VHF antenna for sport boats

[Redhook98]

I concur. My reasoning in my previous message is due to the height factor mostly. Mounting a lower gain antenna on a sport boat really is not that great. A 6-db 8-ft antenna is far better, but doesn't look that great. Most VHF radios are putting out around 25-watts. They vary from manufacturer to manufacturer, but all advertise 25-watts (or 1-watt for close-in comms). As for antenna gain, the higher the gain, the smaller the angle of use is.

There are many factors. The general-use antenna for boating has been 8-ft with 6-db gain for a long time. It is a great all around size. 4-ft is good for light coastal use (within clear sight of land) or lake use. Otherwise buy the 8-footer and raise/lower it as you use it. (I had a 16-ft extension with a 8-ft antenna on my old Roamer. I could see quite a distance).

I now have 2 14-ft 8-db antenni mounted on the upper half of the 3/4 tower on my flybridge. Not sure what the effective range is yet, but it's up there. If comms are important to you (and they should be), don't skimp on this. It could save your life or someone elses someday.

There are actually three (3) variables that come into play that determine the transmission distance of your VHF. They are:

1. Power
2. Antenna gain
3. Height

It seems that all the textbook discussions focus on the "height" part and from experience, the power and antenna gain play a bigger role for boaters. As an example, you can stand on the highest mountain with a 5 watt radio with a 2 dB gain built-in antenna and you are only going to be able to transmit a few miles.

If you look at this fancy diagram, you can see the effect of antenna gain on the radiation patterns from the antenna:

Take that cross section shown and the radiation patterns are essentially rotated around the antenna and look like a donut. A small SS whip has a gain of about 3 dB and looks like a fat donut and a 16' fiberglass antenna can push up to 10 dB gain created a stretched out donut. However, the smaller gain antenna is not as sensitive to having the antenna straight up and down like a smaller gain antenna is... Add more power to the antenna, and the distance increases subject to the height.

My experience with a 6 dB antenna is that it'll transmit about 10-12 miles with a 20 watt radio. That is far less than what a height calculator will show... You need a higher gain antenna (9-10 dB) and some serious power to transmit the full "line of sight" that textbooks discuss. My 4 dB antenna on my dinghy is good for about 3-4 miles.

My 2 cents.

 

[Four Suns]

One of the points I was also trying to make was the whole "hand held is good enough" discussion that comes up from time to time around here. Hand held VHF radios suck when it comes to transmitting. If you can get 1-2 miles you are doing good. They have low power, a low antenna gain and are only a few feet off the water. A combination of the worst properties. People get fooled by them because the can receive transmissions (i.e. listening) from a long way away (like from the coast guard) but they can't transmit worth a poop. I guess as long as you want to listen and not talk they are fine.... otherwise, they are good for around the dock or if a helicopter is right over your head.

A good built in high power radio with a good antenna can't be beat.

 

[Redhook98]

Yeah, I use my hand-held only for coordinating with the marina I am coming into, the gas dock, or toher members of our yacht club when trying to get them all docked up on a trip. Otherwise, it is not much good for anything. It's even the "high power" 6-watt model! It's only of use on the water when I am on the bridge. The irony there is that I already have 2 higher-end Icom VHF's already installed up there (one is a back up I can switch in to the main antenna with an RF switch).

It all depends on your priorities and what you are willing to accept astheticallly on your boat.

 

[Woody]

We don't really know what kind of boating the OP is doing. Maybe he hangs around in some bay cluttered with other boats and good CG coverage. In that case maybe anything would do. But if he's taking his 190BR to the Bahamas....