From ad3e149c0d09c6e0f6ca633f37b84783e04c2c79 Mon Sep 17 00:00:00 2001 From: Annika Schramm Date: Wed, 13 May 2026 12:28:16 +0200 Subject: [PATCH] Fix output power limit on heltec v4 board The HelTec WiFi LoRa 32 V4 board features a SX1262 connected to a builtin radio frontend module (FEM) containing both a PA for the TX path and a LNA for the RX path. Previously the maxium allowed SX1262 TX power was set to 22 dBm. This is dangerous since it exceeds the maximum PA input power limit for all revisions of the board. V4.2 of the board uses the GC1109 FEM with an absolute maximum PA input power specfied as +5 dBm. [1] V4.3 of the board uses the KCT8103L FEM where absolute maximum PA input power is unclear due to a lack of publicly available datasheeets. Consequently configuring a TX power of more than +5 dBm on the SX1262 is dangerous as it can damage the FEM. Furthermore it does not - while not directly dangerous - make any sense to configure a TX power of more than +0 dBm on the SX2162 since the GC1109 used in board revision 4.2 specifies a maximum PA output power of +30 dBm and also a typical PA gain of 30 dB. Similarly the KCT8103L used in board revision 4.3 specifies a maximum ouput power of +28 dBm [2] and a PA gain of 32 dB [3]. Thus using an input power of more than +0 dBm and -4 dBm respectively from the SX1262 would overdrive the PA causing distortions and thus degrading TX performance. Limit maximum SX1262 TX power to +5 dBm so the FEM will not be damaged and set default TX power to -4 dBm so the FEM PA is not overdriven. [1] https://www.geochipinc.com/uploads/23940615_1756826539.pdf [2] https://kxcomtech.com/en/application/info/1774 [3] https://www.ftelectronic.com/Uploads/Temp/news/20251209/69379d0d25b27.pdf --- variants/heltec_v4/platformio.ini | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/variants/heltec_v4/platformio.ini b/variants/heltec_v4/platformio.ini index 6f6bf2b538..2e4d489e4c 100644 --- a/variants/heltec_v4/platformio.ini +++ b/variants/heltec_v4/platformio.ini @@ -26,8 +26,8 @@ build_flags = -D PIN_USER_BTN=0 -D PIN_VEXT_EN=36 -D PIN_VEXT_EN_ACTIVE=HIGH - -D LORA_TX_POWER=10 ;If it is configured as 10 here, the final output will be 22 dbm. - -D MAX_LORA_TX_POWER=22 ; Max SX1262 output + -D LORA_TX_POWER=-4 ; -4 dBm output power from SX1262 is maximum that will not saturate PA on V4.3 board, V4.2 only saturates at +0 dBm + -D MAX_LORA_TX_POWER=5 ; Absolute maximum TX input power rating of GC1109 FEM, exact maximum of KCT8103L unclear -D SX126X_REGISTER_PATCH=1 ; Patch register 0x8B5 for improved RX -D SX126X_DIO2_AS_RF_SWITCH=true ; GC1109 CTX is controlled by SX1262 DIO2 -D SX126X_DIO3_TCXO_VOLTAGE=1.8