Topband: GAP Vertical Question

[Tom W8JI]

> Just to be clear, since the discussion drifted to half-wave radiators, my 
> comment above was specific to the modeling of thick v. thin half-wave 
> radiators, including the 180-195 degree radiators of some well-known 50KW 
> AM stations.  The results when modeling the base Z of approx. 1/4-wave 
> radiators is less affected by thickness, probably due to a lower base Z to 
> start with.

The theory (or cause) behind this is twofold:

1.) A thicker radiator at an end has a larger area for the electric field. 
This results in a less concentrated e-field discontinuity or boundary at the 
open end of the element. In other words the electric field is not as 
intense, and that means voltage is less.

2.) Any antenna is also a transmission line.  The surge impedance of that 
transmission line is lower with a larger effective diameter conductor. The 
mismatch of the open circuit at the far end is transformed through that 
transmission line, by the standing wave in the antenna, to a new lower 
impedance at the center, just as the center can be transformed back up to an 
open end.

This means a thicker antenna element doesn't have the low and high impedance 
extremes at the open end or along the antenna that a thinner antenna has. 
The standing waves are muted.

If the antenna had no radiation or loss, it would have no end current. It 
would have infinite impedance.

This applies to counterpoises also.

We can see how difficult it is to have useful things in antennas (antennae 
are found on insects, and can have zero current) that have no radiation and 
no loss.  :-)

73 Tom