My initial dual band patch feeds were built by making the individual patches, already described, then mounting the patches (S & L) side by side as shown at the right, but the pointing error was unacceptable. My recent dual band patch feeds have been axially mounted, and perform very well.
6' or larger center fed dishes on AO-40 do well with dual band (2401 MHz and 435 MHz - S & U bands) patch feeds. There is a surplus metal dealer south of Denver that has a good supply of aluminum sheets for $1.60 a pound. I cut these sheets on my bandsaw to make a great looking dual patch system. Sometimes it takes a bit of effort get the SWR down on the 435 MHz patch.
I use a $700 graphic SWR meter from AEA called a 140-525 Analyst to get a very close match. Some may question whether it is worth the effort to achieve circularity on the transmitting patch. AO-40 operators have found that stations with circularly polarized uplink signals show considerably less QSB, especially during high squint angle times. I trim the reflector on the 2401 MHz to 3 1/2" diameter to minimize the blockage it presents to the 435 MHz or 1269 MHz patches.
Making a Dual Band 435 MHz and 2401 MHz Patch FeedMaking a Dual Band 1269 MHz and 2401 MHz Patch FeedA TRI Band Patch Feed System for TVRO Dishes
AO-40 currently has 3 bands in common use. 435 MHz (Mode U) is the most popular uplink frequency, and 1269 MHz (Mode L) is growing in uplink popularity, especially when the Squint angle is in a favorable position. 2401 MHz is the downlink frequency in common use. While most AO-40 hams with BUDs just use them for receiving, a large center fed dish makes a great L & U band transmitting antenna also if the BUD is 5' in diameter or bigger.
My measurements found the following
Tri Band Circular Patch Performance from my 10' BUD:
435 MHz - 18 db of gain - 20 degrees beamwidth (a 40 el KLM cross yagi has 15 db of gain)
1269 MHz - 31 db of gain - 6 degrees beamwidth (My 45 element Directive Systems Loop Yagi has 20 db of gain)
2401 MHz - 35 db of gain - 3 degrees beamwidth (3' BBQ grill dish has ~20 db of gain)
A 7 1/2' dish is 1/2 the area of a 10' dish and will be 3 db down on each measurement; a 5' dish is 1/4 the area and will be 6 db down. My new 14' dish is 3 db stronger than the 10' dish measurements.
Making a TRI Band Patch Feed System for TVRO Dishes EME Patch Feed Systems for the BUD
If you have a 10' or larger dish, you should consider using it for Moonbounce. The Moon is quite a ways from Earth, about 224,000 miles or so, and not much of the energy sent its way is reflected. But it will be your most distant contact if you can make it happen. Not too many years ago, EME was accomplished with big antennas and big amplifiers. But recent developments make it possible to receive and send a signal from the moon using that old BUD in the back yard.
There are a number of formulas for predicting the probability of sucess in you EME efforts. A number of different VHF and UHF bands are used, but in this discussion I will limit the bands to 70 CM (432 MHz) and especially 23 CM (1296 MHz).
1296 EME is accomplish by sending a right circular polarized signal (like AO-40) to the Moon, and receiving a left circular echo from the Moon (the echo inverts the circularity). Now considering the fact that the dish also inverts the polarity, what we are trying to accomplish is achieved by generating a left circular polarized signal in the feed system, send it into the dish where it reflects into a right circular signal beamed at the Moon. The Moon reflects part of the energy sent its way, and the inverted reflection is now left circular. The BUD reflects this signal, and the feed system collects the signal as right circular polarized. Gets a bit confusing, but the bottom line is that the feed must produce a left circular signal and receive a right circular signal.
The generation and reception of these two polarities is not too difficult to accomplish if the feed system for the BUD is made with dual polarity. This can be done using the previous patch systems by either using a patch with a single coax connector and a enabling a polarity injecting capacitor on either side of it, or a single polarity injecting capacitor with a pair of coax connectors on either sside of the polarity injector. The dual coax connector technique seems simpler so is the system chosen for the designs that follow.
Two Coax Connectors, One Circularity Injector Patch FeedStart the duality by placing the two coax connectors 90 degrees displaced from each other.Then the circularity injecting capacitor is placed between the two connectors at 45 degrees. Sending the RF to the connector at the left results in a left circular signal. Receiving a signal with the right connector will favor a right cicularly polarized signal. By also placing capacity injectors diametrically opposite the transmit and receive connectors, each circularity and be individually adjusted and matched.
So the actual addition to an existing patch feed shown above is one coax connector and one additional frequency trimming capacitor in return for an ability to work the Moon.
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The picture at the left shows the front side of a standard left circular S & L bands patch feed, a dual polarity L band patch feed, and a dual S & L band patch feed with dual circularity on the L band and left circularity on the S band. The dual circularity is not so obvious from the front side of the patches.
The picture at the right shows the rear of the three patch feed systems, and shows the slightly greater complexity added with dual circularity. The S band feed hole was moved 90 degrees to assure circularity symetry of the dual circularity L band feeds.
Going one step further, a tri band circular S, L, & U band patch feed can be redesigned so that it becomes an S, 2L and 2U patch feed. This now lets a single large dish function as an EME antenna on 432 MHz and 1296 MHz, and operate as an AO-40 antenna using the left circular S band (2401 MHz) feed to the AIDC 3731 downconverter (to 145 MHz or 123 MHz) shown below the patches with your choice of an L band (1269 MHz) or U band (435 MHz) uplink. That's getting a lot of bang for your buck! Components were carefully placed to assure maximum dual circularity symetry.
Making a Dual Circularity 1296 MHz Patch Feed for EMEMaking a Left Circular 2401 MHz & a Dual Circularity 1296/1269 MHz Patch Feed for EME and AO-40Making a Left Circular 2401 MHz, a Dual Circularity 1296/1269 MHz, and Dual Circularity 432/435 MHz Patch Feed for EME and AO-40Metric BoltsSeveral foreign builders have requested information about converting the USA standard bolts that I used to metric sizes.
DJ9PC has built and is using the dual band S & L with his dish and has sent the following conversions:
4-40 bolts for holding connctors = 3 mm metric bolt. Drill oversize with a 4 mm drill.
6-32 bolt for tuning 2401 MHz patch circularity = 4 mm. Drill undersize with a 3.2mm drill and tap with a 4 mm tap.
10-24 bolt used to hold the disks together at the center = 5 mm. Drill oversize slightly with a 5 mm + drill.
1/4-20 bolt used for circularity and frequency tuning = 6 mm. Drill undersize with a 4.8 mm drill and tap with a 6 mm tap.
The reflectors and patch diameter and spacing dimensions can be converted from inches to mm by multiplying by 25.4. Thanks Peter.
ConclusionsWhen all is said and done, I have made up these dish & feed system rules:
1. Measure the F/D ratio of your dish. (F = Focus distance D = Diameter) Use any measurement system you wish. Determine focus point by attaching 2 small mirrors near the opposite edges of the dish, and, aiming the dish at the sun, see where the 2 reflections converge. My 10' dish (120") focuses 42" from the center - it has an F/D of .35
2. If the F/D is less than .4 (typical of center fed TVRO dishes), use a Patch feed. If less than .35, don't use a scalar ring. My 3' barbecue grill with 1/4" fence wire added is .3 F/D. It really works great with a soup can lid Patch.
3. If the F/D is greater than .4 (typical of homebuilt center fed dishes & offset fed dishes), use a Helix feed. Be sure the Helix is Left Circular wound (Counterclockwise forward when looking forward from reflector) for AO-40, since the dish reflection reverses the polarity, just like a mirror.
4. Make the Helix # turns = 1 turn for each .1 of F/D. My Primestar offset fed 40" x 30" & DirecPC 35" x 21" (pictured) have a .55 F/D. I use a 5 1/2 turn wide spaced Helix on them. My 18" dish NETWORK hubcap is .6 F/D. A 6 turn close spaced Helix would be slightly better for it.
5. Small dishes may have too much blockage generating undesirable side lobes to be used for transmitting, but a dual (L & S bands) will work on a 3' (1 meter) dish. If you have a 4' or larger BUD, a dual (U & S or L & S bands) or triple (U, L, & S bands) feed systems make a lot of sense, and will produce more gain than a long yagi with 40 elements. Dishes under 6', when used on the uplink, will require external amplifiers. 500 watts ERP is needed on 435 MHz and 1500 watts ERP is needed on 1269 MHz.
6. 6' or larger BUDs will work AO-40 barefoot. Smaller dishes will require an external amplifier on the uplink if used for transmitting on the U or L bands. L2 will take 4 times more power or twice the dish's diameter compared with L1 ERP power levels.
7. A 10' or larger BUD puts you well within range of EME, so consider one of the dual circularity patch feed systems mentioned in this section.