[TowerTalk] Antenna Height above Ground

[Jim Lux]

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 
that's important.

(Actual pattern formation is a bit more complex that a simple raytracing 
model, but it's a good first order approximation.)