After using the selfmade portable satellite back-pack station from out of many countries through Europe, I wanted to update it with 13cm RX possibilities to include AO40 to the list of workable satellites. One of the problems with 13cm is the antenna size. Many amateurs say that 13cm is the ideal frequentie for easy acces with an satellite. From my experience I know this is not 100% true.
To have acces to the AO40 satellite, no simple antenna’s can be used. Single patch antenna’s work, but the result was not enough for me. With an low gain antenna only the stronger stations are hearable, and with the medium 70cm uplink it will be impossible to hear my own uplink.
The solution had to be found in an higher gain antenna. The problem with higher gain 13cm antenna’s is their size and weight. The only solution seemed to be an dish antenna, but they are impossible to carry in my back-pack, and extreem heavy !!!
So I did look for some idea to have an dish that can be made very little to carry and is very light in weight. I tried to make the well known umbrella design, but my several attempts failed, and the umbrella length was still to long.
Then I wanted to have an try with several patch antenna’s stocked together. While I was looking for some material in my magazine there was an inflatable beach-ball that was in my way. I kicked it away, but while I kicked it away an new idea came up in my mind. This beach-ball was not parabolic in shape, but when it is designed to be parabolic then I would have an inflatable dish.
Now I had the idea, I needed to find some reflective material that is light in weight and flexible as plastic. I tought maybe rescue blankets will work, but I could not find any of them to try the material. Then suddenly I found another material behind the heat-radiator in my home. Behind it there was an special flexible aluminium-foil that reflects the heat toward the room instead of going outside through the walls. I took an little sample of it and tested it at 10 GHz. The material was very reflective !!!! Later I bought more of this material in the store and tested it also at 2m and 70cm and it reflects very good. Measurments showed no difference between an solid aluminium element and an PVC element covered with this foil.
Below you can see an picture of this material. On the left you can see the top side which has an metal colour. On the rigth you can see the bottom side which has an foam to keep the heat. This foam can easily be removed.
Now I have the foam, the next problem is the design. I first tried an small size model to see how it works. Below I will show some of my little prototypes.
Below my first attempt with the relfective foil and an plastic cover to make in air-tight. This try failed because the parabolic shape turns into the shape off an soccer-ball when it is inflated under some more presure.
To solve this problem I made an new design. This design has an copper wire at the outside to keep the parabolic shape when it is inflated under pressure. This design was very good.
An picture from beside. The shape of an dish is clearly visible.
After these tests I started to design an much bigger dish. With the plastic material from large plastic bags I made an dish with an diameter of 1,6 meter. After finishing this dish I find out that it was not air-tight. The plastic material was not usable because it was very fragile, and little holes did occur very easy. The dish was not usable, but I used it to test the parabolic shape, what was very good.
Now I had the idea, the reflective material, but the problem was to find and strong, light, and air-tight material to make the “balloon”. After looking around in the local store I found the solution. What I found as the solution is an plastic protection tarpaulin that can be used over a trailer. This material is very strong, and it is air-tight. One of the problems with this material is that it is not glue-able because it is made of polyethylene. The only way to make an air-tight connection is by heat-welding.
My next dish design with this material was much better. Below you can see an picture of this dish.
Below an picture from the back-side of the dish. The reflective material can easily be seen, because the sun-light can not go through it.
Below: The inflatable dish besides its solid dish sister.
The inflatable dish is very easy to carry. The complete inflatable dish weights only 750 grams, compaired to 5kg of its solid dish sister. I am still working on an good feed system. It must also be very little an easy to carry. The tests I did with AO40, I did use the feed that is mounted in front of the solid dish. I used it to find signal differences between both dishes. I found no differences ! Both dishes are SHF equal !
To give an idea how flexible this dish is for portable use, look at the pictures below:
1. – Untight the strings, take out the inflatable dish and let out the air.
2. – Role up the infatable dish, disassamble the yellow plastic circle in its smaller peaces.
3.- Put the parts in the antenna bag. (This bag now has: 2m/70cm handheld oscar antenna, 10m dipole, and an 13cm inflatable dish è all of them 100% self-made antennas !!)
4. – Ready to go. Let’s go travelling to the next dx-locations J
The total weight of the antenna bag is 3kg. Even lighter than the solid dish J The size of it is smaller than my 3-person iglo tent. The length of the bag is smaller than 54cm. Because of this I can mount the antenna bag horizontal on top of my back-pack without having problems when entering small entrances. These antennas are very compact to carry and have the same electrical features as a medium satellite station.
The costs to make this dish is not very much. The total costs of the material for this dish is about 20 EURO.
People who want to make an inflatable dish by themselves, I advise them first to test the material. Test the reflective material if it really reflects, and before starting to weld the plastic material do some tests before on the wast. With the wast you can test the best welding time and welding heat to be used.
There are still many more technics to be explored. For portable (back-pack) stations is buying not always the best soltion, and designing new technics is the only option…
73 de PE1RAH, William