LNA with the bypass relay and RF VOX

This LNA is assembled based on chip MMIC SPF5122z. HF356 AXICOM relay is used for path tracing. The main distinction from previous designs consists in RF VOX availability (HF voltage detector in the output of LNA). Such decision allows protecting amplifier from random transmitter power feed to their output. RF VOX functions correctly in CW, SSB modes, off delay is nearly 300 ms though it is not desirable to use RF VOX with SSB and CW. Relay will switch over under HF power in the time of every pause in transmission (TX). It is possible to energize LNA through feeder cable or from the external output. Max admissible power through relay with RF VOX is 50 watt, in case of DC power commutation with sequencer is 100 watt.

LNA protection from random transmitter power feed to their output based on RF VOX functioning. LNA without energizing is every time in bypass mode. In case of energized LNA relay switches to bypass when transmitter power feeds to their output. FM transceivers owners should use this LNA permanently, if external devices commutation is not supported by transceivers construction and switched voltage generation is joined with difficulties. It is enough to cut LNA into the feeder next to the antenna and energize it by separate wire (feeders shield will be used as common wire). RF VOX is fitted perfectly for the working with FM transceivers. Owners of transceivers with external LNA mode have to switch on mode of DC power to feeder supplying in the menu. For example for FT847 it’s necessary to set «Ant» in the menu 29 for 144 MHz (VHF-AMP). DC energy for LNA will be supplied by feeder from transceiver. Transceiver switches off DC energy in the transmission mode and LNA will be de-energized switched to the bypass mode. If you need to energize LNA by feeder in case of sequencer usage it is necessary to use power injector or supply DC energy by separate wire. LNA main characteristics are not different from their analog on base of chip SPF5122z without relay. Gain coefficient is lower because of 3db input attenuation. LNA should be energized from the stabilized DC power source 12-13.8V. Inner stabilizer is assigned for SPF5122z. Relay HF356 should be used with voltage 12V. Stabilizer for this part of scheme is not assigned.

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