On Mon, 19 Jan 2004 03:35:58 GMT, Brian Whatcott
wrote:
On Sun, 18 Jan 2004 16:44:18 -0800, wrote:
Every time you double the height of your VHF antenna you gain 6 db in
signal strength
On Sun, 18 Jan 2004 23:46:55 GMT, Gary Schafer wrote:
Never heard this...any sources I can refer to? None of my books suggest this.
///
6 db signal strength increase at 30 miles accounts for about 3 extra
miles in range.
Can you please show the calculations? Or a pointer?
Thanks,
Norm
This is what he had in mind:
If you increase an antenna's height from 40 feet to 80 feet you
increase its line of sight range to the horizon by 3 or 4 miles
( root 2)
And, if you are in free space and are power limited, then doubling the
power transmitted will increase the range by root 2 using the
ordinary inverse square law of surface area [ = power density]
versus radius from a point
So, the argument continues....if the power limited range were 9 miles
and you doubled the power, the power limited range would increase to
12 miles about.
OR
if you doubled the antenna height, the line of sight to the horizon
would increase by about the same amount.
SO
you break even if you lose three dB for an extra 40 feet of height,
and you win if you use less lossy cable than that.
Now in fact, in some cases, VHF communications is not power limited,
but line of sight limited, and it takes MORE than twice the power to
reach an extra 1.4 range or three miles "round the corner" [i.e.
below the horizon]
But using this straight-forward model, you are led towards using
cable which loses less than 3dB per 40 feet or about 7 dB/100 ft.
because that is the breakeven point.
Take home message at VHF:
try to place the antenna as high as possible
and don't kill yourself worrying about cable losses, unless money is
no object. But on a tall boat, better to avoid RG58 if posssible, if
it loses 6 or 7dB per 100 feet.....
Brian W
It is not a linear function though. The amount of additional range
that you get is in the area of 10% to 30% depending on the heights
that you start at.
True line of sight loss (free space) is equal to 6 db every time you
double the range.
When figuring maximum range when changing antenna heights, refraction
of the signal is involved. The above formula also comes into play
slightly too. But it works out to around 6 db of signal increase at
the maximum range when ever you double the antenna height.
Doubling the power and doubling the antenna height do not give the
same results. Doubling the power results in an increase of 3 db in
signal strength. Doubling antenna height results in a 6 db increase in
signal strength. So you would have to increase the power by 4 times to
equal a doubling of antenna height.
Below is a link to a line of sight calculator.
http://www.vwlowen.demon.co.uk/java/horizon.htm
Here is a path loss calculator. It uses 20 miles as a default for the
range.
You can put in some heights and note the amount of path loss in DB
that you get. Then double the height of one of the antennas and note
that the path loss you get decreases by 6 db.
http://www.decibelproducts.com/Calculators/qegli.htm
The "break even point" in cable loss is also a non linear function.
With a moderate loss cable it does not "catch up" to the height
increase until you have reached a couple of hundred feet. With a high
loss cable it can "catch up" at a much lower height. Giving you a
negative return as you go higher with the antenna.
But the point is that the worse the cable loss the more percentage of
power and range you are giving away the higher you go.
Regards
Gary