[Lf] Beacon Power control.]

[Andre' Kesteloot]

Johan Bodin wrote:

> Hello,
>
> Compensating an off-resonance antenna with increased TX power
> seems like a brute force method to me. Why not build a simple phase
> measuring thing that outputs a +/- voltage that is proportional to the phase
> relationship between voltage and current in the feeder? After all, the
> theoretical definition of resonance is that the voltage and current are in
> phase with each other.
>
> Assuming zero output at 0 degrees, the output from the "phase
> sensor" can be used to control a servo motor on a variometer which
> brings the system into perfect resonance whatever the wheather...
> An op-amp in the servo loop, surrounded by suitable R's and C's, will
> make the system slow and stable (dominant pole). It doesn't have to
> be faster than the changes in wheather ;-)
>
> One of my ideas for a "phase sensor" is:
>
> The feeder voltage and current can be sampled with a resistive (or
> capacitive, or both) divider and a current transformer, respectively.
> These two signals are then amplified and clipped to square waves.
> If these signals are fed into an XOR gate (74HC86 etc), the output
> from the gate will be a PWM signal that is proportional to the absolute
> value of the phase error. The PWM signal can be converted to a DC
> error signal by a simple RC lowpass filter (136 kHz >> the servo loop
> bandwidth). The sign of the error signal can be found by connecting
> these two signals to a D-type flip-flop (74HC74, 4013 etc).
> The current and voltage squarewave signals are connected to the
> clock input and the D inputs respectively. The sign bit will appear on
> the flip-flop Q output. This sign bit controls the direction of servo motor
> travel and the filtered PWM from the XOR gate is the magnitude of the
> error.
>
> Depending on actual implementation, some kind of "gating" may be
> needed to prevent the servo loop from "run-away" during key-up periods..
>
> Just an idea...
>
> 73 de Johan SM6LKM