Dual-Band RF Path

The LR2021 can handle both bands, but I still split the external RF front end into separate 915 MHz and 2.4 GHz gain/filter/routing chains. Only one band and one direction are allowed to be active at a time.

Functional TX Chain

Stage	915 MHz path	2.4 GHz path	Design intent
Radio	LR2021 LF output	LR2021 HF output	Generate the commanded LoRa waveform
Gain	External PA	External PA	Raise radio output toward the approximately 1 W system target
Harmonic filtering	Band-specific LPF	Band-specific LPF	Reduce PA-generated harmonics before the antenna network
RF sensing	Directional coupler	Directional coupler	Sample forward/reflected traveling waves
T/R + diversity	RF switch network	RF switch network	Select direction and antenna path
Radiation	915 MHz antenna	2.4 GHz antenna	Send the selected band to the ground station

The ~1 W number is a system target, not 'the PA datasheet says 1 W so we're done.' Final LR2021 drive, PA compression, filter loss, coupler loss, switch loss, trace loss, and connector loss must be measured together before the antenna-port power is considered verified.

Functional RX Chain

Stage	Function
Antenna/diversity selection	Choose the desired physical antenna path
T/R switch	Disconnect the TX chain and route the antenna toward the receiver
LNA	Improve weak-signal level before subsequent loss
Band-pass filter	Reject out-of-band interference before the LR2021 input
LR2021	Demodulate the selected 915 MHz or 2.4 GHz signal

A big LNA gain number by itself does not tell me much. I still have to care about NF, filter/switch loss, blocker handling, compression, and how much garbage the CM5/power converters spray into the receive side.

Why Half-Duplex Switching

Switching Invariant

I am treating the RF switches like a state machine, not a bag of GPIOs that software can flip in any order.

Step	Required action
1	Disable the active PA and LNA
2	Allow RF output to decay
3	Change T/R and/or antenna-selection switches
4	Wait for switch settling
5	Enable only the amplifier needed by the new state
6	Begin transmit or receive operation

The whole point is to make it hard to hot-switch a live PA or accidentally dump RF into a bad/unknown path.

Diversity

The antenna switch network is intended to give the CM5 a second path when the selected antenna is poorly matched or the link quality degrades. Diversity only helps if I can tell 'bad antenna/path' from 'the PA or radio is the thing that's broken'; this is one reason forward/reflected RF telemetry is part of the architecture.

PCB Requirements

Controlled-impedance routing from the LR2021 through switches, filters, couplers, and connectors
Continuous RF return paths with no accidental plane gaps beneath critical transmission lines
Via fencing and compact ground-return geometry around RF launches and switching nodes
Physical separation between sensitive RX regions and CM5/Ethernet/switching-power noise sources
Short, low-inductance decoupling around PAs, LNAs, switches, and the LR2021
Thermal copper and via structures sized around PA dissipation rather than RF routing alone
Layout access that still permits VNA, spectrum-analyzer, and conducted-power validation

Open RF Items

Final PA/LNA/filter selections for both bands
Measured PA gain/compression versus LR2021 drive setting
Final T/R and diversity switch insertion loss, isolation, and power handling
Antenna-port conducted-power target after all downstream losses
Receiver cascaded noise figure and blocker tolerance
Final controlled-impedance geometry from the production stackup