Slot Antenna

Why slot antennas rather than strip antennas?

An entire 7842 thinsat, 1.1 million slot array emits perhaps 3 watts of RF power - 2.7 microwatts of power per slot, 5.4 microwatts peak. If a stripline dipole antenna had an impedance of 100 ohms, then 5.4 microwatts would be a peak voltage of 23 mV, a peak-to-peak voltage of 46 mV . This is a very small voltage to synthesize from a 1.2 volt VDD rail, without other impedance transformations in the signal generation path.

According to this antenna theory website, Babinet's theory states that the impedance of a slot antenna z_s is inversely proportional to the impedance of a free space dipole z_c like so:

/large z_c ~ z_s ~ = ~ 0.25 {\eta}^2 ~ ~ ~ ~
where \eta = 377\Omega;

So, in the same space as a narrow free space strip dipole of 100\Omega;, we can get a 355\Omega; slot antenna. A 50\Omega; strip antenna would have a dual of 711\Omega; That would take a 124 mV peak-to-peak signal - still a small fraction the of 1.2 volt rail, but about 3 times as efficient with our power.

Thinsat geometry

A thinsat must emit power either from the front or back. While two stripline antennas on each side of the ground plane can do this, stripline antennas are very low impedance (they are segments of signal stripline, pretty much) and we will need two (or maybe 4) bondpads to feed each antenna. A slot needs only one or two bondpads to feed it.

Either the stripline or slot antenna interdicts solar cell and radiator area. However, the slot passes light, it does not add useless thrust to the thinsat.

The stripline does have the advantage that signal wires can route underneath it - they must go around the slot.

SlotAntenna (last edited 2015-06-01 22:12:52 by KeithLofstrom)