[BC] FM antenna radiation patterns

[Richard Fry]

Steve Michaels wrote:
>I am curious-after looking at Richard Fry's paper, it would appear that
> the ring-stub radiator has a drastically different pattern from the rest.
>Without using names (they will be obvious from his presentation) why
>would anybody want that design?

The first approach to implement "c-pol" in FM broadcast transmit antennas 
was to interleave v-pol dipoles with h-pol elements.  The ring-stub design 
was the next developmental step from that, and it enabled radiating both 
polarizations from a single element.

But both of these approaches suffer from the same two basic engineering 
realities: an h-pol element also generates v-pol at elevation angles not 
equal to zero (ie, the horizontal plane), and secondly,  the net v-pol 
radiation center is not space-coincident with the h-pol radiation center.

The result of those two realities is that, although v-pol & h-pol might 
have ~equal fields at every azimuth in the horizontal plane, the H-V ratio 
can vary widely at other elevations and azimuths, which, of course, is not 
desirable for a c-pol radiator.  It also displaces the v-pol nulls expected 
from a v-pol element at the zenith and nadir, so that some locations at 
elevation angles hitting the ground near the tower site might receive 
little or no v-pol, and other locations near the tower might receive much 
more v-pol than h-pol..

Added to this is the fact that the h-pol part of the ring-stub elements 
have 100% h-pol relative field at all elevation angles -- so they make for 
high power densities on the ground at and near the tower base.  Cue the 
EPA.

Later c-pol element designs like the normal-mode helix, the opposed vee, 
and the "rototiller style" addressed these issues, at least as to the 
patterns generated by the basic element.  However the antenna feed system, 
mounts, and the nearby tower can add considerable distortion to their 
native patterns (see paper 6 at http://rfry.org).

>The website is http://rfry.org , presentation link on the bottom of the
>page pertaining to zero sidelobe antennas.

There is a paper there by that title, but the pattern comparison that 
started this thread is in paper 10: "FM Antenna Pattern Evaluations using 
Numerical Electromagnetics Code."

RF