[Top] [All Lists]

Re: Topband: GAP Vertical Question

To: <>
Subject: Re: Topband: GAP Vertical Question
From: "Tom W8JI" <>
Reply-to: Tom W8JI <>
Date: Mon, 17 Dec 2012 10:14:31 -0500
List-post: <">>
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
Topband reflector -

<Prev in Thread] Current Thread [Next in Thread>