The term "Break-In" (and the eponymous button on the ESDR2 screen) is misleading here and has nothing to do with the real "Break-In", which has been used in amateur radio terminology since at least 1940 and means "Full Break-in", otherwise referred to as QSK. This is half-duplex operation, where it is possible to interrupt the transmitting station because the operator can listen between its elements (ie. between dots and commas). Due to the fact that this half-duplex operation is technically demanding, the term "Semi Break-In" has been introduced, which means the ability to listen between letters. It appeared sometime around 1970 in transceivers, which were manufactured on the mistaken assumption that with the expansion of SSB, CW operation would disappear. However, the exact opposite happened, there was a renaissance of CW and transceivers labelled "SSB Transceiver" on the front panel became difficult to sell. As it became standard to use an electromechanical antenna relay that was noisy and slow, manufacturers began to introduce the term "Semi Break-In", which meant a de facto acknowledgment of the inability to design a sufficiently fast, purely electronic receive/transmit switching system, namely switching antennas from the transceiver receiver to its transmitter.
Today, it is possible, structurally feasible and inexpensive to switch the antenna using a switch with PIN diodes. Such a switch can switch at times of the order of microseconds, is completely noiseless and has, unlike an electromechanical antenna relay, unlimited lifetime. Nevertheless, most transceivers (including SunSDR2 transceivers) still use an antenna relay for unknown reasons, which must be considered a design flaw today. The reason is the ignoring of CW mode and very often the ignorance of developers who have only brief and often misguided ideas about CW.
Unfortunately, the control of external coaxial relays, which are used with low-noise preamplifiers mounted directly on the mast, is related to the switching system. This arrangement is a standard for DX operation on VHF bands, including eg EME. If the delay were not dictated by the slow antenna relay in the transceiver, it would be possible to control these coaxial relays independently of the T/R switching system, which would greatly facilitate the development of LNAs, transverters and PAs for VHF/microwave bands. Thus, the use of a T/R switch with PIN diodes would become an advantage even for users who do not operate CW.
73,
Martin, OK1RR