home-made Q-meter

How did you make that home-made toroid? Over charcoal on the Weber or on a propane grill?

:-)

Your sampling scope probably has a digital voltage read out, but for you or others that want to use this method, I set the amplitude to seven units at resonance then adjust the frequency until I have 5 units. 7 x .707 = 4.949 or 5 units by my eye. At resonance I either adjust the oscillator amplitude or the variable amplitude adjustment on the scope to get the seven units. Stop reread. Oh, your using -series resonance- I always used parallel resonance. Hmm, is that a 100 ohm termination resistor for your sig gen? It doesn't make sense to put 0.1 ohm across your sig gen. So you end up measuring the voltage across the capacitor?

Uuhh, 0.5 pf scope capacitance, that works.

Ok, John, Run me through your series resonant Q measurement method.. I know I'm missing something.

Thanks, MikeK

You could try the bifilar or Litz-thing (to see if it's copper or core loss).

-- Cheers, James Arthur

I set the resonant frequency voltage to 2 volts p-p, by adjusting the generator amplitude, then find the high and low frequency points that are 1.414 p-p.

No, 0.1 ohms.

It does to me!

Yup.

At resonance, the voltage across the cap is Q times the voltage across R, except that it's kinda hard to measure the small Vr, so I do the resonance bandwidth thing. 0.1 ohms is small enough that it doesn't kill the Q of the network under test. Xl is about 50 ohms, so 0.1 ohms limits the Q to about 500, and I'm measuring values around 70. Actually, Q would be a little higher if R was lower. I should hack another 0.1 in parallel.

I think the old Boonton Q-meters worked this way, with a vacuum-tube rectifier to detect the voltage across C.

If you go for parallel resonance, you still have to inject the signal generator into the LC without disturbing the Q. A 50 ohm generator and a really tiny coupling cap, a fraction of a pF, could do that.

John

Is Litz worth using at 50 MHz? More Q would be nice.

John

It sure is:

I have pounded copper sheets around cores to get around the skin effect but this was for power stuff. The difference versus the regular wire solution they had before was that theirs unsoldered itself while mine didn't, and mine didn't have an "amperage smell" :-)

Yikes! 48-gauge strands at 2.8 MHz!

John

In reality it isn't quite that bad but this is the reason that even SMPS transformers are wound with litz wire if the last milliwatt in efficiency has to be squeezed out.

Even induction ranges with their low frequencies use litz wire, usually.

IIUC Litz wire is all about reducing eddy current loss in the windings. The skin effect verbiage is all nonsense.

Cheers

Phil Hobbs

The VLF radio station at Annapolis (NSS) was on 21.4 kHz and ran at 1 MW. It fed the antenna with litz wire that was 2 inches in diameter.

tm

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I'm not sure, now that you mention it. It's not commonly done at

Here's a toy--a supposedly quite comprehensive inductance coil calculator:

Shooting for 200uH, I guessed 7 turns of 1mm copper wire (#18), 8mm in diameter, air core, 10mm long and got darn close. L=3D195uH, Q calculated ~=3D275.

Stretching the coils apart raises Q and inductance a lot. Or, you can use the 10mm spacing and 0,4mm wire (#26AWG) to get Q=3D270 and L=3D200uH.

So, it seems the coils induce eddy currents in their adjacent colleagues by proximity (capacitively-coupled?), and that produces much of the loss. Spacing turns widely avoids that.

Cool.

-- Cheers, James Arthur

Oh, FWIW, I once cobbled together a 910MHz cavity filter from some copper tubing and a Clabber Girl Baking Powder can. I needed to block some interference deep down in the 800's that was messing up a field test. 3dB bandwidth was 3 MHz--I still have the screen shot.

-- Cheers, James Arthur

I don't think it's practical that high (~ 50 MHz). Terman indicated ~1 MHz as an upper practical limit due to stranding irregularities and inter-strand capacitance. That was then, this is now, and we've made advances in construction, but nothing remotely approaching

Ed

age

u or

by my

The 0.1 ohms resistor to exploit series resonance is a nice idea! Perhaps some generators would be happier with this, since it provides a better match:

----------------------+--- ...

If you are using parallel resonance, you may also use a coupling resistor (Rc) (which is what I have been using till now)

---------------+---+----gnd

First: Measure L from known Ct (wit Ct >> probe capacitance).

Second: At resonance you have a resistive divider with known 50R, Rc and the unknown parallel equivalent resistance R=3DQ=B7XL. If you first measured the open circuit voltage (or trust the generator) you get R from the output voltage. You may even tweak Rc to achieve 1/2 of Vc to avoid cumbersome math :)

The bottom line is: you don't _have_ to insert a huge Rc if you are willing to factor out its effect.

Pere

PD. I had to post through google groups since nntp.aioe.org says 'banlist'. Any hints?

Frankly, I don't have a GEN any where that can handle a 0.1R on it's output to make it useable for anything.. Even my HP gen would complain about that.

Also, the Boonton meters we used at Semco did not sample the specimen that way..

Maybe they made different models that we did not have..

It won't give you the full benefit up there but some. In RF it's often all about gradual improvements :-)

But in general you are right, for power stuff the preferred structure is the "rolled sheet of copper".

Yes. You can thank that idiot AlwayWrong/Nunya/Etc. for that. His infantile nym-shifting to avoid killfiles must have raised a number of complaints, so the owner of aoie.org does not allow posting to SED.

There's always one inconsiderate pinhead who spoils it for everyone else...

Yes.

Go to:

aioe.news.helpdesk

and pose your question there.