[time-nuts] LHCP patch antenna

[Jim Lux]

On 1/21/13 9:14 AM, jmfranke wrote:
> Would anyone happen to have a LHCP patch antenna, with or without
> preamp, they would be willing to sell? I want to use it as the feed for
> a 4 foot diameter F: 0.375 dish antenna for a dedicated WAAS receiving
> set up.
>


why a patch?   Patches are nice when you need low profile or small size, 
but they tend to be narrow band and lower efficiency (unless you're 
using air dielectric).

Why not a 2 or 3 turn helix.  Design is VERY non critical, they're very 
wideband.  Space the turns about 3/4" inch, on 2.5" diameter form. 
Plastic cups work well.

1575 MHz is close to 20 cm lambda, so if you want to follow the design 
rules laid down by Kraus, circumference should be around lambda, so 
diameter is 6.1 cm (2.4"). Spacing should be 0.25-0.35 lambda (0.72 inch).

You can get the match to 50 ohms (if you want) by moving the last turn 
closer or farther from the ground plane behind the helix.  It acts like 
a taper transformer.

One of those metal burner bowls for a gas stove works quite nicely as a 
ground plane.  You'll then have the classic JPL Helibowl antenna.  Pick 
something where the diameter of the ground plane is at least twice the 
length of the helix.

The only problem you might have using a helix for your dish is that it 
would under illuminate.. Just take a couple turns off the helix.