VHF Radio Antenna Length

[jpmurphy]

I'm shopping for a VHF radio and I see 2 lengths of ants. 36" and the big boys @ 8'. I understand the "line of sight" concept but cannot relate to the diferent ant lengths. I will be boating in fresh water lakes in Kansas and want to be able to get the weather info due to our weather changing so quickly here. Don't wanna be a mile from the ramp when the 40 mph gusts start screaming across the water.

Any insght on ant lengths, metal/fiberglass, pros & cons etc ??

Thanks..

 

[Nova II 260]

Re: VHF Radio Antenna Length

Jp, I bought a cheap 5' and she works fine with my Eagle EVR-150 DSC at 8 miles..maybe more, dunno.
My boat is aluminum and that could possibly make a difference with ground plane over FG. Buyers guide to marine antennas

 

[mthieme]

Re: VHF Radio Antenna Length

Picking up a weather station shouldn't be an issue. Go here and find out how close you are to the nearest tower.

http://www.weather.gov/nwr/listcov.htm

I use a handheld and am easily 20 miles away from the tower and it comes in clear as a bell. The towers are usually high enough to where you're antenna ain't gonna make any difference.
Now, if you wanna transmit for help, this is would be a different story.

 

[jpmurphy]

Re: VHF Radio Antenna Length

Picking up a weather station shouldn't be an issue. Go here and find out how close you are to the nearest tower.

http://www.weather.gov/nwr/listcov.htm

I use a handheld and am easily 20 miles away from the tower and it comes in clear as a bell. The towers are usually high enough to where you're antenna ain't gonna make any difference.
Now, if you wanna transmit for help, this is would be a different story.

Outstanding Site !!! By transmitting: are we saying the bigger the better ?

 

[triumphrick]

Re: VHF Radio Antenna Length

The towers are usually high enough to where you're antenna ain't gonna make any difference.
Now, if you wanna transmit for help, this is would be a different story.

Good point. We run 20-30 miles off shore, and all I need is to be able to reach Sea Tow!! They have tall, strong antennaes. I am always making a radio check with them as we venture farther out to make sure we remain in radio range.

 

[jpmurphy]

Re: VHF Radio Antenna Length

Jp, I bought a cheap 5' and she works fine with my Eagle EVR-150 DSC at 8 miles..maybe more, dunno.
My boat is aluminum and that could possibly make a difference with ground plane over FG. Buyers guide to marine antennas

Thats a great site. Years ago I was into CB radios and thought I knew a considerable amount about radios. This however, is my first shot at a marine radio and I just learned a bunch. Thanks for the link.

I also have an alum rig. 15'1" gregor. I plan to mount my little rubber am/fm ant on the console, for lack of a better place.
I'm thinking about mounting the VHF ant on one of the corners of the transom where I have a fair sized triangle shaped ledge. I'll be installing the fold down type of plastic mount I think.

Questions: Where is your ant mounted ?
Did you attached directly to the alum or did you put an insulating "spacer" between the mount and the metal surface ?

I think I want the entire boat to act as the ground but I'm a little fuzzy on this.

Many thanx................

 

[reelfishin]

Re: VHF Radio Antenna Length

The bottom line with a VHF signal is that it is basically "Line of sight" if the antennas can't see each other, communication will be spotty to say the least.
The taller the antenna, the farther the range.
Mounting a shorter antenna higher on the boat will also help.
I run all Shakespeare 8' antennas, and mount them at the highest point possible on the boat.
Check out the Shakespeare faq page, it answers some questions pretty well there.

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

 

[dingbat]

Re: VHF Radio Antenna Length

LOS calculator
http://www.qsl.net/kd4sai/distance.html

 

[Nova II 260]

Re: VHF Radio Antenna Length

Mine is mounted on back on the starboard side, there plenty of room to lay it down also. Ditto on the AM/FM ant. on the dash.

 

[jpmurphy]

Re: VHF Radio Antenna Length

Mine is mounted on back on the starboard side, there plenty of room to lay it down also. Ditto on the AM/FM ant. on the dash.

Looks like we're on the same page with mount location......

Many Thanx......

 

[mthieme]

Re: VHF Radio Antenna Length

My boat had two 8' whips mounted on either side of the windshield when I got her. They could lay down towards the stern. Both are gone now as the radios have died though the years. Now I use a Motorla Nautica handheld and cell phone. I have added an AM.FM which has one of those little 2' whips. I've reused one of the whip holes.

 

[jpmurphy]

Re: VHF Radio Antenna Length

My boat had two 8' whips mounted on either side of the windshield when I got her. They could lay down towards the stern. Both are gone now as the radios have died though the years. Now I use a Motorla Nautica handheld and cell phone. I have added an AM.FM which has one of those little 2' whips. I've reused one of the whip holes.

My boat, which is stock, has no ant mounts at all. I think my buddy, who gave the boat to me last year, bought it with no intent of ever even using it at all.

Being of the military frame of mind that I still am, I want it done right and I want it done right the first time. I've already gotten a little too aggresive on the bottom paint and had to go back and fix that moment of impatience. So I'm moving slowly and (hopefully) methodically through this rebuild step by step.

I sincerely appreciate everyones' input and guidance that I've received so far.

This is an exceptionally great site ! The humor is great too.

 

[mthieme]

Re: VHF Radio Antenna Length

If you go with a longer whip, you'll need to drill some holes through the topside. I always hate drilling holes in my boat but it's part of life.
Consider where and how it will fold down. Typically antennas are forward for this reason and the fact you get a little extra height. Also, a rear mounted antenna stands a good chance of getting in the way.

 

[jpmurphy]

Re: VHF Radio Antenna Length

Yupe. I'm still mulling over the location for the whip. With a 15'1" alum boat I have to be cautious. I've ordered an 8' fold down. Mounted at the console it will hang out past the transom when folded down. Mounted at the transom seems the logical place for it. When folded down it will lay along the gunwhale secured with a velcro strap when trailering.

Right now we are flipped over for painting so I can't measure the transom corners properly. I'm thinking starboard corner, but thats where the aft running light plugs into. Hope to have enough surface space there. I think the cable will reach to the port corner if I have to mount it there. I want to have all cables concealed and out of the way. A neat boat is a safe boat, etc. etc. etc.

For some reason I have this "right handed" mentality. Don't know what thats all about.

 

[mthieme]

Re: VHF Radio Antenna Length

While they have different bases available, the bigger the better so as to minimize flex on the aluminum when running and from the wind. I had a ~4" round base for mine.

 

[pvanv]

Re: VHF Radio Antenna Length

All seems right on the mark. Higher is better, and more dB gain goes further. On sail vessels, we advocate the 3dB 3-footers at the masthead, as they tend to outperform the 8+ footers on deck. The higher the dB gain, the more "pancaked" the radiation pattern becomes. Not so much of an issue on a go-fast boat, but on a heeled sailboat, that could make the pattern point to Neptune.

 

[jhebert]

Re: VHF Radio Antenna Length

Most VHF Marine Band radio antennas for use on small boats are a half-wavelength in length, or about 37-inches long. In some cases the antenna is contained in a much longer fiberglass tube, sometimes as long as eight feet.

The range of a VHF Marine Band radio is primarily determined by the height of the antenna above the water level. To improve range, increase the height.

Antenna gain is always welcome, but it comes with a price. The radiation pattern of an antenna narrows as its gain increases. In the best case the main lobe of the antenna is oriented in the most useful direction. In small boats which are often in complex motion when in a seaway, the antenna orientation may not always be optimum, and the main lobe of the antenna may not always be pointed in the most advantageous way. In these situations, an antenna with slightly less gain but a broader main lobe may prove to be more effective. Antennas with gain also have deep nulls in their pattern, and if you happen to have the antenna oriented so the desired direction of transmission falls into a deep null, the signal will be reduced, not increased, as a result.

Marine antenna makers are extremely generous is allocating gain to their antennas, and in most cases claims of gain are exaggerated. Marine antenna makers are extremely shy about publishing any information about the radiation pattern of their antennas, leading to some skepticism about how clean those patterns are and how many spurious lobes and nulls they contain.

For a discussion of antenna gain and pattern as it relates to the vertical monopole antennas used in the VHF Marine Band, see

VHF Marine Band Antennas
http://continuouswave.com/whaler/reference/antennaVertical.html

For a discussion of how to use remote NOAA Weather Radio broadcast stations as a known signal to check radio receiver and antenna performance, see

Assessing VHF Marine Band Antenna Performance Using Remote NOAA Weather Radio Broadcasts
http://continuouswave.com/whaler/reference/antennaRange.html

For my own small boat I use a three-foot antenna which I have mounted on a four-foot extension mast. This combination gives me the highest antenna mounting that is reasonable on my boat without a complex mechanical support for the mast. I use an antenna with an exposed metal radiator. This antenna can be replaced in component parts. Unlike an expensive eight-foot fiberglass antenna when you break the top of it, you won't have to discard the whole antenna if you hit something. Actually, because the antenna is a metal whip, you probably won't break anything. I described my installation in detail in

VHF Marine Band Antennas For Small Boats
http://continuouswave.com/whaler/reference/VHFAntenna.html

I have no reservations about recommending this type of installation to others, and many have adopted it with good results.

 

[jpmurphy]

Re: VHF Radio Antenna Length

Most VHF Marine Band radio antennas for use on small boats are a half-wavelength in length, or about 37-inches long. In some cases the antenna is contained in a much longer fiberglass tube, sometimes as long as eight feet.

The range of a VHF Marine Band radio is primarily determined by the height of the antenna above the water level. To improve range, increase the height.

Antenna gain is always welcome, but it comes with a price. The radiation pattern of an antenna narrows as its gain increases. In the best case the main lobe of the antenna is oriented in the most useful direction. In small boats which are often in complex motion when in a seaway, the antenna orientation may not always be optimum, and the main lobe of the antenna may not always be pointed in the most advantageous way. In these situations, an antenna with slightly less gain but a broader main lobe may prove to be more effective. Antennas with gain also have deep nulls in their pattern, and if you happen to have the antenna oriented so the desired direction of transmission falls into a deep null, the signal will be reduced, not increased, as a result.

Marine antenna makers are extremely generous is allocating gain to their antennas, and in most cases claims of gain are exaggerated. Marine antenna makers are extremely shy about publishing any information about the radiation pattern of their antennas, leading to some skepticism about how clean those patterns are and how many spurious lobes and nulls they contain.

For a discussion of antenna gain and pattern as it relates to the vertical monopole antennas used in the VHF Marine Band, see

VHF Marine Band Antennas
http://continuouswave.com/whaler/reference/antennaVertical.html

For a discussion of how to use remote NOAA Weather Radio broadcast stations as a known signal to check radio receiver and antenna performance, see

Assessing VHF Marine Band Antenna Performance Using Remote NOAA Weather Radio Broadcasts
http://continuouswave.com/whaler/reference/antennaRange.html

For my own small boat I use a three-foot antenna which I have mounted on a four-foot extension mast. This combination gives me the highest antenna mounting that is reasonable on my boat without a complex mechanical support for the mast. I use an antenna with an exposed metal radiator. This antenna can be replaced in component parts. Unlike an expensive eight-foot fiberglass antenna when you break the top of it, you won't have to discard the whole antenna if you hit something. Actually, because the antenna is a metal whip, you probably won't break anything. I described my installation in detail in

VHF Marine Band Antennas For Small Boats
http://continuouswave.com/whaler/reference/VHFAntenna.html

I have no reservations about recommending this type of installation to others, and many have adopted it with good results.

There's a wealth of information here ! Thankyou for taking the time to post this reply.

 

[waterinthefuel]

Re: VHF Radio Antenna Length

Something else you need to be cautious about is that in a small boat that rocks violently at times, a tall antenna can snap off a mount. I have a 14' fiberglass boat and I went with a 3' whip mounted to the side simply because my boat rocks and rolls with such violence sometimes that a big, long, tall heavy fiberglass antenna would really put stress on it's mount. The range difference between a 3' whip and 8' fiberglass mounted at the same height is negligible, don't care what people tell you. 8' antennas don't work miracles. VHF is line of sight, period. Altitude, or height of an antenna, is much more important than the length. Trust me, I know, I'm a licensed HAM radio operator who operates exclusively on the VHF bands. (my license allows me to go to HF, but my pocketbook states otherwise) I am also a pilot, and I have flown planes with 3 inch VHF antennas and at altitude can talk 100+ miles. I used a handheld 5 watt radio from inside of an airplane at 1000' and talked to another plane at 1000' 60 miles away, crystal clear. My antenna was a few inches tall, as was his. Line of sight is the only limiting factor in VHF. You can use a handheld 5 watt HAM radio to talk to the international space station!

I would recommend no more than a 3', but that's me speaking from experience, others have different opinions. The post that you quoted about putting the shorty 3' on an extension pole is a much better option than an 8' antenna FOR YOUR APPLICATION ONLY. I totally agree with him on that. If you had a big wide steady houseboat on a river, the 8' would be a much more logical choice.