Optimal antenna orientation for High altitude balloon?

Question

(First of all, I hope this is the right subform, please correct me if I'm wrong)

I am using a 433 Mhz LoRa Radio to communicate with a high altitude balloon and am using an antenna like in this guide

I know many people have the antenna facing straight down (with radials attached to the bottom of the payload). However I want to use a satellite tracker as a backup, which means the tracker should always face the sky. If the antenna were at the bottom, the payload would probably fall over when landing... Would it be bad for reception, if i just put the antenna on a side of the payload instead of the bottom? (meaning the radials would be taped to one side of the payload and the antenna would be horizontal). As the HAB will be bobbing around a lot in the wind, would it even matter in which direction the antenna faces?

EDIT: Might the way of attachment shown in the picture be a solution?

Answer 1

It sounds like you really have TWO questions. 1) should I use the existing standard or is there a better choice. 2) will the Sat Tracker antenna receive interference from the part 1) antenna.

I'm going to address these in reverse order. The satellite tracker should probably use a fairly wide patch or sector antenna mounted on the top of the cube pointing up. You could mount it inside as well since your cubesat design is essentially rf transparent.

Sheets of metal are opaque to RF which is why I asked if you were using aluminumized foam. [You may need change this depending on you're local aviation regulations for untethered balloon launches.] You want to not block the GPS receiver from seeing the sky. The balloon will be effectively transparent.

If you are using the monopole with radials, then these antennas are both on different frequencies and mostly in the null of the monopole so you shouldn't expect much interference.

For your ground pointing antenna. The 1/4 wave monopole is probably your best bet from a cost and weight standpoint. A package lifted by balloon has a stable up and down but rotation is impossible to predict. This is why the monopole is useful because it's radiation pattern perpendicular to the antenna element is the same in all directions. Its also a vertically polarized signal. Meaning receiving it on a normal vertically polarized antenna on a vehicle keeps the polarization the same for best reception. With this type of antenna the null or worst place to be is directly underneath the balloon.

Answer 2

Advantages of the upside-down monopole include:

1. It's omnidirectional in the azimuthal plane, so as the balloon rotates the antenna gain doesn't change much.
2. The gain is higher in the down direction towards Earth.

The ideal radiation pattern of a monopole like this is a bagel sliced in half, with the monopole sticking through the hole in the center. With a ground plane of finite size it changes a bit but this is still the general shape.

With the antenna mounted on the side, half that bagel will be pointed up to the sky where you'd just be receiving noise (though not much of it since there aren't many radios up there) or wasting transmitter power. And as the balloon rotates the pattern changes: when the antenna is on the far side of the balloon relative to your base station, gain will be pretty low.

This may or may not be a problem. Ultimately it depends on the reliability you require and your link budget. You can use antenna modeling software to estimate the antenna gain, or build a prototype and test it empirically.