Design

4. Design Specifications

The design specifications that outline the details of the designs are included within this section. The design specifications are split among the two chosen designs, the tape measure dipole antenna and the nitinol memory wire folded monopole whip antenna.

4.1. Tape Measure Dipole Antenna

The tape measure dipole antenna is the most feasible design because of its simplicity. A similar tape measure design has also been launched, proven and tested in space as seen in the Buccaneer Risk Mitigation Mission [4]. 

4.1.1. Antenna Length and Enclosure Dimensions

Design length for the dipole antenna operating at 30 MHz for ½ wavelength is L=5m. Considering this is a dipole antenna that will be divided by 2 to give a length of 2.5m for each tape measure element. 

 

The tape measure dipole antenna utilizes the TunaCan CubeSat configuration. TunaCan enclosures are limited by the following criteria to ensure they are compatible with the NanoRacks CubeSat launchers.

 

  • Max Rail Length on NanoRacks CubeSat launchers- 340mm [5]
  • Max Space on top of CubeSat – 10x10cm 
  • Less than 800g in weight.

Total size of TunaCan H<40mm: W<10mm: L<10mm

4.1.2. Tape Measure Material

Carbon steel is the material used for the antenna elements as it is a good material to coil and conduct RF signals. Carbon steel conductivity and resistivity can be seen in Table 4,1 showing it will work great as an antenna. Wrapping carbon steel creates a spring and will want to return to its extended length. All the parameters will make it a good choice to spring open and release to full length.

Table 4.1: Carbon steel electrical specifications [6]

4.1.3. Input Impedance and Impedance Matching

The input impedance of the balun was chosen to be 10 times the operating impedance. 10 times the operating impedance is a good enough threshold to choke the unwanted RF currents. This will isolate the transmission line from the antenna, therefore, an impedance of around 5k Ohms for the balun is needed.

4.2. Nitinol Memory Wire Folded Monopole Whip Antenna

The design specifications for the nitinol memory wire antenna design are described in this section by its size, material, impedance matching, connections, and deployment mechanism.

4.2.1. Whip Antenna Length

The size details for the nitinol whip antenna are:

  • The total height should be ¼ wavelength (L = 2.5m).
  • One end of the antenna will be connected to the source, and the other should connect to the ground plane.

4.2.2. Whip Antenna Material

The material details used for the monopole are:

  • Will use Nitinol wire (Flexible and has shape memory).
  • The transformation temperature is set by the manufacturing process.

4.2.3. Impedance Matching

The impedance matching design specifications of the nitinol memory wire antenna are:

  • Match the 140-150 Ω from the antenna to the 50 Ω receiver.
  • 3:1 transformer acting as a balun
  • L-type matching network
  • T-type matching network
  • Pi-type matching network

4.2.4. Antenna Connection

The connection details for the antenna are:

Antenna->SMA connector->3:1 transformer(146ohm->50ohm)->Signal source

4.2.5. Antenna Deployment

The antenna deployment mechanism is described by heating using current calculated in Appendix A.

5. Design Concept Development & Evaluation

The literature review was used to develop five possible design concepts. These designs were the tape measure dipole antenna, nitinol memory wire antenna, wire monopole antenna, tent pole dipole antenna, and telescopic dipole antenna. These designs were evaluated using the weighted criteria matrix as seen below in Figure 5.1. The tape measure dipole antenna and nitinol memory wire monopole antenna designs proved to be the most viable concepts and it was decided to only proceed with these two.

Figure 5.1: Design concept evaluation chart

5.1. Tape Measure Dipole Antenna

The literature review was used to develop five possible design concepts. These designs were the tape measure dipole antenna, nitinol memory wire antenna, wire monopole antenna, tent pole dipole antenna, and telescopic dipole antenna. These designs were evaluated using the weighted criteria matrix as seen below in Figure 7.1. The tape measure dipole antenna and nitinol memory wire monopole antenna designs proved to be the most viable concepts and it was decided to only proceed with these two.

5.1. Tape Measure Dipole Antenna

The tape measure dipole antenna is a simple and cost-effective antenna design. It consists of a tape measure divided into two halves. The length of each half is typically a half-wavelength for the desired frequency. This type of antenna offers portability, affordability, and wideband operation. Half-wavelength dipole antennas are desirable for its radiation pattern and impedance characteristics. Construction involves cutting a tape measure to a length of 2.5m and deploying it from an additional storage compartment on the end of the CubeSat called a “TunaCan.” A wire burn releases the antenna and deploys it to full length. 

 

The tape measure dipole antenna scored the highest out of the five proposed design concepts with a score of 24/25. This antenna design takes advantage of the TunaCan CubeSat design to minimize its internal volume consumption. The vast majority of the deployment mechanism is integrated in the TunaCan portion of the CubeSat, requiring only the balun, impedance matching, and burn wire circuits to be included within the CubeSat.

5.2. Nitinol Memory Wire Monopole Antenna

Nitinol memory wire is considered a viable option for an antenna due to its property of remembering its original shape when heated to 50 degrees [16]. One of the advantages of using a memory wire is that it retains its shape and does not move freely compared to traditional flexible wires.

 

The antenna can be put into a monopole configuration of 2.5m (¼ wavelength) and then folded in half (⅛ wavelength). The size reduction is a desirable attribute since space is limited in a CubeSat. Multiple methods can be used to return the memory wire to its original shape. These methods include using a heater, utilizing sunlight or passing an electric current through a wire. 

 

The nitinol memory wire achieved the second-highest score in the evaluation, 18/25. The key advantage of this antenna is its stability during the deployment process. To restore the antenna’s shape, two heating options will be considered: utilizing a feed circuit to pass current through the nitinol wire or relying on natural sunlight to heat the wire.

5.3. Wire Monopole Antenna

The flexible wire monopole antenna was considered for its convenient storage within the CubeSat, where the wire could be wound onto a bearing-mounted reel. A small mass would be attached to an open end for deployment. To keep the wire extended, a constant rotation force is required. This rotation prevents the wire from tangling or collapsing during operation.

 

The wire monopole antenna scored high in all criteria except deployment feasibility with a total score of 15/25. In this antenna design, a constant rotation force is necessary to prevent the wire from tangling or collapsing during operation. However, achieving reliable spinning in the space environment is challenging, and deployment feasibility is an important criterion. Due to these factors, the design did not score highly overall.

5.4. Tent Pole Dipole Antenna

The tent-pole dipole antenna is a portable and convenient variant of the dipole antenna design. It utilizes tent poles or similar structures as the radiating elements, eliminating the need for additional support. The antenna offers easy setup and portability. Construction involves attaching the feedline to the tent poles. Tuning is done by adjusting the 2.5m length and spacing to achieve resonance at the desired frequency.

 

The tend pole dipole antenna scored 13/25. This antenna design poses challenges in terms of mechanical design, as it requires a time-consuming process to fit the antenna inside the CubeSat and ensure proper deployment. Additionally, achieving reliable electrical contacts can be challenging when the antenna parts are folded. Due to these challenges the tent pole dipole antenna was rejected.

5.5. Telescopic Dipole Antenna

The telescopic half-wavelength dipole antenna was considered due to its suitability for applications where space is limited. Two different ideas were explored for the deployment mechanism. The first idea involved attaching a mass to the antenna end and spinning the CubeSat to launch it. The second idea involved using a launcher, such as springs, to propel the antenna.

 

The telescopic dipole antenna scored the lowest, 12/25. Testing the antenna on Earth is the most important requirement for this project. However, the telescopic dipole antenna design poses challenges due to the potential variation of forces acting on the antenna elements in different environments (Earth and space). Additionally, achieving reliable electrical contacts between the telescopic antenna elements in space can be difficult due to harsh conditions and mechanical vibrations.

5.5. Telescopic Dipole Antenna

The telescopic half-wavelength dipole antenna was considered due to its suitability for applications where space is limited. Two different ideas were explored for the deployment mechanism. The first idea involved attaching a mass to the antenna end and spinning the CubeSat to launch it. The second idea involved using a launcher, such as springs, to propel the antenna.

 

The telescopic dipole antenna scored the lowest, 12/25. Testing the antenna on Earth is the most important requirement for this project. However, the telescopic dipole antenna design poses challenges due to the potential variation of forces acting on the antenna elements in different environments (Earth and space). Additionally, achieving reliable electrical contacts between the telescopic antenna elements in space can be difficult due to harsh conditions and mechanical vibrations.

6. Chosen Design Concepts

The development of the two chosen design concepts, the tape measure dipole antenna and the nitinol memory wire monopole antenna is described below.

6.1. Tape Measure Dipole Antenna

The tape measure dipole antenna is the most feasible of the two chosen design concepts. This design was constructed as a prototype while the monopole antenna was decided to be too time and cost intensive

6.1.1. Enclosure

The tape measure design of the dipole antenna will be a top-mounted TunaCan enclosure as seen in Fig. 6.1 and 6.2. The TunaCan enclosure will be housing the two elements coiled up. Carbon steel is the chosen material for the tape measure as it behaves as a natural spring. This will allow the tape to be coiled up and released from within the TunaCan without a motor or propellant. The antenna release process will require a wire burn to cut a fishing line holding the tape measure around the tuna can.

 

The enclosure diameter is the maximum diameter allowed for TunaCans in the NanoRacks CubeSat deployer, 88mm. The height is one millimeter lower than the maximum, measuring 45 mm including the burn-wire circuit and fishing line on top. The space allowed for the tape measure to be rolled around is 6 mm wide, and 28 mm tall, allowing for plenty of room for the tape measure. This prevents significant friction during deployment.

Figure 6.1: CAD rendering of Tuna-Can enclosure internals

Figure 6.2: CAD rendering of Tuna-Can with top cap

6.1.2. Tape Measure Antennas

The total length of the antenna can be calculated using the below equations. Design length for the dipole antenna operating at 30 MHz being fed by a 50Ω coaxial line is 4.563 m.

6.1.3. Balun

When a dipole antenna is connected to a coaxial cable (an unbalanced system), there is an imbalance in the coupling between the inner conductor and the outer shield [30]. This results in unequal currents flowing through the two arms of the dipole antenna as seen in Figure 6.3.

Figure 6.3: Dipole antenna attached to an unbalanced coaxial cable [30]

The current imbalance between I2 and I(common-mode current) causes one arm of the dipole to carry a different current compared I1 current. This imbalance results in unequal currents flowing through the two arms of the dipole antenna. A balun is a device that is used to convert an unbalanced transmission line to properly feed a balanced component.

A lossy choke balun will be used to suppress the common-mode current. This type of balun consists of two elements, a ferrite and a coaxial cable. First, a ferrite mix and size have to be selected, followed by determining the appropriate size of the coaxial cable.

6.1.4. Burn Circuit

Burn circuit is needed to cut the fishing lines holding the antenna from releasing. The burn circuit is placed at the top of the enclosure and the fishing lines are run over the resistors. This allows high current to high up the resistors and cut the fishing line releasing the antenna. Figure 6.4 shows the schematic for the burn circuit using MOSFETs to switch the current flow when needed to the resistors. A second MOSFET was added for redundancy of the burn circuit. The two resistors of 8.2ohms in series will both burn under the 731mA current. The resistors are rated for ¼ Watts of power and will receive a total of 8.7 Watts.

Figure 6.4: Burn circuit diagram

3D rendering of the burn circuit is shown in Figure 6.5 The rendering shows the board layout and mounting screws. The total size of the PCB is 25x25mm and has M2.5 mounting holes. A typical fan connector can be used to power the board.

Figure 6.5: 3D rendering of burn circuit

6.2. Nitinol Memory Wire Folded Monopole Antenna

The nitinol memory wire folded monopole antenna design was extensively developed but it was decided to only proceed with a prototype for the tape measure dipole antenna concept due to the complexity, time requirement, and cost of the monopole design. The design remains a viable solution for deployable HF antennas. 

6.2.1. Antenna Geometry

While researching folded monopoles, it became apparent that this term describes a large variance of antennas with various construction methods like PCB antennas for UHF or large tower antennas for LF. The search was narrowed down to 2-wire folded monopoles and this resulted in two different configurations. The second antenna configuration is described by J. Oh et. al. in wideband planar folded monopole antenna with tapered resistively loadings [31]. This antenna is known as an open-folded monopole and is seen in Fig 6.6. Most literature regarding folded monopoles was this configuration with the total height being less than ¼ wavelength. After presenting this configuration to the Cfar team it was decided to pursue the second configuration.

Figure 6.6: Open folded monopole [31]

The second configuration is called a terminated folded monopole and can be seen in Fig 6.7. The antenna starts at the source and ends connected to the ground plane. The height is ¼ wavelength and is essentially half of a folded dipole. There are very few resources on determining the optimal spacing between vertical wires. M. Ehrenfried specified in his article “Terminated Coaxial Cage Monopole a New Design of Broadband HF Vertical Antenna”, that the spacing should be 1/10 of the wavelength, but he does not provide adequate support for this claim [31]. 

Figure 6.7: Terminated folded monopole

6.2.2. Antenna Specifications

The shape of the designed antenna will match Figure 6.6. The height of the folded monopole should be 1⁄4 wavelength for the centre frequency of the amateur satellite band, which is roughly 29.4MHz and 2.55m. This is ignoring the velocity factor of nitinol, which could not be found. If the suggested space of 1/10 wavelength between the antenna wires is used, then the antenna top wire will be over 1m wide. This is not practical for mounting on a CubeSat, so a spacing of 5cm is selected as a starting point. The total length of the wire would then be 5.15m, without tuning for resonance. It is difficult to select the diameter of the nitinol wire without having handled it, so a diameter of 1.5mm was chosen based on another folded monopole made of copper [32]. With nitinol’s density of 6.5g/cm3, the total weight of this antenna would be 60 grams.

 

 

Making electrical connections between the nitinol and feedline/ground plane can be done by soldering, although it is difficult because of nitinol’s titanium-oxide layer. Special fluxes and solder can make this easier.

 

6.2.3. Nitinol material

Nitinol has two interesting characteristics that make it a good potential space-saving rigid antenna; its superelasticity and its shape memory. Either of these properties could potentially be useful in the deployment of the antenna. The characteristics of nitinol depend heavily on its composition. It is an alloy of nickel and titanium, hence the name. Nitinol is roughly 55% Nickel by weight [33]. Slight adjustments in the composition are used by manufacturers to create nitinol wire with the desired characteristics, such as transformation temperature. The transformation temperature is the threshold for when the nitinol changes from being malleable to forming its trained shape.  The transformation temperature should be the biggest consideration when choosing a wire because it determines the properties of the wire at the current temperature. If the transformation temperature is below the surrounding temperature of the wire, the wire will be super-elastic. On the other hand, if the transformation temperature is above the surrounding temperature, the wire will be malleable until it reaches the transformation temperature. 

 

 

Super elastic nitinol antennas have been used on cubesats before by the US Naval Academy for the RAFT1 and MARScom. There were no available examples of heat-deployed nitinol shape-memory wire antennas 

6.2.4. Deployment

The antenna would be stowed in a coil that would allow it to unravel while maintaining connections at the feedline and ground plane. Shape-memory nitinol is quite malleable when it has not reached its transformation temperature. A nitinol composition that has a high transformation temperature such as 80℃ or higher would limit the chance of an accidental transformation during transit. To prevent accidental deployment of the antenna, the coil should remain secured with a fishing line until deployment is ready. The antenna can be deployed by activating the burn-resistor circuit and heating the nitinol wire with current.

 

A simple Joule’s Law calculation shown in Appendix D, ignoring the environment of LEO and heat radiation away from the wire, shows that the wire can be heated to 80℃ by 1.26A in 30 seconds. Due to neglecting heat radiation, this estimated current is low. Either a higher current or a longer time would be needed to heat the nitinol to the transition temperature. If using a higher current, it will be necessary to check that the coaxial cable and PCB traces can carry the necessary current.

 

The DC current can be supplied over the coaxial cable through the use of a Bias Tee network. This allows a DC bias to be put on the antenna without interfering with RF signals. Off-the-shelf bias tees are not suitable for this due to their large enclosures and a bias tee network would need to be designed, taking into account the antenna’s operating frequency.

6.2.5. Impedance Matching

The method of impedance matching suggested by the Cfar team was the 3:1 turn ratio transformer seen in section 4.2.3.1. The folded monopole has an impedance of 146Ω and the coaxial it connects to is 50Ω. The 3:1 transformer will match these lines to minimize reflection.