What is the optimal antenna for a high altitude weather balloon?

Question

I was thinking of creating a 2.4 GHz WiFi wireless link with a directional, circular polarized antenna but I am not sure if that would work for such a long distance. The antenna cant weigh more than 1 pound which is a major limitation. I appreciate any suggestions

Answer 1

2.4 GHz is most definitely not the way to go, unless you very much know what you're doing.

The free-space path loss over a fixed distance increases with the square of your frequency.

Whilst one could argue that an antenna with fixed area would at the same time increase its directivity with the square of frequency, this doesn't help you very much, because it'll be very hard to steer an antenna on a floating balloon.

So, go for lower frequencies.

Note that I'm really not saying that you can't use the 2.4 GHz band at all - one could very easily implement a transponder that uses dsss with a massive spreading factor, and just work at an extremely low rate.

Paired with a high tower and thus, line-of-sight channels and little interference, this could work well (I'm not making this up. I know 2.4GHz systems that operate nlos over multiple km with maybe 20dBm). But your wording, your use of the tag "wifi" and the overall tone suggested you expect existing consumer standards to work - bad news, they won't.

Answer 2

I've never launched a balloon, but I'd guess gravity would keep the antenna oriented vertically so circular polarization is unnecessary weight. If you did want circular polarization, put it on the ground station. It will mean a 3 dB penalty to your link budget.

Free space path loss, with $r$ being the distance in meters, and $f$ the frequency in Hz, is:

$$ P_{r(\mathrm{dB})} = P_{t(\mathrm{dB})} + G_{t(\mathrm{dB})} + G_{r(\mathrm{dB})} + 147.6 - 20 \log_{10} (rf) $$

$P$ is the transmit power and $G$ the receiver and transmitter gains.

WiFi transmit power might be 1 watt, or 30 dBm at most (local regulations and equipment capabilities vary). A monopole antenna on the balloon may have a gain of 5 dBi, at the optimal orientation. The distance to the balloon is at a minimum 20,000 meters, if it's not very high, and directly overhead. And a reasonable directional WiFi antenna at the ground station might be 20 dBi. So:

$$ \begin{align} &30\:\mathrm{dBm} + 5\:\mathrm{dBi} + 20\:\mathrm{dBi} + 147.6\\ &- 20 \log_{10} (20,000 \cdot 2,400,000,000)\\ &= -71\:\mathrm{dBm} \end{align}$$

This would work, assuming an otherwise quiet environment, and you can reasonably point the ground antenna in the right direction. As a rule of thumb, WiFi requires a received signal strength of:

• -60 dBm: excellent signal (typical design goal for commercial installations)
• -70 dBm: good signal
• -80 dBm: marginally working, at reduced data rates and with some packet loss (probably sufficient for basic telemetry)
• -90 dBm: buried in the noise floor, will not work at all

Keep in mind, this is a best case estimation. In reality:

• the balloon will probably be farther away (if it's below the horizon, losses are very much higher and I doubt it will work at all)
• there will be some noise (microwaves, neighbor's WiFi equipment, etc)
• the antennas may not be aligned optimally

Improving the antenna gain with a larger dish on the ground or a collinear array would provide additional margin. If you have an amateur radio license and the right equipment, you may be able to operate at higher powers and on a frequency not available for public use.

You'll have to run these numbers for the maximum distance you expect, and add some "fade margin" to account for noise, misalligned antennas, etc. Before launching the balloon you should get some attenuators and test the equipment at the expected signal levels.

If you're unable to make it work, and assuming you need just basic one-way telemetry, WiFi is not really the best way to go. The slower the data rate, the less power is required, and WiFi is probably far faster than you need. WiFi also requires bidirectional communication to send acknowledgements, which means added complexity and weight on the balloon. Using a lower frequency will make aiming the antenna less critical.