Yagi positioning for high altitude balloons  - Page 1
Forum section: electronics / rf-microwave

author: Deku Tree
author_rank: Contributor
timestamp: August 07, 2019, 03:22:55 pm
content: I'm working on a project using high altitude weather balloons and recently we've ran into issues with our reception past around 25k feet. For context, we are using an Digi XTend module which outputs 1W at 915mHz and a modem on the ground. We were originally using the supplied dipole, but at these ranges I figured we would want to go with a directional antenna. So I purchased a high gain Yagi.We launch from a salt flat, so there's really nothing around to get in the way, but I wanted to make sure that my antenna mounting made sense. Is there a vertical distance off the ground that I should be trying to mount the antenna at? Or can I get away with just about any mounting as long as there's line-of-sight?We are also looking to move to LoRa modules in the future for their lower power/higher sensitivity/smaller size and so making some kind of permanent yagi mounting solution makes sense for now. We usually drive a truck out to the field, so a mount on the truck itself would be very convenient.Thank you guys for any guidance!

author: radiolistener
author_rank: Super Contributor
timestamp: August 07, 2019, 03:45:54 pm
content: What is the size of largest dimension of your antenna?You can put it into this formula and get safe distance from antenna to objects (from any antenna side):R = 2 * D^2 / lambda,where lambda is wavelength, which is 0.327642 m for 915 MHz.For example half wavelength dipole has size about D = 0.164 m, so R = 2 * 0.164^2 / 0.327642 = 0.164 m. It means you should keep half wavelength dipole away from objects for at least 0.164 m distance.You should keep your antenna from any objects on this distance, because antenna directivity is formed at some distance from the antenna (in the far field region). When you get closer (to the antenna) than this distance, there is no directivity. Antenna has very complicated field structure in the near field region. And antenna is very sensitive to energy loss and field distortion at near field region.

author: denverpilot
author_rank: Regular Contributor
timestamp: August 07, 2019, 03:46:10 pm
content: A yagi for the balloon or the ground station?You’re going to get more bang from your buck by going lower in frequency, but if you’re not hams you may not be able to.You can probably find photos of almost a couple hundred strings* here if they haven’t taken them down... don’t think they’ve ever flown a yagi. The ballon spins.http://www.eoss.org/* Looks like they’re at number 292.

author: Deku Tree
author_rank: Contributor
timestamp: August 07, 2019, 04:22:47 pm
content: Oops, to clarify I have one ofthese antennasin the air, and the yagi on the ground.

author: Deku Tree
author_rank: Contributor
timestamp: August 07, 2019, 04:44:11 pm
content: Quote from: radiolistener on August 07, 2019, 03:45:54 pmExcellent, this was just what I was looking for. Where does this formula come from? Are there any more resources you could point me towards?

author: radiolistener
author_rank: Super Contributor
timestamp: August 07, 2019, 05:10:28 pm
content: Quote from: Deku Tree on August 07, 2019, 04:44:11 pmConstantine A. Balanis, ANTENNA THEORY ANALYSIS AND DESIGNThere is limitation for very small antennas. If antenna is very small, radiating near field region may not exists and this distance is limited by reactive near field region which is commonly taken to exist at a distance λ / (2*pi) from the antenna surface for very short antenna.

author: Deku Tree
author_rank: Contributor
timestamp: August 07, 2019, 05:17:04 pm
content: Quote from: radiolistener on August 07, 2019, 05:10:28 pmThe second diagram makes sense, but what about mounting the antenna up on a pole/tripod? Does the pole/tripod interfere and is there anything you can do with shape or material selection to help mitigate this?

author: radiolistener
author_rank: Super Contributor
timestamp: August 07, 2019, 05:25:17 pm
content: yes, it interfere a little, but antenna is designed in such way to have a mount point at the least critical point. So it should not affect much. The more significant problem here is to setup proper direction of antenna

author: Deku Tree
author_rank: Contributor
timestamp: August 07, 2019, 05:58:31 pm
content: Quote from: radiolistener on August 07, 2019, 05:25:17 pmThank you very much for your help, hopefully one day I'll be this knowledgable so I can continue to help noobs like me on the forums!