VHF Antenna

[motion30]

Does it matter. How long.the antenna.is. or.it just.about how.high..it is mounted for.best performance ?

[Cheechako]

how high.

[Teknav]

The length of your antenna is dependent on the radio frequencies used in transmission; a decrease in frequency results in a longer antenna. For VHF, mast's height has no noticeable effect on efficiency.

Mauritz

[Group9]

VHF is line of sight. See how far you can see from the deck, and then go up on you mast and see how much farther you can see. VHF works the same way.

I have two VHF antennas. One on top of the mast, and one mounted on the solar panel frame. A really high masthead antennae doesn't do you much good if the mast is in the water.

[Cheechako]

Originally Posted by Group9

VHF is line of sight. See how far you can see from the deck, and then go up on you mast and see how much farther you can see. VHF works the same way.

I have two VHF antennas. One on top of the mast, and one mounted on the solar panel frame. A really high masthead antennae doesn't do you much good if the mast is in the water.

"efficiency" BS, it's all about line of sight...

[motion30]

so regardless of the installed height I still use the standard 8 foot antenna ?

[btrayfors]

No, not if it's a sailboat.

Use a standard 3db antenna atop the mast. This will give you good performance, even when heeling.

The longer VHF antennas achieve their higher gain performance at the cost of a narrower vertical radiation pattern. That's fine for a power boat. But in a sailboat when you heel, much of the power is radiated right into the water or straight up into the sky.

Bill

[motion30]

I have a powerboat with an 18 foot bridge clearance with my antenna.down

[btrayfors]

Well, why didn't you say so?

Mount it as high as you can, on a folding mount for going thru bridges.

Bill

[motion30]

Bill. that's what I am doing now , but it's not great climbing out on the sundeck top. to raise or lower the antenna especially when I am single handed

[btrayfors]

Know what you mean. I had two 8' VHF antennas on my very active houseboat, both on 8' extensions and mounted high up on the flybridge. Single-handed a lot. Usually remembered to lower the antennas before going thru low bridges.

But not always. Over 17 years, I managed to break both antennas about 3-4 times, as I came back thru the four 17' clearance 14th St. Bridges over the Potomac (after partying up-river).

No much choice. Mount it lower and lose a bit of distance capability, climb up each time to lower it or, better, get a comely young crew member to do the climbing for you :-)

Here's the houseboat, anchored in the Washington Channel circa 1984 before the two big VHF antennas were fitted. Two Crusader 350s with 20"x14" 4-blade props. She could make 27 knots wide open; about 10 knots cruising. Absolutely the most "active" boat on the Potomac, with 3-4 overnites out each week....winter or summer! Most fun you could have with your clothes on! Even better without :-)



Bill

[pbiJim]

Longer antennas will give a transmission pattern that is wider & flatter. If your boat always sits flat in the water, then a longer (higher db rated) antenna will give you better reception & transmission range. If you are healing, then a shorter antenna, with it's rounder reception zone, will perform better because a flat pattern will direct your radio signal into the water or into the sky where the other boats with VHF radios are not located. Most mast mounted antennas are short ones that have round (low db) patterns. Mounting it higher almost always helps, regardless of what antenna you have.

Some antennas (especially the wire ones) have small length adjustments on them that are usually controlled by a set screw. This is for tuning the antenna. When the antenna is properly tuned, more of the RF energy is used in sending the signal & less is reflected back into the transmitter. Making this adjustment requires a device called a standing wave resonance meter that only ham radio operators & serious electronics geeks will have. Most people don't ever bother with this adjustment. Most people just pull the antenna out of the box, mount it as high as possible & call it good. I've seen improvements of around 15-20% at most on the antennas that I've tuned over the course of the last 30 years.