Selecting an Antenna

As usual, I underestimated the importance of the antenna and the difficulties involved in selecting what ultimately needs to be a compromise. We can’t have everything! We need to decide upon the antenna for the HAB and the ground antenna.

HAB Air Antenna

What we need out of an antenna

The things to consider are:-

• Profile – horizontal and vertical – we want to to 360 degress around and require vertical radio to be maximum directly below and still be fairly strong up to 45 degrees and not so strong at the horizontal.
• Impedance – It needs to match that of the XBee modules which is 50Ohms.
• The gain – well, this is something we can’t control, because we need sufficient coverage (Profile above). It would be great if we can have a ground plane above the antenna to reduce radiation above the antenna.
• Bandwidth requirements dictated by XBee is 902MHz to 928MHz

Other considerations

The cable and connectors are also very important because we lose a fair amount of the signal in these components, before it is even radiated.

• Connectors need to be 50Ohm
• Connectors needs to be easy to connect to cable
• Connectors need to have suitable bandwidth
• Cable needs to as low loss as possible (minimal attenuation)
• Cable needs to be able to bend sufficiently to attach to antenna and XBee.
• Cable needs to be 50Ohm to match the xBee modules.

Where to obtain

The antenna here might do the job. Still investigating this.

Ground Base Antenna

It is possible we might be best getting two types of antennas for the ground base station. A:-

• Yagi
• A spiral antenna

We consider what we need in these two antennas separately. I have read that during the initial stages of the flight, when the ground station is under the balloon, the radiation pattern is more circularly polarized. Use of a Yagi in this stage of flight would lead to highs and lows (absence) of signal, whereas a Spiral will be 3db down, but will not lose signal altogether.

As the HAB moves further away, the radiation coming from the HAB is closer to horizontally polarized, and so we can then employ the YAGI.

Yagi

What we need out of an antenna

The things to consider are:-

• Profile – Highly directional
• Impedance to be 50ohms
• The gain to be as great as possible. (VFT suggested high gain was better and that a lower gain (to increase profile angle) probably wasn’t the best way to go.
• Horizontally polarized
• Bandwidth of 900 to 928Mhz

Other considerations

• Connectors need to be 50Ohm
• Connectors needs to be easy to connect to cable
• Connectors need to have suitable bandwidth
• Cable needs to as low loss as possible (minimal attenuation)
• Cable needs to be able to bend sufficiently to attach to antenna and XBee.
• Cable needs to be 50Ohm to match the xBee modules.

Where to obtain

The antenna here might do the job. Still investigating this.

Spiral Antenna

This is sometimes referred to as a Helix antenna.

What we need out of an antenna

The things to consider are:-

• Profile – Rotation
• Impedance 50ohms
• The gain needs to be as high as possible.

Other considerations

• Connectors need to be 50Ohm
• Connectors needs to be easy to connect to cable
• Connectors need to have suitable bandwidth
• Cable needs to as low loss as possible (minimal attenuation)
• Cable needs to be able to bend sufficiently to attach to antenna and XBee.
• Cable needs to be 50Ohm to match the xBee modules.

Where to obtain

I’m considering making this antenna myself. I’ve found a good website that describes how to make a Helix Antenna. It also mentions how to get the impedance close to 50Ohms using about 6 different techniques!! A shop in town has an impedance meter and if push comes to shove, I’ll ask them if they can measure the impedance for me. I’m currently waiting for some 2mm solid copper to arrive.

The payload which I refer to here is all the box and all the gadgets we send up at the end of the balloon. It includes all the necessary pieces to ensure that they remain functional at all times. The best way to describe the payload is with some photos and some commentary. Read on.

The Styrofoam box

This is a small Styrofoam box that we were very lucky to acquire in a purchase my wife made on the Internet (allergy friendly chocolates). The Styrofoam box is an ex-Broccoli box. It was chosen because the dimensions are almost perfect for fitting all the components and the thickness of the walls is good to allow the cameras to JUST poke out enough.

The boxes dimensions are:-

Length: 385mm

Height: 160mm

Width: 290mm

Wall thickness: 20mm

A few pictures of the box (with components within it) are shown below. You will notice that we have some blue foam inside the box. This is to provide a cushion for the components and to help reduce the chance of water getting to components and to also reduce air flows that might take heat away.

Sensors/Communications Electronics

This a fairly complicated part of the payload. It consists of several components:-

• Main electronics polycarbonate box (which is described in another blog)

• External PCB – used to interconnect the Linksprite camera, the external temperature sensor and the Status LED

• Xbee antenna cable

the components are all sandwiched between special water resistant foam-like material purchased from Clarke Rubber and some other foam normally used to make cushions. This material has been chosen because of its ability to cushion the sensitive electronics against knocks, help the payload retain warmth so that the electronics can continue to work at the very high altitudes (30km) it should reach. The foam is very light which is an obvious requirement.

Below are some pictures:-

The “HackHD” camera

The HackHD camera (purchased from LittleBirdElectronics) is installed standalone with it’s own 3 x AA battery pack. We purposely kept this separate from the rest of the electronics to reduce chance of failure of all electronic systems. It is shown pictured below:-

We installed a 32GB Class 10 microSD card, the largest size we can install in the camera to give us about 3.5hrs of video footage. We choose Class 10 because we wanted to be sure that the camera could write to the microSD fast enough. The idea with the microSD is that we can start it up just before flight and if we are able to recover the payload, then be able to play the whole launch from a rather good vantage point.

The Abrasion Jacket

The styofoam container is a great insulator and is light but it does not have the strength to withstand a long possibly windy flight, the balloon popping. So what we do is put this styrofoam package (with all the electronics within it) into a ‘jacket’ made out of spinnaker cloth (ripstop nylon). This is the same sort of material used in kites, yacht spinnaker sails.

We are closely following a design described in the document:-

http://www.parallax.com/dl/docs/cusapps/NearSpace1.pdf

Below are some photos taken during construction:-

This jacket provides a means of attaching the nylon rope from the parachute to the payload.

Materials

I have just acquired 4 metres times 1.5 metres of ripstop nylon fabric also known as Spinnaker clothe. I also purchased 30metres of thick polyester thread, some attachment material (like Dacron tape) and one hundred pack of 1″ rings. (I only need 8 of these rings). The rings are the sort you use for curtains. I purchased all these items from Spotlight in Cairns for about $40.00.

Construction

I initially asked someone to create the jacket but was advised that it could not be done on a standard sewing machine; that an industrial sewing machine was required. I didn’t quite believe this and discovered that it can be done on a standard sewing machine. I made the decision to construct the jacket myself.

I was generously loaned a ‘new’ home sewing machine which I will use to construct the jacket for the high altitude balloon. For the last week after all the day chores have been done, I have been cutting sections off and familarising myself with the sewing machine and trying to do some seams.

After about a week of trying to perfect the stitches I realised I wasn’t going very well. The sewing machine couldn’t load the bobbin (so I did it manually), the thread was very thick and hard to thread, the material was crunching up and it looks liked the top thread was appearing in the bottom, i.e. incorrect tension. Purchasing a bobbin winder was $129 and a new sewing machine of the same caliber was about $250.  So I decided to purchase my own sewing machine. This has been one of my better decisions. I haven’t looked back.

The Design

The jacket design I have been using is from Parallax Inc document which has a lot of good information on HAB designs. Because I am very new to sewing I have cut out pieces of paper and used a stapler to work out how to ‘construct’ the jacket. I’ve realised that making this jacket is a little more involved/complicated then I anticipated, so making a prototype out of paper/stapler helps me to visualize how it will all fit together. The document from Parallax Inc was a bit difficult to read and I did modify a few things, in particular the way the tube was constructed and the way the bottom was attached. I also suspect that I the top will be simplified. I do think that the people who designed the jacket may have made things more complicated than necessary. That is something I guess I’ll find out.

The opening for the cameras

We have two cameras and they need to have full access to the skies. We did this by cutting a square hole in the fabric and then using some Birch Bias Binding to strengthen this area and stop the fraying of the fabric. See the pictures below.

The Lid

I detoured off the design in the lid in the Parallax document. The doc said to wrap some ripstop nylon around the lid and hand sew. This seemed awkward, so my next idea was to hot-glue the ripstop nylon on to the styrofoam. I tried this out on a piece of sample syrofoam and fabric. It worked well. I then decided even that was a little ‘over the top’. The whole aim was to try and keep the lid attached and to ensure the jacket is tight/firm and doesn’t cover holes. I got some Velcro with sticky side and stuck this on top of the syrofoam lid. Then I sewed the other Velcro material to the jacket. See image below.

There is the possibility that the velcro may come off the syrofoam at high altitudes dues to the evaporation of glues in the low air pressure environment. This I guess I will find out, should the HAB be recovered. One can be said, is that it is a lot simpler then the original design.

Acknowledgments

I must also thank my neighbour for his suggestions with the Velcro-to-Syrofoam suggestion.

I must not forget to mention the help my eldest son Jeremy is provided in the construction of the HAB. He provided some fairly frank advice regarding the sewing of another seam to protect the bottom edge of the side tube. I accepted this advice. Thank-you Jeremy.