Bill W4ZV wrote
You must enable one of the CONFIG:KRX3 ANT options (ANT=BNC or ANT=ATU) even if you never plan to use it. Check item #3 on the KRX3 Manual ERRATA.
After the KRX3 is enabled above, you can then toggle between MAIN and one of the above ANT choices by enabling SUB and then toggling ANT.
Make the last sentence:
After the KRX3 is enabled in CONFIG as described above, press SUB then hold BSET, and then toggle ANT to choose MAIN or the choice you set in CONFIG above. This is confusing the first time you do it and I missed it because I had not pressed SUB. All 3 steps must be done...CONFIG:KRX3, SUB, and BSET before toggling ANT.