[TowerTalk] Circularly Polarized Receive Antenna

[Brian Beezley]

Fading can be a problem on 160. When it is due to polarization rotation 
of the incoming signal, a circularly polarized receive antenna can 
eliminate it. An elliptically polarized antenna can reduce it. I'll call 
both CP.

I describe a CP antenna for 20 meters here:

https://www.eham.net/community/smf/index.php?topic=143601.0

A compact CP antenna for 160 can use the same principle but with a tuned 
loop replacing the dipole. I modeled an untuned square loop 3 feet on a 
side. Optimizing for minimum axial ratio at 20 degrees elevation angle 
yielded an antenna center height of about 71 feet and a loop tilt of 
about 23 degrees.

Geometry image (height above the X-Y ground plane not to scale):

https://i.postimg.cc/QN4dtMZd/geom.png

Axial ratio plot:

https://i.postimg.cc/6pQSQ9wt/cp.png

For this model I used Hagn generic ground constants for pastoral ground 
at 1.8 MHz (permittivity = 20, conductivity = 3.6 mS/m). Constants for 
other ground types are here:

https://k6sti.neocities.org/hfgc.htm

I don't know whether this idea has merit. Fading on 160 may be mostly 
due to factors other than polarization rotation. A tower supporting the 
loop may interfere, as may other nearby antennas. It may be difficult to 
adequately decouple the loop feedline. Your local ground constants might 
be quite different than those you model.

I haven't operated on 160 for many years. But I recall the frustration 
of waiting for a callsign during a slow fade only to have it disappear 
into the noise. Maybe this antenna can help.

Brian