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On Sat, Sep 30, 2023 at 07:29 AM, Evan Hand wrote:

Do you know the apparent power out when driving the antenna?? I have found that the QDX is very susceptible to antenna loading.? My inline QRP power meter shows much higher power when the SWR is farther away from 1:1.? As high as 6 watts measured on the antenna when only 4.5 watts measured into a 50 ohm dummy load.? The supplied current goes up with the power change.? I can overcome the issue with a tuner that I first tune with my IC-7300 and then switch to the QDX.

That's not a problem. But I'll explain anyway so that you know won't be bothered, but in reality it is a non-issue.

The actual output impedance of many QRP rigs are not tightly controlled to 50ohm but can be a bit away. Also, a final amplifier will generate more RF power if the load impedance given to it is lower than designed, not because it works better but that's just how the amps work. So, it is best to adjust the antenna tuner to minimize the reflected power rather than maximize the forward power, although that makes little difference in most practical situations. If the power is too high, you could drop the supply voltage, but that can change the transmitter's output impedance so you might touch up the tuner dials to minimize the reflected power once again.

I don't need to start a new round of inline SWR sensor vs resistive bridge debate, but the resistive bridge is not measuring the SWR but it allows you to adjust the apparent antenna impedance close to 50ohm. If the transmitter's output impedance is close enough to 50ohm, that should be fine. If not, you might find some discrepancy. Whether it matters in practice or not, you decide.

One way to get some useful insight is to adjust your antenna tuner for min reflected power (and that is probably close to max forward power) and then replace the transmitter with a VNA and measure the complex impedance. Let me know if you manage to do that.