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Steve,

Yes and maybe.? You need to consider the application and expected Q to be measured and do some calculations to see if it is important to you.? If Q is less than a couple of hundred, the losses in an air-spaced variable capacitor are probably too small to worry about unless it is old (most are), has signs of corrosion, and/or the grease for the rotor bearings/contact fingering has dried out.? Most Q meters have special? arrangements to minimise contact resistance and keep it stable.

If you think you have to worry about that, you also need to consider the rest of the circuit between the inductor connections and the capacitor.? If you look at photographs in the HP Journal for the HP4342A or the manual for the Marconi TF1245A you will see that the connections are substantial silver-plated brass.? But for a home brew Q meter to do A-B comparisons rather than absolute measurements, a well made air-spaced variable capacitor in good or restored condition and thick wire or braid/brass sheet will do.

Wes Hayward, W7ZOI , carried out many careful measurements on inductors and air spaced variable capacitors and found most of the capacitors had a Q of around 350 at 1MHz with his "best" toroid.? (See "Experiments with Coils and Q measurements" amongst several articles he published).

PeterS??? ??? G8EZE

-----Original Message-----
From: Steven Greenfield AE7HD via <alienrelics=[email protected]>
To: [email protected]
Sent: Tue, 9 Aug 2022 13:02
Subject: Re: [Test Equipment Design & Construction] Making a Q-meter /

I don't think I've seen anyone mention the impact of the variable capacitor on Q. I understand that for lower Q, the losses in the inductor swamp losses in the capacitor. But at what point do capacitor losses begin to impact the measurements?

In other words, will the garden variety tin coated, brass, or aluminum plate variable do? Does it need to be silver plated for lowest losses?

Steve Greenfield AE7HD