A New Antenna for KickSat-2
In addition to looking into new radios for KickSat-2, I've also spent some time over the last couple of weeks redesigning KickSat's antenna. KickSat-1, like many CubeSats, used a simple quarter-wave monopole antenna made from a piece of Stanley tape measure (yes, cut from an actual tape measure). The beauty of this design is that the tape measure can be folded flush against the CubeSat structure for launch and will snap out straight once the CubeSat is deployed.
While the tape measure monopole is simple and elegant from a mechanical perspective, it doesn't provide very good RF performance. One reason for this is that a monopole antenna needs to be mounted perpendicular to a large ground plane, and a CubeSat structure isn't quite big enough to be effective in that role. Another reason is that there are impedance matching issues when connecting a monopole to a radio. Basically, without careful engineering, a lot of RF energy is reflected back into the transmitter by the antenna. The reflections result in a loss of efficiency and, in extreme cases, can actually damage the radio.
Because radio waves interact in complicated ways with the metal structure of the satellite, we have to use a computer to simulate different antenna designs. Here is a simulated gain pattern from KickSat's original monopole antenna showing where the radiated signal is strongest and weakest:
Even more important than the gain pattern is a number called the standing wave ratio, or SWR. The SWR tells us how much reflection is going on between the antenna and the transmitter, with a value of one corresponding to the ideal no-reflection case. The higher the SWR, the more reflection, and the lower the power tranfer efficiency between radio and antenna. The monopole antenna simulation gave an SWR of nearly 4000 - not so good.
Here is the new design I came up with this week. It consists of two pieces of tape measure forming a slightly bent dipole antenna, like the "bunny ears" many of us used to have on our TVs.
The simulated SWR of this new antenna is 1.2 - close to a perfect match! That should help us squeeze much more efficiency out of our radio, ultimately making KickSat-2 easier to hear and easier to command than KickSat-1 was.
That's it for this week. I'll be back with more news next Friday. Thanks again for your support!