BoatBanter.com - View Single Post - Expected range increase from 6db vs. 3db vhf antenna

On 22 Jun 2004 21:38:22 GMT, (BOEING377) wrote:

Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?

I have both and to tell you the truth the only difference I see is that a boat
on the fringes (noisy signal, low S/N ratio) on the 3 dB gets a bit clearer on
the 6 dB. I assume it does the same general thing for my transmittedf signal.
It isnt as big a deal as you might think, no dramatic range increases. I think,
from my experience,that antenna height is the more dominant factor, not antenna
gain.

As a rough rule of thumb, the length of the antenna must be twice as
long for it to have a 3 db gain over another one. A 6 db gain antenna
would be twice as long as a 3 db antenna. A 9 db antenna would be
twice the length of a 6 db antenna.
If they aren't then they probably don't have the promised gain.

Another way to get gain is to increase the height of the antenna above
ground / water. When you double the antenna height you pick up 6 db
gain at the range limit. Every time you double the height again you
pick up another 6 db.

The only problem is that the height gets out of hand quickly. Doubling
from 20 feet means going to 40 feet. Doubling that means going to 80
feet etc.

Here is a link with a chart that lets you estimate range with
different antenna heights and gain.

http://www.artscipub.com/simpleton/simp.range.html

Regards
Gary