Originally Posted by malthouse
But the relationship between feet and miles is no the same as that of meters and kilometers.
If my antenna is a big 7ft job then (sq rt of 7x1.5) the range is 3.2 miles.
If my antenna is the same size measured in meters then (sq rt of 2.13x1.5) the range is 1.8km which is 1.1 miles.
The metric one should be 12.75 and not 1.5, I think.
I thought the best way of working out the value of a longer antenna was to take the height above sea level as two thirds up the length of the whip, therefore a 7ft one is about 4ft higher than a 1ft mounted in the same location.
In any event the effective height of the antenna is calculated by multiplying the square root of the height in feet by 1.23. So our fixed radio at work which is 50ft up has a radio horizon of 8.7 miles, assuming the receiving station is at sea level.
My apologuies for not making my post clearer,
When i refer to "height of antenna", I'm talking about its height above water - not the antenna lenght.
To be perfectly honest, its not my calculations either, if you search for VHF propagation, you will find these, as they are taught internationally.
So sorry about that, but guess your big 7 ft Job will need to be recalculated to see how good it is.
This is also why coast radio stations are built on top of hills and up whopping great towers, to increase height, and to give better coverage to the horizon.
If you want to calculate 2 stations of differing height, i suggest you google "dipping height" and it will broadly allow you to do it, really for visual light but
between you and me you can add on a few extra meters for vhf and you'll be fine !
But it matters not how big your stick is, its how high you hold it up.
Good luck with the calc's again