Antennas for a High Altitude Balloon

[Ryank]

Hey, I'm looking for the optimal antenna to transmit in the 900 Mhz range from a high altitude balloon. The transceiver I have says it is capable of 40 miles LOS with a 15 dBi antenna. Can achieve this? Can I just hang say a 100ft wire from the bottom of the balloon? Do I need a different type of antenna for receiving/transmitting. Remember this balloon will be a maximum of 20 miles away through open air and will be almost directly above it's target. Obviously we'd also like to be able to receive when the balloon get back to the ground too, so the antenna can't be too directional. If anyone can just point me to a good resource on explaining how antennas work that would work too.

Any advice is very much appreciated,
-Ryan

 

[berkeman]

Hey, I'm looking for the optimal antenna to transmit in the 900 Mhz range from a high altitude balloon. The transceiver I have says it is capable of 40 miles LOS with a 15 dBi antenna. Can achieve this? Can I just hang say a 100ft wire from the bottom of the balloon? Do I need a different type of antenna for receiving/transmitting. Remember this balloon will be a maximum of 20 miles away through open air and will be almost directly above it's target. Obviously we'd also like to be able to receive when the balloon get back to the ground too, so the antenna can't be too directional. If anyone can just point me to a good resource on explaining how antennas work that would work too.

Any advice is very much appreciated,
-Ryan

Welcome to the PF. You don't want to use a longer antenna, that does not help you get gain. Does this antenna need to be omnidirectional? Or can you have it aim itself somehow toward the receiver?

 

[skeptic2]

15 dBi is a high gain directional antenna. It would help to know how much power the transmitter is capable of and the sensitivity of the receiver. Fortunately when transmitting over land there are a lot of losses such as multipath and absorption that you won't have, so I don't think your antenna will need quite so much gain.

Will you be transmitting simplex or duplex? Are the transmit and receive frequencies the same? The same antenna will work equally well for both transmit and receive if the transmit and receive frequencies are the same.

My first thought for an antenna for this application is a helical with a reflector behind it for both under the balloon and on the ground.

 

[sophiecentaur]

Hi
You seem to have a bit of a conflicting requirement here. If you want your antenna to be almost omnidirectional (elevation and azimuth) then it can't have much gain - because gain relates to directivity and how narrow the beam is. You would need to radiate (downwards) almost over almost the whole of a hemisphere - which implies only a bit more than 3dBi of gain. You'd have an advantage of great height, most of the time, of course, so your horizon would be a great distance away but I guess you might want comms when you're near the ground, also. Perhaps the +15dBi requirement is a bit pessimistic under your circumstances.
The sort of radiation pattern you need (in 3D) is a bit like a squashed pumpkin. There would be less power directed downwards because the ground is never very far away and an omnidirectional bulge out towards the sides - so that the not-too-distant horizon gets a good signal. You need a mixed / circular polarisation so that, as the balloon changes its heading, you don't lose signal due to cross polarisation with the receiver.
I think you could get a pattern of that sort of shape with a pair of crossed horizontal dipoles hanging a bit less than a quarter wavelength underneath a groundplane / reflector of light mesh. The whole thing wouldn't be more than about 50cm square with 160cm dipoles suspended about 6cm underneath. It could be lightweight construction, hung over the side and retrieved before hitting the ground. If your gondola is a basket, you might even get away with mounting it in the floor. The details of dipole size, spacing and how to feed it would be a bit but not too critical.
What you need is an enthusiastic Radio Ham with a workshop near where you live . . .

Skeptic - I initially thought of a helical antenna but could you get the right pattern with a dent on axis?

 

[skeptic2]

Remember this balloon will be a maximum of 20 miles away through open air and will be almost directly above it's target.
-Ryan

I assumed from this statement they would track the balloon somehow and be in a vehicle that would stay beneath it. One would hope the balloon wouldn't get caught in the jet stream.

A helical antenna, in addition to being directional and capable of high gain, has the advantage of maintaining the same polarization between transmitter and receiver even as the balloon rotates.

 

[sophiecentaur]

"A helical antenna, in addition to being directional and capable of high gain, has the advantage of maintaining the same polarization between transmitter and receiver even as the balloon rotates."
As would crossed dipoles - suitably fed, of course. I did consider a helix, initially, but I'm not sure about the radiation pattern being wide enough.

For the balloon to be 20miles away and 1mile high, the elevation is only about three degrees - i.e. you need a very wide vertical beamwidth (i.e. a reasonable level even at over 80 degrees from vertical boresight). Can you get that with a helix? If you can, then it's a good solution - even if just because it's very easy to feed.

I think the '20 miles away" and "overhead" figures won't apply at the same time in a hot air balloon.

 

[skeptic2]

Hey, I'm looking for the optimal antenna to transmit in the 900 Mhz range from a high altitude balloon
-Ryan

For a hot air balloon they wouldn't. When I read the above I was thinking 20 miles straight up.

 

[sophiecentaur]

According to Wiki the present altitude record is 4.1 miles.
We really need to know more details about actual requirements.

 

[skeptic2]

http://en.wikipedia.org/wiki/High_altitude_balloon

"High altitude balloons are unmanned balloons, usually filled with helium or hydrogen that are released into the stratosphere, generally reaching between 60,000 to 120,000 feet (18 to 37 km)."

 

[sophiecentaur]

That's useful info but it's essential to know what sort of peripheral performance may be needed. If it really doesn't matter about horizontal response then go for a directive antenna. But when the balloon is coming down you could lose it if the signal off boresight is embarrassingly low.
A specification is essential.

 

[skeptic2]

This is probably the best reference there is.
https://www.amazon.com/dp/0872599876/?tag=pfamazon01-20

 

[skeptic2]

http://n1vg.net/balloon/

http://balloons.space.edu/habp/project_4/airphotos.html

http://www.treehugger.com/files/2010/03/amazing-photo-of-earth-taken-with-point-n-shoot-camera-and-balloon.php

http://itn.co.uk/25d6a847270f95bc83f25740ee26282e.html