RadioBanter - View Single Post - radials vs radialless @ 144/430 MHz

Richard Clark · October 21st 09, 06:33 AM posted to rec.radio.amateur.antenna

On Tue, 20 Oct 2009 16:31:28 -0700, (Dave Platt)
wrote:

In an antenna of this sort, I believe that the primary function
of the radials is to act as a sort of feedline choke... they minimize
any tendency of RF coming up the inside of the feedline shield to flow
back down along the outside of the feedline.

Hi Dave,

I've modeled the conventional 4 radial GP, in free space; and, if
anything, the down lead (added as a 1/4 wave drop wire) supports more
current than the "radiator."

When I did the same thing with a ground plane of 120 radials (and the
added drop wire), the down lead supported nearly as much current as
the "radiator."

Neither radial configuration exhibits any/much choking.

In essence, these models were simply vertical dipoles with a star of
neutral elements. Of course, changing the length of the drop wire
complexifies the outcome; and would, I suspect, merely mimic nature.

Going further by following up on that last thought:

When for both radial configurations I vary the length of the down lead
(starting at 0.1 then adding 0.05 wl increments), the down lead shows
the current distribution of any inverted radiator that correlates to
that length (up to and including 3/4ths). However, the drive point
Z's R varies considerably ONLY around resonances (quarter and three
quarter wl) while the Z's X barely shifts value except (and again) at
those resonances.

What the elevated radials appear to do is dominate (buffer) the feed
point Z against variations of a feedline's length. Without these
radials, then this would be nothing other than a vertical Off Center
Dipole - if the feedline doesn't find ground, that is. The presence
of ground takes all bets off the table, but I don't think ground will
introduce choking action to the list of qualities for elevated
radials.

73's
Richard Clark, KB7QHC