AUTHOR: LEVENTE BUZAS

The in-house dipole antenna of the ORCASat TT&C subsystem has gone through many iterations of testing - previous updates covering these can be found here, here, and here. These tests were designed to give as much confidence in the performance of the antenna as possible, while making the best possible use of the resources available to the TT&C team for antenna testing at and around the University of Victoria.

With the final, flight candidate version of the antenna now available, the antenna testing was revisited again. This time, instead of the makeshift in-house facilities, the decision was made to get the antenna professionally evaluated, at a suitable anechoic chamber. For this, Antenna Test Lab Co. of Raleigh, NC, was selected in the USA, due to the affordability and rapid turnaround of their services. The purpose of this test was as follows:

Determine the optimal antenna length for operation in free space while mounted in the spacecraft by length trimming in a suitable anechoic chamber
Validate the antenna input characteristics (VSWR, return loss, input impedance etc.) results obtained during previous in-house tests
Accurately quantify the impact of the spacecraft body on the gain pattern of the antenna by obtaining a 3D scan of said pattern
Validate the assumptions made about the antenna gain in the ORCASat link budget
Determine the efficiency of the antenna

To this end, a flight candidate antenna board was shipped out for testing, inside of a spacecraft mockup (shown below) which resembled the final spacecraft as closely as possible at the time the testing was conducted.

At the test facility, first the antenna was tuned in the anechoic chamber to the optimal length for operation in free space. Below are a set of graphs illustrating this process. The optimal length was found to be 124 mm (measured from the solar panel PCBs, for each dipole arm), where a return loss of -41.92 dB (VSWR 1:1.02) was obtained at the measurement point (437.1 MHz) closest to the ORCASat assigned frequency of 437.06 MHz. This is an excellent result, as it indicates that the antenna is tunable, it is nearly perfectly matched to the 50 Ohm transmitter output. Examining the data also reveals that the 3dB return loss bandwidth of the antenna is 2.1 MHz centered at 437.1 MHz, which is more than sufficient than what is needed to support the ORCASat TT&C link, which has an assigned frequency band of 49738 Hz.

Next, the tuned antenna was mounted on a computer controlled positioner in Antenna Test Lab’s anechoic chamber as shown below, and its gain was measured in all directions, with 10 degree resolution to obtain the 3D pattern, which is also shown below. As seen, the pattern overall is as expected, a doughnut centered on the dipole.

From this 3D gain data, the gain in any direction can be obtained. The overall maximum of gain was found to be 3.89 dBi, towards the ram face at the measurement frequency (437 MHz) closest to the frequency of interest (437.06 MHz). This is higher than the theoretical maximum of 2.16 dBi for an ideal dipole, but it can be explained by antenna pattern distortion created by the satellite body.

The standard slices along the E and H plane are shown below, normalized to the overall maximum gain of the antenna, 3.89 dBi. The E plane pattern further illustrates the higher than expected gain referred to earlier - the pattern is clearly distorted compared to that of the ideal dipole.

In terms of the operation of the ORCASat TT&C link, the gain is most crucial in the nadir direction, as nominally the ADCS points this face towards the Earth where the ground station is located. The measurements indicate, as per the unnormalized plot below, that the gain in this direction is 2.88 dBi, which is 2.88 dB higher than the isotropic assumption in the ORCASat link budget. This is great, as the results indicate that the conservative assumption in the link budget is valid, which gives further confidence in the correct operation of the ORCASat RF link.

From the gain data introduced above, the test facility also obtained the antenna efficiency by computation. This is a measure of how much power presented at the antenna input is radiated, and how much is dissipated as heat. For the ORCASat antenna, this was found to be 95.28 percent at the frequency of interest.

Overall, the antenna test was a success. While not in the final spacecraft, the results obtained greatly increased the confidence of the TT&C team that the antenna will be suitable for flight. To obtain final confidence, antenna testing on the flight spacecraft would be the ultimate solution. However, this is not feasible, due to the project schedule and risks related to handling the flight unit. Instead, this testing will be conducted on the near identical twin of the flight spacecraft dubbed ProtoSat, at the ATF2 facility of the David Florida Laboratories in Ottawa. This will be covered in a blog post at a later time.

Antenna Testing Revisited