Making a Dual Band 435 MHz and 2401 MHz Patch Feed

Several months ago I made a dual band (S & U) patch, shown at the left, for my 10' dish. The 2401 MHz patch's presence has negligible effect on the resonant frequency and the impedance of the 435 MHz patch. An AIDC 3731 from K5GNA is held to the back of the 435 MHz reflector with 2 wire ties. A type N right angle adapter is attached to the AIDC 3731, and a type N double Male is attached to the right angle connector and the 2401 MHz patch. The 2401 MHz adapters go through 1" holes in the center of the 435 MHz reflector and 435 MHz patch. I tried two 2401 MHz patches on the system. The ringless one's radiation angle is 150 degrees wide, and one with a 1" high ring, made from a 4" diameter peach can is 130 degrees wide. My dish's feed angle is 140 degrees. The 435 MHz patch tests out at 1.1 : 1 at the rig after 70' of 1/2" hardline over 435 MHz to 436 MHz. I tried a strong beacon signal and a weak beacon signal while putting 75 watts into the hardline going to the 435 MHz patch. No sign of any desensing with no filter used other than the built in comb filter in the front end of the AIDC 3731. The exact dimensions are are given below. I tested the 2401 MHz patch types by screwing on my various 2401 MHz feed designs and seeing which one gave the strongest signal on the AO-40 beacon. I also tried a 3 turn helix. All simply screw onto the type N double male adapter, so the test is quick & easy. The patch with just a simple 4" plate reflector produced the best signal strength. The 3 turn helix was definitely the poorest performer. The "peach can patch" should be used if you are experiencing side lobe QRM. Although the "peach can patch" under illuminates the dish slightly, hence has slightly lower received signal strength, it also has a significantly lower level of spillover loss.

The Doppler effects on 2401 MHz are bad enough. Most downconverters' crystal oscillator also drift. Keep the downconverter always turned on, and as shown in the picture at the right, covering the downconverter with some pipe insulation really stabilizes it especially during days with broken clouds. A California Amplifier downconverter from WJ6T, shown in the picture at the left, also works well on the dual patch with no sign of desense.

Circular Polarized Dual Band (S & U) Patch Feed

There are seveal possible mechanical configurations, but in the picture at the right, the plates are held together with a strong center bolt and stout bushings, making this dual band S & U patch very strong mechanically. The main differences from the single band 435 MHz patch described previously are matching 1" holes bored in both the 435 MHz reflector and 435 MHz patch plate so the coaxial feed from the 2401 MHz patch can pass through to the 2401 MHz downconverter mounted behind the 435 MHz reflector plate. I have made versions of this S & U patch in which the 2401 MHz patch was completely separate but in front of the 435 MHz patch. I felt they were mechanically weak but they did permit rapid interchange of 2401 MHz patches to test which one worked the best.

Start by cutting out the 4 disks. The 2401 MHz patch (the smallest disk, in the front) is made from 1/32" (1/16" would be easier to tap) brass for solderability, the 2401 MHz reflector and the 435 MHz patch disks are made from 1/16" aluminum, and the 435 MHz reflector is made from 1/8" aluminum. Cut the rearmost 435 MHz reflector to 16 1/2" diameter. Cut the 435 MHz patch to 14" in diameter. The 2401 MHz reflector is next at 3 1/2" in diameter, and finally cut the 2401 MHz patch out of a piece of brass to 2 3/8" in diameter. I use K & S brass from a local hardware or hobby store rack. You could use a soup can lid but it's tough to tap.

Center Hole in all four Disks

Drill a 3/16" hole in the center of the 4 disks which will be used for axial alignment of the disks when a 10-24 bolt, 2" long, is passed through the disks. Make a mark anywhere on the brass disk that is 11/16" out from the center hole. This will be the point of attachment of the 2401 MHz coaxial connector and the centering hole for the larger holes in the bigger aluminum disks. Temporarily bolt the four disks together tightly with the brass disk on top. Now drill a hole through the mark in the brass disk and the 3 aligned aluminum disks below the brass disk. Remove the 10-24 bolt and mark one side of each disk as "UP".

2401 MHz Patch Plate

Put a mark on the brass disk 1/2" clockwise from the 1/8" feed hole, also 11/16" out from the center hole. Drill a 1/8" hole in the brass 2401 MHz patch at this point. Then tap this hole with a 6-32 tap held perfectly perpendicular to the surface of the 2401 MHz patch. Screw in a 1/2" long 6-32 flat head bolt from the "down side". Put a 1/4" wide hex 6-32 nut on the "UP" side of the bolt, with about 1/8" extending downwards. Finger tighten the nut to avoid stripping the threads in the brass disk. This screw will be used to tune for best received circularity and strongest received signal (hopefully the same point). Set the brass disk aside for now.

2401 MHz Reflector

Now the 2401 MHz reflector is drilled out to attach a type N male long chassis connector. I have 2 male sizes in my connector collection. This design requires a connector that is about 1 1/8" long from flange bottom to barrel top. The shorter 3/4" version is not long enough to directly attach a downconverter. Drill out the off center 1/8" hole previously drilled to 3/8" exactly centering the drill on the 1/8" hole. Some chassis connectors have a bottom taper that may require as much as a 1/2" hole for center pin clearance. This type N male chassis connector is attached from the "DOWN" side with four 4-40 flathead 1/4" long bolts inserted from the "UP" side. Holding the connector centered in the 3/8" (1/2") hole with the outer edge of the connector parallel with the outer rim of the 2401 MHz reflector, mark where the holes will be drilled. Drill four 1/8" holes at the marks. Then turn the 2401 MHz reflector over and counter sink bevel the holes from the "UP" side so the four flathead screws will attach as flush as possible. Attach the type N Male long chassis connector to the "DOWN" side of the 2401 MHz reflector using the four 4-40 flathead 1/2" long bolts and 4-40 1/4" nuts. Another disk done.

435 MHz Patch Plate

The 435 MHz patch and reflector plates get 5 more holes on their "UP" side. Draw a line on the 435 MHz patch plate from center to edge, 90 degrees clockwise to the 1/8" feed hole previously drilled for the 2401 MHz feed hole. Then put a mark on this line 3 3/4" out from this center. This will be the 435 MHz patch's feedpoint, 90 degrees out of phase with the 2401 MHz patch feedpoint. Bolt the center of the two 435 MHz disks together with the "UP" sides up, patch on top. Temporarily lock the two plates' orientation together by putting a short 4-40 bolt through the 1/8" holes already drilled in each. Now drill a 5/32" hole at the mark on the line for attaching the 435 MHz coaxial center pin.

The 435 MHz patch plate is held 1" off the surface of the 435 MHz reflector by four 1" long nylon spacers. The exact pattern is not particularly important, but I like things squared off, so after making a 4 square pattern, I hold this pattern on the 435 MHz patch plate so that the sides of the square are parallel/perpendicular an axial line drawn between the two feed holes in the plate, and centered. Then I place a mark at each corner and drill a 3/16" hole at the four marks through both the 435 MHz patch and the 435 MHz reflector for mounting the nylon spacers with 1/4" long 8-32 bolts.

Unfasten the two plates and drill a 3/8" hole in both plates, centered on their 1/8" alignment hole. Dig out your old trusty 1" Greenlee tube (you remember those things, don't you) hole punch and punch a 1" hole in both the 435 MHz reflector and patch plate for the 2401 MHz feed to pass through. Set the 435 MHz patch plate aside.

435 MHz Reflector Plate

The disk with the most holes will be the 435 MHz reflector. Orient the disk with the "UP" side upwards, and the 1" hole to the left so we are all playing from the same sheet of music. The 5/32" hole drilled in the previous section should be at the top. Place a mark 2 3/4" clockwise from this 5/32" hole, also 3 3/4" out from the center. This hole will be used for the circularity adjusting bolt. Drill a 3/16" hole at this mark. Now draw a line across the top of the disk, starting at the 5/32" feed hole continuing through the center hole to the opposite edge. 6" out from the center, place a mark on this line for the frequency adjusting screw. Drill a 3/16" hole at this mark too. Tap out both 3/16" holes with a 1/4-20 tap, making sure you keep the tap perpendicular to the surface of the plate.

Drill out the 5/32" hole to 3/8" for the type N female chassis connector, keeping the 3/8" (1/2") hole centered to the 5/32" hole. While holding this female chassis connector centered in the hole and the edges parallel with the rim of the disk, mark the four holes that are used to attach the connector. Drill the four holes with a 1/8" drill. From the "UP" side counter sink bevel the 4 holes so that four 4-40 flathead 1/2" long bolts will lie flush.

The final holes in the 435 MHz reflector plate are used to attach this dual band patch to a dish using the original mounting arm(s) used with a Chaparral C Band feed. The original Chaparral feed was attached to the arms with four 1/4-20 bolts in a 4 1/8" square pattern. If you have a single tube mount for the Chapparal feed like on my 10' Janeil dish, then you need to put in two holes 4 1/8" apart, 2" out from the center for attaching the rear of the 435 MHz reflector to the single tube mount.

Otherwise, you will have a four tube mount for holding the Chapparal feed in front of the dish. Since the 435 MHz reflector is 16 1/2" in diameter instead of the 7" on the Chapparal feed, the geometry puts the patch about 3" too far away from the focus point. A simple adjustable cross frame can be used to put the dual band S & U patch feed back in focus, unless you want to shorten the length of the four tubes. Either way, drill four 1/4" holes in a square pattern, 1/2" in from the edge of the 435 MHz reflector. They will be about 11" apart, on a 15 1/2" diameter. Put on four 1/4-20 hex head bolts from the "UP" side and fasten with a nut on the "DOWN" side, along with an additional nut and wingnut on each bolt for precise focusing.

Attach the type N female chassis connector to the "DOWN" side of the 435 MHz reflector plate using four 4-40 flathead 1/2" long bolts and 4-40 1/4" nuts. From the "UP" side of the 435 MHz reflector plate screw in a 1" long 1/4-20 flathead bolt into each of the two 1/4-20 tapped holes from the "UP" side. Leave extended about a 3/8" high and place a 1/4-20 nut on the "DOWN" side of each bolt to lock it in place after calibration. Finger tighten only, to avoid stripping the threads in the plate.

435 MHz Circularity and Frequency Adjusting Bolts

The frequency adjusting bolt for the 435 MHz patch is a standard 1 1/2" long 1/4-20 flathead bolt.

The circularity adjusting bolt for the 435 MHz patch is a standard 1 1/2" long 1/4-20 flathead bolt with a circle of scrap 1 1/8" brass center soldered to the flat head.

Assembling the Dual Band S & U Patch

In reverse order, or "bottom up" sequence, place a 2" long 10-24 bolt through the center hole in the 435 MHz reflector plate from the "DOWN" side. Fasten with a 10-24 hex nut and firmly tighten. Now put another 10-24 nut on the the 10-24 bolt and adjust so that a 1/2" space between the plates will result. The center conductor the the type N female chassis connector has to be extended. I cut the head off a brass 6-32 bolt to 1 1/2" long and sharpen down the cut off end on a grinder so that the end result is about 1 1/4" long. I tin this sharpened end the center conductor of the type N female chassis connector. Holding the sharpened end on the center conductor, I solder the two together, making sure this center conductor threaded extension is standing up straight, perpendicular to the surface of the 435 MHz reflector. Try to be neat about it.

The next thing to do is mounting the four 1" nylon spacers to the 435 MHz reflector "UP" side. The spacers came from a Surplus Electronics place called Saunder's in Boulder, Colorado. These nylon spacers are 1" long, 1/2" in diameter, and have a 1/8" hole bored through the center. I tapped the hole out to 8-32 on either end of each spacer and attached them to the 435 MHz reflector using four 1/4" long 8-32 bolts.

Screw in the circularity adjusting 1/4-20 bolt with the 1 1/8" brass head into the 1/4-20 tapped hole nearest the type N connector, from the "UP" side. Screw it in so that the head stands about 3/4" off the surface of the reflector.

Screw in the frequency adjusting 1/4-20 flathead bolt all the way into the 1/4-20 tapped hole axially opposite the type N connector, from the "UP" side, so that its head is near the surface of the reflector, for minimum capacitance.

Place the 435 MHz patch plate on the center mounting bolt and align the patch plate so the 1" holes match and the feed screw goes through the 5/32" feed hole in the patch plate. The holes in the top of the spacers should also appear. When appropriate adjustments are made if needed, remove the patch plate, put a 5/16" 6-32 hex nut on the brass screw coming from the type N connector and a 10-24 nut on the center bolt so the patch plate will be spaced 1" from the reflector, put back the patch plate and add another 10-24 nut and 6-32 nut to hold firmly in place. The 435 MHz patch plate should be spaced 1" from the 435 MHz reflector all around and the brass screw should not be pulled up, potentially breaking the solder joint.

Put another 10-24 nut on the center screw, placed so a 1/2" of separation from the 435 MHz patch plate to the back of the 2401 MHz reflector plate occurs. Place the 2401 MHz reflector on the center hole, with the type N male long chassis connector extending through the center of the 1" holes in the 435 MHz reflector and 435 MHz patch plate. Now put another 10-24 nut over this center screw to lock the orientation perfectly centered.

Put another 10-24 nut on the top of the last nut, snug it up and verify that you have 1/4" spacing. If your nuts are not exactly 1/8" thick like mine, then use washers to achieve 1/4" spacing between the 2401 MHz reflector and the 2401 MHz patch plate. Align the 1/8" hole in the 2401 MHz patch plate with the center conductor underneath. Add a final 10-24 nut to the center bolt to lock the 2401 MHz patch plate to the assembly. You can use a small wire to bridge a gap if your center conductor is short, but I usually just form a solder bridge between the center conductor and the 2401 MHz 1/8" feed hole.

At this point you should have an interesting stack of four disks that looks like it might communicate with ET directly.

Attach this S & U dual band patch in place of the C Band Chaparral feed horn as detailed above. Connect 435 MHz coax to the type N female connector, and put a downconverter on the type N male long connector. A California Amplifier from WJ6T is shown in the picture at the right. A cross piece is shown which provides the correct focal spacing when used with four arm feed support systems on most TVRO dishes.

Calibration

I find that if I have exactly followed the dimensions and set the 3 tuning bolts as will be detailed, the patch will work very close to perfect without further adjustments.

Loosen the 6-32 locknut on the 2401 MHz patch plate and turn the front screw so that it is spaced .020" or the thickness of 3 "bingo" postcards above the reflector. Loosen the 1/4-20 locknut on the frequency tuning bolt furthest from the 435 MHz type N connector. Turn the 1/4-20 flathead bolt so that it is as far from the 435 MHz patch plate as possible. Loosen the 1/4-20 locknut on the circularity tuning bolt with the 1 1/8" brass head nearest to the 435 MHz type N connector, and turn the bolt so that it is just touching the back of the 435 MHz patch. Note the position of the slots on this flathead bolt and turn the bolt back exactly 2 turns (.1") or 14 "Bingo" postcards separation. These setting will be very close, but you can use the other 1/4-20 bolt furthest from the 435 MHz connector to trim the 435 MHz patch to exact frequency. I achieved an almost perfect match when this frequency adjusting bolt was 3 turns back (.150") from touching the rear of the 435 MHz patch.

Does it work?

I find that when the squint and distance to AO-40 is reasonable, the beacon from AO-40 will be coming in about 20 over S9 on my 10' dish with this dual patch. When I compared the return strength of my 40 element circularly polarized KLM 435 MHz yagi (15 db gain) to the 435 MHz patch on my 10' dish, it took twice as much power (3db) to the KLM to equal the return signal from the 435 MHz circular patch feeding the 10' Janeil dish.

When you have achieved perfection at the patch feed point, be sure to measure the SWR at the rig end end of the coax too.