On 10/16/11 2:32 PM, Gary Reich wrote:
> Good afternoon, what is the actual antenna height above ground when a
> antenna is mounted on a roof ? I have a hex beam antenna mounted on a 30'
> tilt-over mast which is mounted above the roof of my house. Due to security
> issues I had to install a frame of welded rebar under the roof to prevent
> robberies. This framework is attached to 5- 8' ground rods. The roof is 10'
> above ground level which puts the antenna at 40'. So in theory is may antenna
> mounted 30' or 40' above ground ? Thanks' in advance
> Gary Reich, YS1GR
40 ft... assuming you're asking about far field patterns for elevation
angles below, say, 45 degrees. what's directly under the antenna
doesn't make much difference in the pattern (it DOES affect things like
tuning and efficiency), because what drives the pattern is the soil
properties at a distance.
Imagine a ray propagating from the antenna up at the take off angle of
interest. Imagine another ray that heads down at exactly the same angle
relative to horizontal, strikes the ground, and reflects up at the angle
of the original ray. It's where that "strikes the ground" happens
(Actual pattern formation is a bit more complex that a simple raytracing
model, but it's a good first order approximation.)
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