Mounted onto the side of the Baker rooftop antenna is a service box. This box contains various electrical components which help attenuate unwanted frequencies when receiving. The contents of the service box are as follows:
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- 145 and 435 MHz Dual Band Filter
- RF Relays
- Low Noise Amplifier
- Bias-T
- Triplexer
- Desiccants
- Additional Electrical Wiring
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These components are fastened into the service box according to the following schematic:
Everything to the right of the Tx/Rx component (which is the ICOM IC-9700) is located within the service box with the exception of the UHF and VHF antennas on the right and the 3A @ 12V power supply at the bottom. A breadboard is included to link the electrical components together, including the N-channel MOSFET.
In this configuration, the N-channel MOSFET acts as a switch which controls whether the RF relays receive power. When the ICOM receives transmissions, it supplies power across the coaxial cable, and the Bias-T channels that power into the N-channel MOSFET’s gate (G). When the gate has power, the drain (D) is able to channel its power through the source port (S), supplying power to the RF relays. This switches them to their normally open state (NO), which allows the low noise amplifier (LNA) and dual band filter to be used.
When the ICOM is in transmit mode, the ICOM supplies zero volts, which disables the RF relays, switching them back into their normally closed state (NC). This prevents the transmitted signal from going through the LNA and dual band filter.