A. Antenna testing

1. Preface

1. Introduction: Context, Misconceptions, and System Constraints

The successful execution of the RABSII project, which focuses on measuring the influence of the ionosphere's sporadic E layer, hinges upon the reliability of both signal transmission and, crucially, signal reception.
This report documents the necessary compromises and design constraints imposed by using reduced-size High Frequency (HF) antennas.

A persistent theoretical misconception, often reinforced by current modeling software, suggests that a small-scale antenna can achieve transmission efficiency comparable to a full-size counterpart. In reality, while small antennas may theoretically radiate, their miniaturized form exhibits significantly reduced bandwidth and increased inherent losses, which are often challenging to measure accurately in a real-world environment.

Furthermore, we must distinguish between transmission and reception principles.
While concepts like antenna aperture and reciprocity apply to directive antennas at higher frequencies, the ultimate metric governing our data retrieval is the Signal-to-Noise (S/N) ratio at the receiver.
Achieving a decodable signal involves a trade-off: higher data speed necessitates wider bandwidth and a correspondingly higher S/N ratio.

To navigate this compromise, the RABSII project employs the FT4 protocol.
This choice offers a balance of speed, narrow bandwidth requirements, and decoding capability, even overcoming some Doppler and multipath distortions due to its slow symbol rate.
However, the S/N value remains paramount for accurately isolating the effects of the sporadic E layer.
To protect the fidelity of these amplitude-sensitive S/N measurements, a fundamental system constraint is required: the satellite's antenna must maintain a highly steady amplitude emission directed toward the point of reception (Earth).
This constraint is critical in dictating the final design and characteristics of our radiation pattern.

2. Antenna Selection and Design Implementation

The constraint for maintaining a steady amplitude emission towards the Earth led to the selection of a full-size dipole antenna.
Modeling of various candidates, including loops and reduced-size loops, demonstrated that loop configurations failed to produce a sufficiently steady signal toward gravity due to the satellite's anticipated spin.
While extremely small loops might fall within amplitude tolerances, the significant efficiency reduction and extreme sensitivity to minor changes in resonance frequency and impedance rendered them impractical.

The chosen dipole must be stretched in line with the satellite's motion vector to generate a uniform radiation field directed toward Earth.
With a predicted spin rate of 10 degrees per second, a nutation of up to 20 degrees is expected to generate only a minimal ripple in the signal, which is predicted to be negligible for decoding purposes.

Given the proximity of the antenna to the satellite body and its foldable solar panels, the satellite itself was designed to act as an integral part of the antenna system.
This requirement, combined with the need for a compact deployment mechanism, pushed the design toward using Shape Memory Alloy (SMA) materials for the antenna elements.

The final design utilizes a 0.3mm diameter nitinol wire with an actuation temperature (Ta) of 45c.
This choice was governed by balancing the necessary bending radius for stowing the required total wire length.
The lower electrical conductivity of nitinol, while imposing some RF loss, presents a critical benefit:
it enables the material to be heated by solar radiation to assist in reliable deployment.
Furthermore, at the operating frequency of 28MHz, the skin depth on nitinol is approximately 500um, ensuring that the complete cross-section of the wire is utilized, which minimizes the conductivity penalty compared to highly conductive materials like silver (skin depth 15um).
This resistive property also suggests that some self-heating may occur during transmission, which is expected to have the positive side effect of further straightening the nitinol wire post-deployment.

3. Stowing Area and Deployment Mechanism

As proposed, the two parts of the dipole will be stowed on two opposite sides of the fully deployed satellite.
This configuration sacrifices two Photovoltaic (PV) panel positions: one at the top end of the body and the other at the far end of a deployable solar panel.

The nitinol antenna wire will be galvanically connected to the satellite body and panel PCB. To ensure proper operation, the panel PCBs and all associated wiring will be Radio Frequency (RF) isolated from the antenna body.
The antennas' feed point is positioned at the hinge of the deployable panel, ensuring the satellite body and panel act as the highest current-conducting parts of the antenna.
Given the significant surface area of these components, this design effectively minimizes potential interference with nearby electronics housed within the satellite body, which functions as a Faraday cage.

The physical stowing area available for one leg of the dipole is 48mm x  80mm with a maximum height of 2mm.
The PCBs themselves are integrated into the pocket structure to stow the antenna, thereby reducing overall mass and material complexity.
The walls of this pocket will be constructed from PCB material, soldered directly to the satellite or panel PCBs during final assembly.
All electrical connections must be made on the component/silk side of the panels, as the reverse side houses a PV panel and precludes the use of feed-through components.

For stowing, monoline nylon fishing wire, approved as a space-grade plastic, is used to tighten down the folded nitinol.
The nylon wire is fixed after tightening by crimping a ferrule, eliminating the need for adhesives or heat processes.
During deployment, this nylon wire will be cut using a 0.1mm diameter nitinol knife or wire.
Since both ends of the nylon wire are fixed, this system will not generate any space debris.
The nitinol antenna wire itself is woven between three pins in a specialized pattern designed to self-tighten upon heating.
At one end, the central pin is replaced with the nylon wire and nitinol knife mechanism, allowing this side of the antenna to be released upon deployment.

The current used to sever the nylon wire is the same current used to actively straighten the antenna wire, requiring only a single current-limiting source (expected maximums: 600mA in air, and around 100mA in free space).
Specialized fixtures, a weaving template, and a tensioner fixture are used to manage the nitinol wire during ground assembly.

A unique characteristic of the nitinol wire is exploited during final pocket assembly:
it has been proven that rotating the wire on its axis generates greater forces upon heating.
By bending the end of the wire 90 degrees and utilizing it as a spring, the deployment forces are directed perpendicularly to the pocket, which prevents the wire from being confined by the pocket walls during the stretching phase.
This small bend also securely fixes the wire at the 3-pin connection point against any potential inline forces.

2. Remarks and principles

1. Resonance and Efficiency Philosophy

The core design principle emphasizes maximizing physical efficiency, as this is difficult to model accurately in simulation.

• Resonance Definition:
  The antenna is defined as resonant when its impedance on the Smith Chart has zero reactive component, i.e., Z=R+j0.

• Tuning Priority:
  Achieving resonance by physically adjusting the antenna's length and form is considered the optimal design solution, as this inherently yields the highest efficiency.

• Simulation Limitation:
  It is acknowledged that a definitive efficiency value cannot be simulated correctly.
  While shortened antennas may theoretically show comparable radiated power (dBW) to full-sized antennas in simulation, their real-world efficiency will be lower due to un-simulatable losses.
  Therefore, optimal physical tuning remains the paramount objective.

2. Signal Stability Requirement and Ripple Metric

The success of the RABSII communication link, particularly for weak-signal modes, depends on consistent signal strength.

• Mandatory Requirement:
  The antenna system must deliver a steady signal strength (amplitude) towards Earth.
  This stability is obligatory for reliable FT4 decoding and accurate Signal-to-Noise Ratio (S/N) evaluation.

• Evaluation Metric: Power_to_gravity_dBW

  • This metric quantifies the ripple (variation) in the received power (dBW) caused by the satellite's attitude dynamics.

  • Measurement Protocol:
    The ripple is measured across a 360∘ elevation sweep (simulating a full rotational spin of the satellite's long axis) under two nutation conditions:

    • 0∘ Azimuth: No nutation.

    • 20∘ Azimuth: 20 degrees of nutation.

3. SWR Mismatch Tolerance (Worst-Case Scenario)

The simulation uses a strict SWR threshold to guarantee performance even under mismatch conditions.

• Reference SWR Threshold:
  The acceptable mismatch limit for the system is defined as 1.7:1 SWR.
  This is the practical maximum deviation typically tolerated by solid-state power amplifiers without causing a significant sacrifice in power output.

• Simulation Methodology (Worst-Case):
  For the purpose of the simulation, every deviation from the ideal 50+j0 Ω characteristic impedance (Z0​) will be modeled as an uncompensated SWR mismatch.
  This approach conservatively simulates the worst-case scenario by temporarily excluding the beneficial effect of the fixed-value matching network that will be incorporated into the final construction.

4. Antenna deployment test

B. Payload 

Contact information