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VHF cable type?
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
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