Constructing Cardboard-Box Antennas

for Uplinking to AO-40

 

Anthony Monteiro, AA2TX

aa2tx@amsat.org

 

[© 2002 - Anthony Monteiro]

Overview

In order to access the AO-40 satellite on the UHF uplink, an effective radiated power of 200 to 1000 watts (linear polarization) is required. Since the UHF output of a typical transceiver is only 20-50 watts, a high gain antenna is required. These notes provide directions for constructing antennas that can provide the needed gain. The antennas are unique in that that are constructed mostly from cardboard-boxes and aluminum foil. Two designs are presented a simple version and a high-performance version.

 

The simple version is made from a single cardboard box and has about 9 dBi gain. When fed with 50 watts, it will provide a solid SSB uplink signal to AO-40 out to about the 30,000-km range. It can also be used for CW all the way to the satellite's apogee at 62,000 km.

 

The high-performance version uses the same construction style but is bigger and requires more cardboard. It provides over 14 dBi gain, equivalent to an  8-element, optimized Yagi, and will provide a solid SSB signal all the way out at apogee.

Technical Description

The cardboard-box, transmitting antennas are corner reflectors operating at 435 MHz. They are fed with a simple dipole constructed from 3/4" aluminum tubing and "black" PVC insert couplings. The reflector surface is constructed from cardboard, aluminum foil, and packing tape. These antennas have no adjustments and no test equipment is required.

 

The basic corner reflector antenna is shown below in figure 1, the side-view is on the left and a profile view is on the right. The black rod represents the dipole feed.

 



The technical performance specifications are as follows:

Simple Version

·        Operating Frequency: 425-465 MHz (SWR < 1.5:1)

·        Reflector size: 22" x 22" x 16"

·        Gain: 9 dBi

·        Elevation beam-width: 1 dB = +/- 13 degrees, 3 dB = +/- 23 degrees

·        Azimuth beam-width: 1 dB = +/- 26 degrees, 3 dB = +/- 43 degrees

·        Polarization: linear horizontal

 

High-Performance Version

·        Operating Frequency: 425-465 MHz (SWR < 1.5:1)

·        Reflector size: 27" x 27" x 38"

·        Gain: 14 dBi

·        Elevation beam-width: 1 dB = +/- 8 degrees, 3 dB = +/- 13 degrees

·        Azimuth beam-width: 1 dB = +/- 13 degrees, 3 dB = +/- 24 degrees

·        Polarization: linear horizontal

Materials

The reflectors and the overall structure of the antenna are made of cardboard. A very inexpensive source of cardboard is old shipping cartons available from grocery stores. These are generally free for the asking. The large size standard carton is around 16" x 22" x 22" of 1/8" thick cardboard. You want this size although the exact size is not critical.

 

The reflector 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 dipole-feed is made from aluminum tubing and black PVC insert couplings of the type used in lawn irrigation (i.e. sprinkler) systems.  RG-58 type coaxial cable is used to feed the dipole. You will need to attach an N-connector to one end of the coaxial cable. A minimum of five feet of cable is required.

 

The complete list of materials is as follows:

·        (Quantity 1-3) Cardboard shipping carton(s), 16" x 22" x 22"

·        Roll of 18" wide aluminum foil

·        Roll of 2" wide, clear packing tape

·        1' length of 3/4" aluminum tubing

·        (Quantity 2) #8 x 1/2" sheet metal screws

·        (Quantity 3) 1/2" black PVC insert coupling

·        5' length of RG-58 with N-connector at one end and 1/2" leads at the other


Dipole Feed Construction


The dipole feed is the same for both the simple and high-performance versions of the antenna. The dipole uses a direct coax connection that provides a 50-ohm load when used with the corner reflector and no balun transformer or other matching network is required. The dipole is made from 3/4" aluminum tubing and the black PVC insert couplings. One of the PVC couplings is used as a center insulator for the dipole and the other are used to hold the dipole in place over the reflector. Please see figure 2 below. In the figure, the gray bars represent the aluminum tubing and the black bars are the PVC insulators.

Figure 2

Dipole Feed

 

To construct the dipole, cut two pieces of aluminum tubing, each 5-1/16" long. These can be easily cut with a pipe-cutter or a hacksaw. Push the two aluminum tubes onto one of the PVC insert couplings leaving a 1/4" gap in the center. This will require filing down the insert coupling. A DremelTM tool is helpful for grinding down the insert coupling but a file or coarse sound-paper will work also. The tubes should fit snugly and not move freely on the insert coupling. Note that the 1/4" gap is a critical dimension so measure this carefully.

 

With the tubes 1/4" apart on the insert coupling, drill a 3/16" diameter hole through the top of each aluminum tube and the insert coupling, about 1/8" from the end of the tube. Insert a #8 machine screw in each hole. Tighten just enough to thread the hole, then, back-off two turns. Wrap a coax cable lead around each screw and tighten. The center conductor goes around one screw and the shield around the other. Make sure there are no short-circuits.

 

Push an insert coupling into the open end of each of the aluminum tubes so it sticks out about 2-3/4" as shown in figure 2 and tape in place. The dipole feed is now complete.

 

Simple Corner Reflector

The simple version of the antenna is made from a single cardboard box. Cover the bottom and one of the narrow sides of the box with aluminum foil and tape in place. The bottom and narrow side of the box should each be about 16" wide and 22" long. You will need to cut or fold-over the foil to fit if you use the 18" wide size. The aluminum foil should form a corner reflector shape as was shown in figure 1.

 

Next, cut a 5/8" hole on each side of the box, 7" from the bottom and 7" from the foil-covered side as shown in figure 3 below. These holes will be used to mount the dipole. Also, cut a hole in the center of the reflector for the coax. Mount the dipole by pushing the insert couplings through the holes in the sides of the box and hold in place with tape. Push the coax connector and cable through the hole in the center of the reflector and tape the cable to the box to hold it in place. Congratulations you are done! Your completed antenna should look like the photo in Figure 4.

 

 

 



High-Performance Corner Reflector

The high-performance version of this antenna essentially the same as the simple version, only larger. The reflector sides are lengthened from 22 inches long in the simple version to 27 inches long. The reflector width is expanded from 16 inches wide to 38 inches wide. These dimensions are not critical. They were selected based on the cardboard boxes that are readily available.

 


To make the high-performance reflector, carefully unfold a 22" x 22" x 16" cardboard carton including the top and bottom flaps. This will give you a piece of cardboard roughly 38 inches wide by 76 inches long. It will have a fold right in the center. Cut off 11 inches from each end to make the piece 27 inches on each side of the fold. This will be the reflector surface. Cover it with aluminum foil and tape in place. See figure 4 below.

 

 


Next, flip the reflector over so the aluminum foil side is down. Using the leftover pieces of cardboard and tape, reinforce the reflector so it will hold its shape but make sure the center will still fold. Cut a hole in the center of the reflector surface for the coax connector to pass through.

 

Get another 22" x 22" x 16" cardboard box. Cut off one of the narrow 22" x 16" sides and then cut the box in half down the long dimension leaving two identical "corners" each 8 inches wide. These "corners" are used to hold the reflector surface in shape.

 

Bend the reflector surface along the centerfold so the aluminum foil side is inside and tape it into the two corners so it will hold the corner reflector shape. You may need to do more reinforcing with cardboard and tape.

 

Next, make a holder for the dipole out of cardboard and tape. The holder needs to position the dipole so it is held 6.5 inches from each of the reflector surfaces just as in the simple version as shown in figure 3.

 

An easy way to do this is to cut out the corner from yet another box, punching holes in the sides to hold the PVC insulators and the bottom for the coax connector as was done in the simple version of the antenna. Mount the dipole in the holder and fasten with tape. Push the coax through the hole in the center of the reflector and fasten with tape.


When completed, the antenna should look as shown in figure 5.

Azimuth Positioning

The azimuth beam-width of both antenna versions is quite wide so the azimuth angle need only be adjusted every hour or so during a satellite pass. You will need to track the orbit of AO-40 in order to position 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.

Elevation Positioning

With one of the reflector sides horizontal and the other one vertical, the antenna radiation pattern points up at about a 45-degree angle. This is the same for both versions of the antenna. To lower the radiation angle, raise the back of the antenna. Since the elevation beam-width is quite high, it may be possible to set the antenna to an angle where no additional elevation positioning is required, depending upon your station location. In Massachusetts, for example, setting the elevation angle to 20 degrees provides good coverage for all AO-40 passes with no other elevation pointing. To set the angle to 20 degrees, raise the back of the antenna until the bottom reflector makes a 25-degree angle with the ground. This was done by adding "legs" to the back of the high-performance antenna as can be seen in figure 5. For more southern locations such as Texas, a higher angle, such as 35 or 40 degrees, will be needed.

 

Summary

This article has described antennas that can be used to access the AO-40 satellite on the UHF uplink. The antennas are dipole-fed, corner reflectors using cardboard and aluminum foil for the reflector surfaces. The dipole is made from aluminum tubing and PVC insulators readily available from hardware stores. The dipole can be constructed in less than half an hour using ordinary hand tools. After constructing the dipole, the simple version of the corner reflector can be constructed in about 10 minutes. The high-performance reflector takes about an hour to put together.

 

Do these antenna really work? Yes! The author has constructed both versions of the antenna and has made dozens of contacts on AO-40 using up to 50 watts of transmit power from an FT-847.

 

When the satellite is within about a 30,000-km range, the simple antenna provides a solid SSB or CW signal. At apogee, the SSB signal will be weak but a CW signal will still be quite usable. For reliable SSB operation beyond 30,000 km, the high-performance version is recommended and will provide a solid uplink signal all the way out to apogee. See you on the bird!