Welcome to the
“PE1RAH inflatable dish”
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