Overview

The heart of the software-defined antenna is called the "J-Box". A J-Box is an intelligent antenna switch that's placed along the antenna elements to provide multi-band operation. Each J-Box is configured with its "cutoff frequency", the ham band where it has been placed within the overall antenna structure. The J-Box uses a small pickup wire that's wrapped around the antenna element to couple a small portion of the RF signal into the J-Box.

This coupled signal is used for two purposes:

1) to drive the frequency counter circuits and decision-making logic, and 

2) to harvest RF signal to recharge the J-Box battery.

Power Train

Referring to Schematic 1, the coupled RF Harvest signal enters on lower left, where it's met by a lightning surge protector and two back to back Zener diodes that clip large signals to no more than 24V p-p, before it enters the simple rectifier. Turns out that high-frequency RF signals rectify quite nicely and drive a great DC voltage. From there, the DC voltage flows into the LiFePO4 battery charger circuit. A second source of more frequent charging voltage comes from the solar panel.  BAT1 is a AA LiFePO4 battery that operates at 3.6V with up to 600 maH capacity.

The battery voltage is then regulated to provide a stable 3V supply as VCC3.  A "Switched VCC3" supply is created by using a SiP32510 power switch that's controlled by the main MCU. During ultra lower-power sleep, all non-essential circuits are powered down to conserve battery capacity.  When fully optimized, the target average sleep current usage of the entire board is less than 10 uA.  When the main MCU is awakened from sleep mode, the first thing it does is power up the Switched VCC3 supply for operating the relay H-Bridge, configuration switch reading, etc.

It took a lot of effort to reduce overall current usage to 10 uA or less. These antenna switches are typically raised high into the air as part of an antenna system, so one doesn't want to have to go up there (or take the antenna down) to change batteries!  When not in use, the battery can operate the idle device for up to 250 days without recharging.  The solar panel will typically recharge the battery to nominal levels with about a days worth of sunlight.

The LiFePO4 batteries are the same ones used in outdoor solar powered lawn lights, so they’re tough and can take up to 2,000 recharges with no memory.  This could potentially extend the devices operational life without a battery change 10 years or more, if everything operates as expected, from what I’ve been able to determine from available materials.

RF Frontend

The other path a small portion of the harvested RF signal takes is up into the RF Frontend, which conditions the signal in preparation for the Main MCU to perform its frequency counting and relay control decision-making.

First stop is the low-power Schmitt trigger circuit, which converts the analog RF signal into a CMOS compatible square wave. This signal is in the range of 1.8 MHz (160 meters) up to 54 MHz (6 meters).

Next stop is the Prescaler circuit. To keep costs down and flexibility high, an 8-bit nano-power Microchip PIC processor is used, the PIC12LF1822.  This little jewel only draws about 50 nanoAmps when sleeping and nothing to do.  It’s firmware was designed to operate at a low clock frequency of just 2 MHz, which conserves active power usage at the expense of a minor amount of accuracy loss at higher frequencies.

The PIC uses TIMER1 and its prescaler to divide the digitized RF down...

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