Constructing a Cardboard-box Antenna
for Receiving AO-40 on S-band
by Anthony Monteiro, AA2TX
[© 2002 - Anthony Monteiro]
These notes provide the directions for constructing a Cardboard-box antenna for receiving the AMSAT OSCAR-40 satellite on 2.4 GHz (S-band.) It is constructed from two cardboard cartons, aluminum foil, and packing tape. No coax, connectors, or soldering is needed. In spite of this, it provides 20dBi of gain, equivalent to a two-foot, parabolic dish. The down-converter is directly attached to the horn and a short piece of 14 gauge wire is used to create a coax to waveguide transition coupling. The antenna is a pyramidal horn. The basic pyramidal shape is shown below in figure 1.
Basic Pyramidal Horn Shape
The technical performance specifications are as follows:
· 2.4 GHz (S-band)
· 20 dBi gain
· linearly polarized
· clean radiation pattern with minimal side lobes
· return loss of greater than 20dB (SWR < 1.2:1)
The structure of the antenna is made of cardboard. A very inexpensive source of cardboard is old shipping cartons available from a grocery store. These are generally free for the asking. The large size standard carton is around 16" x 22" x 22" of 1/8" thick cardboard though, the exact size is not critical. Two cartons are needed. One is cut up and used to make the horn side panels and the other is used to support the horn structure.
The horn panels are covered with aluminum foil, which is held in place with packing tape. It is suggested that 18" wide aluminum foil be used to make construction easier. Similarly, 2" wide packing tape is suggested.
The most exotic component required is a 5/8"-24 threads-per-inch nut used to mount the down-converter. This is the size required to screw onto the threads of a female N-connector. This nut is supplied with single-hole, chassis, and bulkhead coaxial (N and UHF) connectors and is also widely used for other types of circular connectors. A modest junk box will often include several of these nuts but they are also readily available from coaxial connector suppliers for $.25 - $.50 each.
The only other component required is a short length of solid, bare, 14 gauge (AWG) copper wire for the coaxial to waveguide coupling probe. This is to be cut as carefully as possible to 1-5/8" inches long and will be inserted into the female N-connector of the down-converter. Note that the diameter of 14-gauge wire is the same as the center pin of a male N-connector and so will fit snugly yet not deform or damage the female N-connector on the down-converter.
The complete list of materials is as follows:
· (Quantity - 2) Cardboard shipping carton, (around 16" x 22" x 22")
· Roll of 18" wide aluminum foil
· Roll of 2" wide, clear packing tape
· 5/8"- 24 nut to fasten N-connector
· 1-5/8" length, solid, bare 14 gauge (AWG) copper wire
Draw the horn panels on the cardboard carton sides as carefully as possible and then cut out the horn panels with either a box cutter or scissors. Be careful not to mangle the cardboard! After cutting out the horn panels, it may be helpful to apply some tape to strengthen the panels.
Next, apply the aluminum foil to one side of each horn panel, roll it over the edges of the panel, and tape it snuggly on the back side to hold it in place. The foil needs to cover all of the edges of the cardboard panels Do not allow the tape to run over the edge of the panels. The foil on the edges will be used to make electrical contact between the panels.
Once you have created the panels, they need to be taped together. The easiest way to do this is the place all four panels on a flat surface foil side down and aligns the long edges together. Make sure to put the side panels in-between the top and bottom panels. This will then look like an unfolded, flat horn. The long edges are all about 27.3" so they should all line up. You will be taping the three long sides that are touching.
Separate the edges from each other by 1 cardboard thickness (~1/8") using a scrap piece of cardboard and then tape the backs (not the foil side) of the panels together. This is important; the long panel edges need to be taped 1 cardboard thickness from each other in order to make good electrical contact when the horn is folded together.
After the panels are taped together, but still flat, carefully pick up the panels, form the pyramid shape, and tape the remaining edge tightly. When you form the pyramid, the cardboard on the edges will slightly crush which will hold the aluminum foil covered edges together making good electrical contact. After taping the fourth edge, hold the pyramid up to a light to see if there are any gaps between the edges. There will probably be a few. The design can tolerate 1/4" to 1/2" gaps without problems. Carefully tape the larger gaps to pull the edges together better. If you end up with any really large gaps, patch them with foil and tape.
The pyramid will not hold its shape at this point so set it aside for now and get the remaining cardboard carton. This carton will be used to hold the pyramid, providing both structure and protection. We will be taping the mouth of the pyramid to the open end of the cardboard carton. The top and bottom of the pyramid are about 15.5" apart at the mouth and will fit nicely into the 16" wide opening on the carton. However, the pyramid will be about 25" deep from front to back, which is too big for the carton. We need to cut a hole in the bottom of the carton to allow the back of the pyramid to fit through. The hole should be centered in the bottom of the carton and will need to be around 5" wide by 3.75" high. You will need to trim this hole to fit the pyramid.
Once you have the hole completed, center the pyramid in the carton. Next, align the front of the pyramid with the open end of the carton so it is flush and tape the top and bottom edges of the pyramid to the carton opening. There should be about 1" on each side between the pyramid sides and the carton sides. Apply some more tape to secure the back part of the horn to the hole in the back of the carton. Please see the photo of the completed horn in figure 4 but note that the back end will still be open at this point.
Front View of Horn
Now it is time to mount the down-converter. Cut three or four 2" x 2" pieces of cardboard from the remaining scrap and cut a 5/8" hole in each piece. These will be used to make a spacer for the down-converter. Slip the cardboard spacers over the N-connector on the down-converter. Being careful not to rip the aluminum foil, push the end of the N-connector through the 5/8" hole in the top panel at the back end of the horn and secure with the 5/8"-24 nut. You will need to compress the cardboard spacers a little to do this. The top of the connector should be just about flush with the top of the nut when it is secured. The idea is to use the resiliency of the cardboard spacers to hold the down-converter nice and tight in the hole but you do not want the connector to protrude very much into the horn. The down-converter will still have a fair amount of play in the hole.
When the converter is mounted, take the 1-5/8" piece of 14 gauge wire and gently push it into the center hole of the N-connector until it won't go in any further. When seated properly, the wire probe should protrude around 1-1/4" past the edge of the N-connector into the horn. Hold the down-converter so that the probe is about parallel with the back plane of the horn and use tape to hold the down-converter in place. Please see the photo in figure 5. Finally, use a piece of aluminum foil to cover the back end of the horn using tape to hold it taught. Congratulations, your AO-40 receiving antenna is now complete!
Back View of Horn with Down-converter
You will need to know the position of AO-40 in order to point the antenna. There are many satellite tracking programs available including some free ones on AMSAT's web site, www.amsat.org . This is a good source for general information on satellites and tracking. Another useful web site specifically about AO-40 is ao40.homestead.com .
The antenna has a beam-width of about 15 degrees both horizontally and vertically so pointing it is not too critical. The azimuth can be set by pointing the antenna using a compass. You will need to know the magnetic declination in your area which is the difference between where the compass points north and true north. This difference may be greater than the antenna beam-width as it is in North Andover, MA (15 degrees West.) You can find your declination and a detailed explanation at www.thecompassstore.com . A boy scout or girl scout can probably help here as well!
For elevation pointing, it may be helpful to tape some additional pieces of scrap cardboard to tilt up the outer carton as can be seen in the photo in figure 5. You can also just prop up the carton with a heavy object. A protractor will be helpful for setting the elevation angle. Most good compasses have a protractor function that you can use to measure the elevation angle.
The easiest way to test your new antenna is to point it at AO-40 and tune around for the middle beacon at (nominally) 2401.323 MHz. A certain amount of tuning around will be needed to find the satellite beacon due to the Doppler-shift and the frequency drift in the down-converter and satellite. With a little experience, it is actually quite easy to find the beacon. The AO-40 beacon is a very strong signal.
So, does this antenna really work? Yes! The author has used this antenna on several occasions to receive AO-40. The complete station setup uses an AIDC-3731AA down-converter running at an intermediate frequency of 123 MHz into a Yaesu FT-847. A laptop computer running AMSAT's InstantTrack is used to track the satellite.
Once you have managed to find the beacon, you can decode the satellite telemetry. A convenient way to do this is using the ao40rcv program available for free download at http://www.qsl.net/ae4jy/ao40rcv.htm This program uses the sound card in the PC to demodulate the signal and decode the telemetry. This was tested with the receiving antenna and worked great.