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CURRENTLY IN DEVELOPMENT!
CURRENTLY IN DEVELOPMENT!
Project Overview
Project Overview
The JHU CubeSat Team ground station uses two military-grade radios for CubeSat communications and HAM radio operations. Currently, switching between antennas requires manually swapping coaxial cables. This is a time-consuming process that creates operational risk during active passes, along with requiring someone in person to manually switch the cables.
This project is a custom-designed RF multiplexer PCB that allows both radios to remotely select any of up to eight antennas from software, with hardware-level protection preventing both radios from ever connecting to the same antenna simultaneously. No commercial off-the-shelf solution exists that satisfies this specific two-radio to multi-antenna requirement with the necessary RF performance and safety interlocking.
The project is currently in active development, with the schematic complete and PCB layout underway.
Design Highlights
Design Highlights
Hardware Interlock:
Physical Protection Against Radio Damage
Hardware Interlock:
Physical Protection Against Radio Damage
A diode bridge between the two switching circuits creates a hardware logic layer preventing both radios from routing to the same antenna simultaneously. Even if firmware fails or enters an unexpected state due to high-energy cosmic particles, the hardware enforces priority, effectively protecting military-grade radios from catastrophic damage.
50Ω Impedance-Matched RF Tracks
50Ω Impedance-Matched RF Tracks
All RF signal paths are designed with track widths calculated to maintain 50Ω characteristic impedance throughout. Relay outputs feed directly into N-type connector footprints, keeping uncontrolled RF path lengths as short as physically possible to minimize reflections and signal loss.
Board as Mechanical Structural Component
Board as Mechanical Structural Component
The PCB doubles as a structural element of the metal enclosure — N-type connectors are soldered directly to the board and mate through the housing face, eliminating all internal wiring. The only external connections are a single Ethernet cable and the coaxial antenna lines.
Design Features
Design Features
1: Circuit Design
1: Circuit Design
System Architecture
System Architecture
PIC microcontroller independently drives two 74VHCI38FT 8-to-1 demultiplexers — one per radio
Each demultiplexer controls 8 channels, each channel: transistor stage → relay → antenna port
Both circuits are structurally identical, keeping signal paths equivalent for both radios
Hardware Priority Interlock
Hardware Priority Interlock
Two-layer protection against both radios connecting to the same antenna:
Software: microcontroller tracks both radio states and blocks conflicting selections
Hardware: a diode bridge between the two circuits acts as physical logic, enforcing priority at the circuit level if both attempt to activate the same relay
The hardware interlock is to account for firmware bugs or single-event upsets, protecting the radios regardless of software state
Full Circuit Architecture
Full Circuit Architecture
2: PCB Layout
2: PCB Layout
RF Track Design
RF Track Design
Track widths calculated to maintain 50Ω characteristic impedance based on dielectric properties, copper thickness, and layer stackup
Deviating from 50Ω causes reflections, power loss, and standing wave issues on the antenna feed line
RF traces kept as short as physically possible: relay outputs placed directly adjacent to N-type connector footprints to minimize uncontrolled path length
Current-PCB Layout
Current-PCB Layout
Dual-Sided Layout
Dual-Sided Layout
Two switching circuits occupy top and bottom copper layers respectively
RF connections feed through both sides via vias; N-type connectors accessible from the top face of the enclosure regardless of internal layer
View of Top and Bottom of PCB from Side
View of Top and Bottom of PCB from Side
Current Status
Current Status
RF connections fully routed
Remaining work: optimizing demultiplexer and microcontroller signal paths, resolving routing congestion, finalizing layer stackup before fabrication
3: Mechanical Integration
3: Mechanical Integration
N-type connectors mount directly to the PCB via solderable pins, aligned with precision through-holes that mate with the metal enclosure face
PCB carries connector loads directly; no separate bracket or mounting hardware needed
Fully self-contained module with zero internal wiring
Only external connections: one Ethernet cable (power + control) and coaxial antenna cables at the N-type ports
Board removal for servicing requires only disconnecting external cables and unbolting from housing
4: Next Steps
4: Next Steps
Complete optimization of demultiplexer and microcontroller routing
Define and finalize the PCB layer stackup
Send to fabrication and assemble
RF performance testing: verify 50Ω impedance continuity, insertion loss per channel, and isolation between channels
Functional testing: verify software antenna selection, hardware interlock behavior under fault conditions, and PoE power delivery
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