Modeling and Simulation of Onboard Dipole Antennas Operating at 2.4 GHz and 433 MHz

Date: 27.07.2025

Author: Dionysis Mourelatos

Context

Antenna simulations provide insight into the performance of an antenna, helping to optimize its design and placement. We will examine two onboard antennas (@ 2.4GHz & 433MHz) and our primary objective is to evaluate their suitability for maintaining robust communication links, particularly for critical telemetry.

Simulation Parameters

• Simulation Software: Ansys Electronics Desktop Student 2025 R2 (Ansys HFSS)
• Antennas:
  • Type: Half-wave ($\lambda/2$) dipole antennas
  • Frequencies: 2.4 GHz and 433 MHz
  • Material: 2 mm diameter, copper wire
  • Feed Gap: 5 mm
  • Correction factor: 0.81, 0.97 (accordingly) {to account for real world effects}
  • Mounting: Side-mounted at mid-length of the rocket body
  • Environment: Air
• Rocket Body Model: Simulated as a Perfect Electrical Conductor (PEC). A worst-case scenario for antenna interaction with the conductive carbon fiber (CF) structure. CF is an anisotropic and dispersive material, making accurate EM modeling particularly challenging.
• Results:
  • Radiation Pattern
  • S(1,1) Parameter Plot
  • Smith Chart

Free Space Dipole Radiation Pattern (For Reference)

![[Torus.png]]

In free space, a half-wave dipole antenna exhibits a characteristic torus-shaped, omnidirectional radiation pattern. It radiates most strongly perpendicular to the dipole axis and minimally along its axis (nulls).

Dipole Antenna @ 2.4 GHz

• Frequency: 2.4 GHz (λ≈12.5 cm)
• Pattern Shape:
  • The pattern is an oblong extending mostly perpendicular to the rocket's axis.
  • It shows clear lobes where the signal is strongest (red/orange).
  • There are deep nulls (blue/transparent areas) extending directly along the rocket's main axis and also behind the rocket body.
• Impact of Rocket Body:
  • The conductive rocket body acts as a significant reflector and scatterer.
  • The ideal torus shape is distorted and radiation is pushed away from the rocket body.
• Implications:
  • Directionality: This antenna becomes directional. It radiates best broadside to the rocket.
  • Limited Coverage: Very poor (or no) signal directly above, below or behind the rocket.
  • High Path Loss: 2.4 GHz signals experience more atmospheric attenuation and path loss than lower frequencies and thus need more transmit power or higher gain GS antennas.

Dipole Antenna @ 433 MHz

• Frequency: 433 MHz (λ≈69.2 cm)
• Pattern Shape:
  • The pattern is much more omnidirectional.
  • There is a wider overall coverage and less severe nulls along the rocket's axis
• Impact of Rocket Body:
  • The rocket still negatively influences the pattern, but it allows more energy to wrap around it.
• Implications:
  • Better Omnidirectionality: The pattern is much more suitable for our rocket, that will have unpredictable, spinning behavior. While some fading will occur, it will be less severe and less frequent than with the 2.4 GHz.
  • Improved Coverage
  • Lower Path Loss

Frequency Shift

A negative consequence of the proximity to large, electrically conductive bodies (such as carbon fiber) is a shift in operating frequency. Their interaction affects the theoretical electrical lengths, making fine-tuning necessary.

Recommendations

• Hands-on experiments are needed, to evaluate the effect of CF

• Custom antennas might be necessary, if off-the-shelf antennas fail to perform.

• For critical telemetry the 433 MHz dipole is superior. A more omnidirectional pattern means a more reliable link, even if the rocket spins or has unpredictable behavior. The 2.4GHz antenna can offer redundancy.

• Mounting on a small standoff from the main body could approach ideal patterns, but this could be impractical (aerodynamically or statically).

• The ground station will need a high-gain antenna (e.g. Yagi) that can be actively pointed (manually) to maintain the link.

• Ground and real time telemetry testing are obviously mandatory to ensure functionality.

Conclusion

In conclusion, for a reliable telemetry, the 433 MHz dipole, offers a more robust radiation pattern compared to the 2.4 GHz dipole, with a higher probability of maintaining a reliable data link throughout the flight. However, the precise effect of the conductive CF surface on the antenna's performance requires more simulations and testing.

Warning

This simulation is not exhaustive, final or complete. Many crucial parameters remain essentially undefined, so it is important that we revisit and reassess our models. Such parameters are:

• Exact dimensions of antennas (Off-the-shelf? Custom?)
• Exact placement on rocket body
• Exact material characteristics of body (PEC is worst case)
• Exact rocket size and analogies