Q of 80m Texas Bugcatcher loading coil

[w5dxp]

I had always heard that a Texas Bugcatcher loading coil was a very high-Q coil. However, my AIM4170D says the Q is 20 at 3.633 MHz. If the RF resistance at 3.633 MHz is really 116 ohms, wouldn't the 80m mobile feedpoint impedance be higher than 20 ohms? TNX, Cecil, w5dxp.com

[w5dxp]

Sorry, I should have read the manual. A Q-measurement measures the Q of a tuned circuit, not the Q of a loading coil.

[Bob]

The AIM can be used to measure the Q of a coil by itself. There is an app note included with the zip package. It's also available here:
Q-Measurement
--73/Bob

[w5dxp]

Thanks for your input, Bob, I completely forgot about the .pdf files included in the .zip package. Just a note on my coil Q measurements.

The calculation of Q is based on the first resonant point. If we measure the Q of just a coil by itself, the calculation is based on the self-resonant point which, for a mobile loading coil, is not at the mobile operating frequency. To measure the Q at the mobile operating frequency, a series capacitor equal to the capacitance of the mobile stinger needs to be used. For instance, I used a variable capacitor to set the series resonant frequency to 3.8 MHz for my 80m Texas Bugcatcher loading coil and measured a Q of 56. Measuring the Q at the self-resonant frequency yielded a value of 23. Alan, K0BG, tells me that an HP4243A will report a Q of about 90 for that particular loading coil. The AIM4170D reports a Q of 90 at 3 MHz.

[Bob]

The Q of the coil by itself can be displayed over the whole frequency range. It is numerically equal to X/R.
This can be displayed by clicking the Setup menu at the top of the screen: Setup -> Plot Parameters -> Q

The full scale value can be selected using the Scales button at the bottom of the screen. This gives you a picture of how the Q varies over a wide frequency range.

The Q value is also displayed in the data area on the right side of the graph.

--73/Bob

[w5dxp]

Bob, with only the coil being measured, the Q graph indicates a Q of 30 at 3.73 MHz. If I add a resonating capacitor and ask for a "Q measurement", it indicates a Q of 54 at the resonant frequency of 3.73 MHz. The app note indicates that a series capacitor increases the accuracy of Qs greater than 200. Is the 54 value for Q more accurate than the 30 value? Here's a graph of my measurements: