OK. Curiosity got the better of me, so I decided to try Dave's (W0LEV) suggestion and make a measurement with my VNA. I happen to have an LZ1AQ loop right under my 80-10 EFHW inverted L. It happens to be about 25 feet from the vertical part, and 25 feet under the horizontal part. The VNA is set up to sweep from 2 to 30 MHz with the output port connected to the EFHW antenna. The input port is connected to the output of my bias-tee. The loop is on a rotator.?
Depending on the rotation of the loop, the various peaks and valleys move around quite considerably. The largest peak occurs when the loop is in alignment with the radiating wire, and is at 3.5 MHz and -18dB. Initially I was surprised by such a large return signal. But then I realized that the LZ1AQ amplifier has a gain of about 29dB. So the actual power into the loop is quite small even at this close distance. I will leave the input voltage calculation up to the mathematicians in the group, but my guess is it's below a volt. Remember that neither the 1m loop's output impedance or the amplifier's input impedance is 50 ohms. According to measurement Steve Ratzlaff (AA7U) recently made on this same loop amplifier design, the input impedance is around 15 ohms at 3.5 MHz. Have fun with the math.
As further background, my LZ1AQ does not have any input protection diodes or an input disconnect relay. I have operated 100W CW on most bands numerous times while the loop has been active without damage. Obviously if I was running a kW and had the loop closer I might have had issues.
73 Tom
