1296 MHz Dual Circularity Patch Feed for EME
Two connector, one circularity screw, 1296 MHz only version
That BUD of can be used for more than just AO-40. 10' or larger BUDs make great EME dishes if a suitable feed system appropriate to EME is added. Most of the feeds in the past have been horns, but the narrow angle of these feeds, designed for long focal length dishes, will underilluminate the dish, resulting in signal loss on transmit and receive.
EME 1296 MHz activities use right circular on transmit and left circularity on receive due to the moon reflection reversing the polarity. Many EME stations use complex polarity reversing schemes or separate antennas. This is a description of a dual circularity patch feed that takes advantage of a BUD's high gain and short focal legth. By mounting a pair of connectors on the patch assembly, one optimized for left circularity and the other optimised for right circularity, a single BUD can outperform a very complex array of yagis for 1296 EME.
This patch transmits left circular polarity to compensate for the dish's polarity reversal, resulting in a right circular polarized signal transmitted to the Moon. On receive, the double reversal (by the dish and the moon) matches the receiving connector's right circularity.
1296 MHz DSP EME using JT44 occurs between 1296.1 and 1296.2 MHz. There is a SETI Moon Beacon transmitted from New Jersey on 1296.000 MHz.
Here are the 1296 MHz dual circularity patch disk's dimensions and
holes:
4 3/8" diameter, 1/16" thick aluminum. Center hole is 3/16" for a 10-24 bolt. Coming out 1 5/16" from center are two 3/32" holes for 6-32 bolts. The two holes are at a 90 degree arc from the center which makes them 1 13/16" apart.
1296 MHz dual circularity reflector disk's dimensions and holes:
6 7/8" diameter, 1/16" thick aluminum. Center hole is 3/16" for a 10-24 bolt. Temporarily bolt the two disks together at the center, and drill matching 3/32" holes also through the reflector plate which defines the center of a type N chassis connector. Unbolt the disks and drill out the two 3/32" holes to ~1/2" to provide clearance for the type N center conductor & housing. Drill four 1/8" holes for mounting each type N chassis connector on the backside of the reflector plate. The receiving Type N chassis connector can be a male chassis connector, as shown in the picture to the left of the patch. Rotate the position of the chassis connectors for maximum flange clearance away from the circularity screw between the connectors. Bevel the inside of the eight 1/8" holes for 1/4" flat head 4-40 bolts that hold the two type N chassis connectors. Drill a 3/16" hole between the two type N connectors, 1 5/16" from the center. Tap with a 1/4-20 tap for the circularity screw. Draw three lines through center to the other edge. One line is from the circularity hole, and the other two are from each type N center hole. Place a mark 1 3/4" out from center on each hole, on the side opposite from center of the connectors and circularity screw. These will be used for the frequency adjusting screws. Drill a 3/16" hole at the three marks, and tap with a 1/4-20 tap. Make a circularity adjusting capacitor bolt by soldering a 1" brass disk to the head of a 1/4-20 flathead bolt.
Assembling:
Put
a 10-24 x 3/4" long bolt through the reflector center hole from the rear. Tighten
down with a 10-24 nut, and put a second nut on the bolt, outside spaced 3/8"
from the front surface of the reflector plate. Solder a brass 6-32 x ~1/2" bolt
extension to the center conductor of each type N chassis connector. Screw in
the circularity adjusting bolt with the brass head facing upwards towards the
where the patch plate will be. The outside spacing of this circularity adjusting
plate will be ~ 1/4" from the front surface of the reflector plate. Place a
1/4-20 nut over the end of the circularity bolt for locking the spacing after
tuning. Place a 1/4" 6-32 nut over each 6-32 extension bolt from each type N
chassis connector, outside spaced to ~ 3/8" from the front surface of the reflector
plate like the center 10-24 nut. Screw all the way in, a main frequency adjusting
1/4-20 x 1" hex head bolt diametricly opposite the circularity bolt, from the
side facing the patch disk. Place a 1/4-20 nut over the end of the main frequency
adjusting bolt for locking the spacing after tuning. Screw all the way in, two
frequency trimming adjusting 1/4-20 x 3/4" hex head bolts diametricly opposite
the two type N connectors, from the side facing the patch disk. Place a 1/4-20
nut over the end of each frequency trimming bolt for locking the spacing after
tuning. Place the 1296 MHz dual polarity circular patch over the center 10-24
bolt and the two 6-32 center conductor extension bolts. secure the patch disk
with a 10-24 nut and two 1/4" 6-32 nuts.
Tuning:
Attach a low power 1296 RF source to an SWR meter and couple to the XMt connector which is the right hand type N chassis connector as viewed from the rear with the connectors at the top. Trim the circularity bolt for best SWR. Fine trim with the main frequency adjusting bolt. There is considerable interaction. Then connect the RF source and SWR meter to the left connector and check the SWR. Use the fine trimming frequency bolts diametricly opposite the driven connecotr to correct any construction asymetry. When satisfied, lock the four tuning bolts with their 1/4-20 nuts.
Mounting the Patch on the Dish:
Drill
four or three (or six) 1/4" holes at the edge in either a 90 degree or 120 degree
pattern (or both), keeping things as symetrical as possible, to match your support
arms coming from the dish. A single center support will only require two 1/4"
holes spaced 90 degrees apart at the edge. Focus the dish on the 1296 patch
for maximum signal. This patch's beamwidth will be about 150 degrees wide, making
it ideal for EME use with centerfed TVRO dishes in the .3 to .4 F/D range.
Using:
Mount a type N SPDT relay behind the reflector plate. Connect the N.O. contact to the receive type N connector with a double male type N adapter. This will then require energizing the relay on receive, a typically safe way to avoid accidentaly transmitting into your preamp. The relay output goes to the input of a EME rated 1296 MHz preamp such as the DEMI 23ULNA PHEMT <.4 db 17 db gain preamplifier. The attached picture shows an ICOM 1296 MHz preamp made to work into the ICOM 910H but the lower cost ($140) DEMI preamp would be better. The transmit type N connector is connected directly to the output of the 1296 MHz amplifier, potentially one of the DL2AM 1296 MHz bricks shown in the left picture below, mounted right at the rear of the 1296 reflector as well. Indoor tube amplifiers such as the 2C39 cavity by W4SM or the GI7B Russian tube in the water cool cavity by W4SM offer a couple of hundred watts, potentially driving a Russian GS-35B to a KW.
Future:
This EME dual patch feed can be combined with an S band patch to produce a dish usable on both EME and AO-40. Going one step further, by adding a U band patch to the assembly we get a triband patch good for EME and AO-40 use under marginal squint conditions. The ultimate might be making a tri band patch with a dual circularity 435 MHz patch too, so a single dish could be used for 432 MHz EME, 1296 MHz EME, 435 MHz uplink to AO-40, or 1269 MHz uplink to AO-40, both with a 2401 MHz downlink from AO-40 without any feed system replacement. Stay tuned for more dual feed multi band circular patch feeds.
Robert W0LMD