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900 mhz Long range antenna selection

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SaberShip

Sun Apr 06 2014, 07:17AM

Registered Member #4546 Joined: Mon Apr 23 2012, 02:46AM
Location:
Posts: 8

It's tough to decide. i'm not so much worried about whether I can keep in contact with a simple quarter wave element or helical antenna any more.

I know that the helical antenna has a good radiation pattern when tuned for 900Mhz and it has 1.5 turns on the helix. (6.5dbi, 85deg Half Power Beam Width) The beam width is an increase over the Patch that we are using now which works fine. On the downside (if my math is right) the ground plane is about the same size (if not bigger) than my patch is now. (about 9.7in.)

On the other hand a simple quarter wave element is probably the smallest, lightest weight, and simplest antenna. However, I'm very concerned with how that will fare after a landing. If its a rigid antenna it would possibly pierce the payload causing some pretty bad damage, if its flexible like magnet wire with a weight on it it would probably crumple up and cease to function as a useful antenna.

Could the quarter wave be placed on top of the payload box? As of now there is a thin layer of aluminum foil on either side of the insulation, but we can easily strip that off or just use a different type of insulation. The box is currently about 9.5" square and there is also a GPS antenna on the top off-center by about 4 inches or so. It's a GS-407 with a helical antenna if that helps.

Ash Small

Sun Apr 06 2014, 12:21PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

What forces are involved in a landing? (ie at what velocity does it hit the ground, etc)

Can you not add some form of lightweight framework that will protect the underslung antenna on landing, and prevent it from piercing the payload? (Maybe lightweight fibreglass/kevlar/balsa, maybe even something that will absorb any 'severe' impact, even if it (the framework) suffers damage in a severe impact, but protects the antenna and payload, but is designed to remain undamaged in a 'normal' landing scenario?

Proud Mary

Sun Apr 06 2014, 01:14PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If you search Google Images for ground plane antenna you will find numerous examples and variations of this simple antenna which you could mount on top of the gondola.

As with my previous suggestion the vertical radiator element should be very slightly less than one quarter wave length to bring it to resonance.

The input impedance can be varied a little for best match by altering the angle of the horizontal spokes.

Googling for ground plane antenna will bring up numerous discussions of the theory and practice of this very useful and very common antenna.

By the way, the most common antenna types used in missile telemetry nowadays are the patch antenna and the similar microstrip antenna, (Google!) but I suggest you go with the groundplane antenna to start with as it is very much easier to implement without UHF test instruments.

SaberShip

Wed Apr 09 2014, 07:33AM

Registered Member #4546 Joined: Mon Apr 23 2012, 02:46AM
Location:
Posts: 8

Thanks every one for all the suggestions and help. I will consult with my friends and show them this forum.

Our 3rd launch will hopefully be some where in June or July, and hopefully this will be the last flight with the heavy patch on board the payload. This flight we are testing a different type of latex balloon that should hopefully get us a bit higher than previous launches. Thankfully costs of launches for us is only the cost of the balloon + a small fee for our lifting gas which happens to be natural gas.

We've been very pleased in our results with this widely underestimated yet cheap lifting gas and if any one is interested in our work and the project feel free to leave me a PM.

Dr. Slack

Wed Apr 09 2014, 08:23AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

CH4 as lifting gas, very clever, roughly half the weight of air, and very easy to get. However, if you're using something flammable, why not go the whole hog and use hydrogen, around twice as much lifting power? (Well OK, much harder to buy, takes a long time electrolytically, or uses a lot of sulphuric and scrap iron). If you're handling a bag full of methane properly, then hydrogen is not going to be much more dangerous in terms of ignition energy, ignition temperature, burn temperature etc, you need to be covered in non-flam clothes, visor etc anyway.

Once you're using lighter antennae, put a Yagi, lambda/4 top and bottom, and a helical on there, and have the transmitter sequence a minute to each. Then you'll see what effect real-world use and propagation conditions have, because the antenna equations are relatively easy, the practical details are what usually bites.

SaberShip

Wed Apr 09 2014, 07:21PM

Registered Member #4546 Joined: Mon Apr 23 2012, 02:46AM
Location:
Posts: 8

We actually did attempt to buy hydrogen from a local company, and yeah our request was denied merely because we were not a normal customer and our first request was hydrogen.

I think making the hydrogen would be very difficult considering we need to fill a roughly 8ft Diameter balloon.

Ah well... CH4 is practically free by comparison (I think less than 10$ per launch!) the best part is we have a gas line right at the launch site so it goes right into the balloon, no need to rent a bottle!

We have many ideas to improve the efficiency of our design to make up for the relatively weak lifting capabilities of CH4, a lighter antenna is one of them. We ultimately hope to rival altitudes reached by many other designs around the internet (80K-100K ft) that use helium or hydrogen.

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