Measuring Unloaded Q of Loop Antenna?

You say unloaded, so you need to get a setup that is stable and away from anything that will cause losses. Get your signal loop as far away from the AM loop as possible, but close enough to couple in a signal large enough to see on the scope. I often put a resistor in series with the scope probe it help isolate the

15pf of scope capacitance. 100k should work. It has been a while since I've done this work, but I think I used a couple of clip leads as a dipole on the end of my signal generator cable. See which gives you the most signal, loop or dipole. I like to peak the signal on the scope at 7 units, by adjusting the output level of the sig gen. Then move the frequency until you have 5 units (the 3db point) then go back the other way to 5 units (the other 3 db point). The math, .707 x 7units = 4.949units This is 5 units to my eye. Hope I explained that ok, I have a writeup, but it's on a different computer I don't have access to. Mike

Yep, each measurement can turn out different. The pleasing upside is acknowledging that the most accurate test setup was the one that returned the highest Q value!.

describes in great detail what Mike has already noted. Also look at the series of "LC experiments".

great detail what

Yes, I'm getting a Q of around 100 at 1 MHz, but I think it's higher. I used a 1 meg resistor in series with the scope probe and the half power points are around 9KHz apart. Using a 100K resistor gives a much lower Q, so it looks like a 100k load effects the circuit significantly.

Thanks for the link.

-Bill

Just a check, you are using half power point and not half voltage point, right? .707 vs. .5 Mike

Yes, half power points, 0.707 voltage points. 0.707 squared is 0.5.

-Bill

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Loathe to do any paying work today, I spent a diverting hour measuring some aerial coils kicking about. A data-point ...

Would seem Q's of a couple of hundred are not unreasonable using Litz wire. (Q's were measured using an 'Advance C.M.1' Q meter)

Yes, I found somebody with a Q meter and the uloaded Q measures 300 at

600KHz, 285 at 1 MHz, and 225 at 1700KHz. Quite a bit higher than scope measurements. The Q meter is a Japaneese copy (MQ160B) of the Booton 260A. The loop is a 15 inch square wooden cross frame with 14 turns of #18 solid copper wire. I added spreaders in the mid sections to separate the odd and even windings to further reduce the capacitance. Resistance is only about 0.5 ohms. Works pretty well and I can hear a 2KW station about 30 miles away in the daytime. Going to do some DX listening tonight to see what I can hear from other out of town places.

-Bill