Optimize RF System Match with VSWR Null Circuit Design | Opamp Help Needed

[dnyberg2]

I'm trying to design a circuit that monitors the rectified and filtered output of a directional coupler that is monitoring the reflected power of an RF system. The higher the reflected power the larger this voltage is. I can make it positive or negative. (Turn the diode around) I need a micro-power microscopic opamp that can control a varactor to null this detected voltage out, thereby minimizing the reflections and optimizing the match of the system. I can't seem to fathom the design of the opamp CKT. Single supply only around 2.6 volts for Vcc right now, but I can steal 5 if I have to. I think I need to be able to set a bias on the varactor to tune the center point of operation. Any opamp gurus out there with a few minutes to spare?

Thanks,

 

[skeptic2]

How much is the forward power?
Is the impedance of your system 50 ohms?

 

[dnyberg2]

Well the system is dynamic, so at the sweet spot it is close to 50 but due to the complexity of the system, it can vary quite a bit. It doesn't go down so much as it can go upwards of 125 ohms I think. The power level here is ~2MBps @ 50 MHz up to .25 watts...

 

[dnyberg2]

My problem is, I can't fathom how to determine if a system goes out of match one way or the other... I can see controlling a varactor with a detected voltage from something but how can you possibly know in such a complex system which way to tune (Up or down) to correct the mis-match?

 

[Averagesupernova]

Current is leading or lagging the voltage depeinding on the mismatch. That should be a clue. I know someone who actually built his own auto matching network for a mobile antenna on the HF ham bands. Sorry I don't recall details. I do recall him mentioning that as he drove by large metallic objects his tuner would auto-tune.

 

[dnyberg2]

UGH! That would be very bad in this case. The lead and lag clue might be helpful if i can figure out how to instrument it @ 49 MHz...

 

[vk6kro]

You can probably null out the indicated voltage in your instrument, but that won't help the actual matching of the antenna.

At 49 MHz, a typical Yagi antenna would be tuned by a Gamma match. This has a capacitor in it which really needs to be readjusted if you change frequency or if a bird lands on the antenna or if it is raining.

So, you could detect a high SWR and have a small motor which rotates this capacitor via plastic gears.

This would then be tuned until the best SWR was obtained.
This can be done manually or it could be automated with a small microprocessor.

Commercially, a long solenoid is wound and used as an antenna. The inside of this solenoid has the insulation removed and a sliding contact moves up and down the inside of the solenoid driven by a motor driving a screw thread.
At some point, the solenoid becomes resonant and the motor is stopped.