435 MHz Circular Patch Feed

This patch is another scaled up version of the S band patch feed. It gets rather large, but on dishes 6' or more in diameter, it produces reasonable gain. Here are the 435 MHz dimensions. I now add capacity from a bolt threaded into the reflector instead of the patch. This results in easier adjustments as hand capacity to the reflector is less disruptive than hand capacity to the patch.

The reflector is 16 1/2" in diameter, made from a piece of 1/8" thick aluminum. A 3/16" hole is drilled in the center for the 10-24 mounting bolt in the center. 3 3/4" out from the center is a 3/8" hole through which the center conductor of a type N female connector extends. The type N connector, on the side of the reflector away from the patch, is attached to the reflector plate by four pan head 4-40 bolts with the heads on the side of the reflector facing the patch plate. The type N center conductor has a 1 1/2" length of a 6-32 brass bolt soldered to it for connecting to the 435 MHz patch plate. Put a 5/16" size 6-32 nut on the brass bolt threads, outside spaced 1" from the reflector. A third hole for the circularity screw is a 3/16" hole, 2 3/4" clockwise from the type N hole, also 3 3/4'" out from the center hole. A fourth hole for a screw for the fine frequency adjustment is another 3/16" hole, axially opposite the type N hole, 6 1/2'" out from the center hole. I then carefully tapped these last two holes with a 1/4-20 tap.

The 435 MHz patch plate is 14" in diameter, and was made from 1/16" thick aluminum plate stock. I start by drilling 2 holes in this patch plate. The first hole is a 3/16" hole in the center of the patch plate and the second hole is a 5/32" hole drilled 3 3/4" out from the center (mates to the type N center conductor on the reflector).

Four more matching 3/16" holes are drilled in the 435 MHz patch plate and the 435 MHz reflector on a 4" square centered around the center screw. These are used for mounting 1" nylon spacers, obtained from an Electronics Surplus place in Boulder, Colorado, between the 435 MHz reflector and the 435 MHz patch to rigidly hold the plates 1" apart. Eight 1/4" long 8-32 bolts are used to hold the plates to the spacers.

I put a 1 1/2" long pan head 10-24 bolt through the center hole of the reflector from the side away from the patch, and fastened the center bolt to the reflector with a 10-24 nut. Then I mount the four 1" nylon spacers to the reflector with four pan head 8-32 bols. This provides the 1" 435 MHz MHz patch spacing. Put another 10-24 nut over the center bolt, so that the patch side of the nut is 1" from the surface of the reflector. You can trial fit the patch to the reflector, verifying all the holes line up. The third hole on the reflector, tapped to 1/4-20, should be clockwise from the 2nd hole for LHCP. If counter-clockwise from the 2nd hole, the patch becomes RHCP. Since the dish reflection inverts polarity, the desired RHCP dish needs the clockwise LHCP location for the third hole.

I made the adjustable bolt capacitor much larger for greater spacing since it will be used for transmitting. I soldered a 1 1/8" piece of 1/32" brass left over from making the 2401 patches to the centered head of a 1 1/2" long 1/4-20 flat head bolt.

Now carefully screw this 1 1/2" long flat head 1/4-20 bolt & with its 1 1/8" brass head into the threaded third hole on the 435 MHz reflector, from the side of the reflector that will be facing the 435 MHz patch. Leave the brass head about 3/4" above the reflector surface for now. Attach a 1/4-20 nut on the other side of this tuning bolt to act as a tuning lock.

Finally, screw another 1 1/2" flathead 1/4-20 bolt used for fine frequency adjust into the reflector from the side of the patch. Attach another 1/4-20 nut on the other side of this tuning bolt to act as a tuning lock.

Fasten the patch to the center bolt with another 10-24 nut, fastening the type N center conductor's 6-32 bolt extension to the 2nd hole in the patch with another 5/16" size 6-32 nut. . I experimentally determined that the best SWR for this 435 MHz patch feed resulted when the flat head of 1/4-20 bolt and brass head was spaced the thickness of six postcards (direct mail "Bingo cards") from the surface of the reflector (.042"). Lightly snug up the tuning lock nut when this spacing is achieved. Later, if you are happy with the performance, you can seal everything with some fingernail polish.

I vary the frequency tuning screw to achieve the best SWR at the desired operating frequency, and snug up the this lock nut to keep the tuning stable.