remember from my CB days as a kid (back when they were so popular) that it was best to match the antenna to the radio using a SWR meter. Does the same apply to marine band radios? I know a shorter antenna will reduce reception some but I cannot stomach that huge whip on top of my boat. Is it possible to use a four foot antenna on a 25 watt radio? They are 25 watt...right? Are you still supposed to get a licence for marine band?

 

[Splat]

Re: Marine Radio Antenna length

Fcc does not require a license for using a Marine VHF anymore. You should read up on proper etiquette and channel assignments tho.

Yes your antennea as well as lenght of antennea cable must be matched to your radios wave length.

You can purchase a 3' antenna with 6db of gain, But I seriously doubt you'll get ten miles of range out of it.

Bill

 

[gcboat]

Re: Marine Radio Antenna length

Here's a list of VHF channel frequency's and their primary uses.
http://www.navcen.uscg.gov/marcomms/vhf.htm

 

[dingbat]

Re: Marine Radio Antenna length

Here is a pretty good tutorial on VHF antennas for boating applications
http://members.toast.net/boatguy/vhfant.htm

 

[wire2]

Re: Marine Radio Antenna length

At 25 watts, a low SWR (a good match) is important. The marine vhf band is just above the amateur 2 meter band, (144-148 Mhz), so a used 2 meter antenna can be trimmed a bit shorter for 156-162. They can be had cheap from an amateur radio flea market, but most are designed to work as a 5/8 wave on a ground plane, (metal car roof/trunk). It's worth experimenting, imho.

 

[Boatist]

Re: Marine Radio Antenna length

Most VHF antenna have a 50 ohm impedence and require no tunning or cutting coax. I always take my SWR meter with me when we go to the Pacific Ocean and I have help many solve radio problems back in the camp we all stay in. The only antennas I have seen with high SWR were either shorted Coax where the conector was soldered on, cut coax where it went thru the deck, broken coax where it bends sharp each time they lay down the antenna. I did see one where the antenna was mounted on the deck of the boat along side a metal cab and it had a high SWR over 3 to 1.

The other problem is the antenna it self. The cheap flexable the radiator inside offten break. Some are just tuned Coax.
The bigger stiff antennas hold up much better over time. They also have better Radators, solid brass or copper and receive and transmit much better even thou they are rated as 6 db the same as the little flexable 8 foot antennas.

Most Trailer boats should use a 6 DB gain antenna.

Sail boats that heel over should use a 3 db antenna mounted on the mast head. This is because the 6DB atennas transmit in a narrow horziontal plane and when the boat is heel over the signal is either going into the sky or into the ocean.

The 9 db gain antenna should only be used on larger boats. This is because they have a even more narrow horziontal plane and when the boat pitches and rolls in ruff water the signal could drop out.

Since all the one I have seen were 50 ohm inpedence you can cut the coax to any length and have no affect on the SWR. This is totally different than most CB or Ham Radio's.
My self I never cut one shorter than 6 to 9 feet. When done I always check the SWR Just to make sure everything is good.
I have two CB SWR meters and neither will work at the Higher VHF frequency, My 2 meter ham VHF SWR works good for 2 meters to 440 ham band and VHF Marine.

http://www.shakespeare-marine.com/faq.asp