'Perturb and observe' comes to mind, a strategy commonly used to find the maximum point of power transfer (MPPT). It seeks to find the optimum way to utilize a power source so as to obtain the most watts.
Suppose we had a 'black box' which does these steps:
1) Apply a certain voltage (1.2V) to the varactor.
2) Then add a slow up-and-down pulsation. Variation is a few percent.
3) Suppose signal strength increases at V=1.25 V. Then make that your new applied voltage.
4) Resume slow up-down pulsation.
The trick is to create a circuit whose response is not based on absolute signal strength, but by comparing to a 'recently remembered' level, however that can be accomplished.
This sort of thing is done all the time by modern radios. You start the scan, the set scans the band at low sensitivity and if a suitable station is found you then memorise it on say channel1. Going for a scan again within a certain time (10 secs?), causes the set to increase its sensitivity and rescan, this normally has to be done four times or so before the set is running at its maximum sensitivity and a massive number of stations can be heard.
So you have a fast wide tuning scan, until a signal threshold is exceeded then you go into the perturb mode.
Frank
Because of the high Q and the directivity of the aerial, I think you will miss some signals unless you are only after S9+40 ones.
Frank
This project requires several building blocks. One is a detector for recent greatest signal strength.
Here is a circuit that detects when signal strength is rising. One input is delayed slightly by a capacitor. The output goes high only when signal strength is rising.
Output goes low when signal strength starts to decline. (The function is an integrator.)
This creates a window which might control something else, maybe a sample-and-hold, or some other memory circuit. It is not needed to remember greatest signal strength, but to record some other condition at the moment when signal strength starts to decline.
This is only a building block. More circuitry will need to be added.
The other alternative would be to use a LOW power oscillator fed to the loop and optimize the SWR before switching back to receive mode. You say 'active magnetic loop' so I assume it is only for receiving, the technique doesn't lend itself to transmission except at very low power. From experience, loops can generate tens of KV at resonance even with only a few Watts drive so be careful if you try it.
Brian.
What I was thinking was to disable the receiver temporarily and feed a signal TO the loop. If the signal is at the same frequency as you want to receive (or use the oscillator in a direct conversion receiver) you can use an SWR or simple voltage measurment to see when it's resonance has been achieved. It should be the same resonanace for radiating and receiving.
Brian.
That would seem to be a pretty fundamental obstacle to any " max amplitude" seeking tuning system.Onlt that it gives a constant signal to use for tuning. Noise alone would be prone to picking up other sources. For example, if there was a stronger
noise source a few KHz away from the frequency you want, it might auto-tune to peak that instead.
Which takes us full circle to my suggestion of using an oscillator as a signal source until the optimum tuning point is reached. :-DI would measure the reflection coefficient and either tune for a minimum reflection power, or tune for a zero degree reflection angle at the start of the matching network
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