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A good impedance matching network should match a load to within a maximum SWR (typically less than 2?).

But with a varying load (i.e. Antenna in varying surroundings), are there any techniques that match a widely varying load to a less widely varying match?

In other words, if the plotted range of a given antenn's loads on a Smith chart were the size, so to speak, of an SWR circle of 3.5, is there a matching technique that results in a matched impedance that is always less than 2?

UPDATE In my particular case, I'm working with a Maxim 7044 transmitter at 320MHz in a wireless sensor. So large SWR voltages aren't the problem. I want to maintain an SWR < 2 under all conditions, with an antenna that occupies most of a quadrant of the Smith chart.

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  • \$\begingroup\$ Have you considered Varactor Diodes, under control of an MCU and a directional-coupler monitoring the reflected energy? \$\endgroup\$ – analogsystemsrf Mar 28 '17 at 3:53
  • \$\begingroup\$ A resistive pad can protect the driving amplifier from damage due to high VSWR, even with varying VSWR at the load. But of course it doesn't help you out with improving radiating efficiency. \$\endgroup\$ – The Photon Mar 28 '17 at 3:59
  • \$\begingroup\$ You could probably find a match that will work. Not sure the best way to do it. An iterative solution might be most practical. \$\endgroup\$ – mkeith Mar 28 '17 at 5:32
  • \$\begingroup\$ Have a look at this dissertation \$\endgroup\$ – JLo Mar 28 '17 at 8:18
  • \$\begingroup\$ I don't have a good way to adjust a varactor because I don't have a good way to measure what the match is without measuring the SWR, requiring yet more circuitry. I can certainly interatively try different matches in circuit, measuring the results with a VNA. And I have noticed some matching networks have ΔSWR reducing effect more than others, but don't have a theoretical understanding of how it's happening. \$\endgroup\$ – Lance Beasley Mar 28 '17 at 13:36
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You can use a stripline return loss DC20 directional coupler to tap -20dB of the reflected power into an RSSI chip or a Schottky diode amp and measure the reflected energy due to antenna loading effects on standing waves.

In the old days I did this to tune a small dipole in VHF band and discovered this method is a great motion sensor for over 10 wave lengths (size of lab)

What you do with the variable phase and amplitude feedback is up to you. It would be theoretically possible to inject the conjugate feedback signal to modulate the Tx, but I doubt it is worth it.

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