NOYB wrote:
wrote in message
oups.com...
NOYB wrote:
"Wayne.B" wrote in message
...
On 13 Jan 2006 09:56:30 -0800, "
wrote:

According to post in this newsgroup, we need a metal plate for
grounding if we use a 3dB antenna, and a fiberglass boat doesn't have
it. My boat is a fiberglass boat. What's the alternative? Does this
mean that I cannot use 3dB antenna?

There is no need for a ground with a VHF antenna.

Either the 3 db or 6 db antenna will be adequate for near shore
coastal operation.


I'm with Wayne on this one.

The dB rating plays a relatively minor role in the distance the signal
travels. The height of the transmitting antenna plays a larger role. But
the height of the receiving antenna is the most important element.

Example:
A 3' antenna mounted on the top of the gunwale of a small boat might be
5'
above the water line. It can travel only 5 miles to another antenna
that
is only 5' off the water...but can travel 15 miles to an antenna that is
100' high (like at a Coast Guard station).

An 8' antenna mounted on the same gunnel can travel 9 miles to another
antenna that is only 5' off the water (big difference!), but only travels
18
miles to that 100' antenna (a not so big difference from the 3' antenna).

If the CG antenna is 250' above sea level (mounted on a tower on a
mountain
for example), the 3' antenna can travel 23 miles...and the 8' travels 25
miles.

So the question is...
Who do you want to call with that radio?

Another boat within 5 miles? Then either antenna is fine.

The Coast Guard when you're 10-15 miles from shore? Then either antenna
is
fine once again.

Here's a good link to explain this further:
http://www.yachtworks.net/Store/antennas.htm

Thanks for the explanation of the reason why a 3-ft 3dB antenna may be
all I need.

I go home and check the possible location for a 8-ft antenna in my
18-ft boat. I have a feeling that a 8-ft antenna is just too long. If
I mount it in the front rail of the T-top and set the antenna at a
45-degree angle toward the stern, the 8-ft antenna will still stick out
covering the entire deck in the stern of my boat. This is going to be
a problem if I want to cast from the deck in the stern of my boat
because my fishing rod "may" hit the antenna. A 3-ft antenna that is
pointing straight up will not have this problem.

A 3-ft antenna mounted on the T-top will be as much as 9-ft above the
water level. According to this formula:
range-in-miles = square-root-of-height-above-water-in-feet x 1.42
I should expect to get the range of up to 4.26 miles.

But that's only to reach another antenna that is right at the level of the
water. If there were another boat with an antenna 9' above the water, you'd
carry at least twice that distance.

I didn't know this (I must have skimmed through the forumla too
quickly). I assumed that the number from the calculation was for
communicating with another antenna that had the same height as the
transmitting antenna. Thanks for correcting this wrong assumption.

I am very glad that you have given me the correct definition of the
formula (to calculate the effective range). Now I know the reason why
the calculated effective range doesn't match the number shown on the
book (because the number shown on the book takes into account of the
antenna height of the receiving party).

Jay Chan