Tesoro Brown Concentric

I have several Tesoro coils. I have 3 white "donut" concentric coils. A 5.75", 8" and 10.5". I also have 2 of the newer brown "donut" concentric coils. I also have some of the much older solid white concentric coils but I'm not really concerned with those at the moment. The 3 white "donut" concentrics all give me about 45uS delay between the transmit waveform and the receive signal at the output of the receive amplifier. This works out to about 194 degrees of phase shift. My brown concentrics, however, appear to have only a 9uS delay between transmit and receive. I had thought these coils were interchangeable. Is this difference normal? Anyone else notice this? I can post waveforms if needed.

Merc

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