Chris, John,
indeed, in theory radiation resistance is proportional to the square
of the height (for a short vertical antenna).
So "jacking up" the antenna even a few meter is worth the efford.
As a bonus the heigher antenna has a larger capacitance (although this
effect is less if you have good topload) and often (but not always) a
lower loss resistance (cfr the footprint theory of G3AQC).
I am in the onfortunate situtation when the loss resistance increases
with antenna height, because the antenna (topload) is over an open
grass area (30 by 45m) surrounded by forrest. As the antenna height
increases the footprint increases and it then includes more forrested
area. This results in a larger loss resistance. But even in this
situation the benefit of a larger radiation resistance overrules the
increased loss:
- antenna height 6m = 35 Ohm loss
- antenna height 11m = 45 Ohm loss (= +29%, but Rrad = +336%)
73, Rik ON7YD - OR7T
Quoting John P-G <[email protected]>:
Chris wrote:
....and how does 'Jacking up' the antenna increase the ERP?
Chris, G4AYT.
Chris,LF,
Presumably by increasing the height one increases the capacity and the
the radiation resistance and thus the ERP will rise for a given TX power
and antenna current? One probably also reduces the loss resistance too,
due to needing less loading L, so the current might also increase,
leading to yet more ERP for a fixed TX power.
John
GM4SLV
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