Antenna ferrite loopsticks verses air core?

Connecting a scope directly to a loop antenna may cause a loss of Q. I'm hoping not, as I am building a very high Q antenna that will depend on a high impedance not sapping it, but it depends on the antenna. The point is if your measurement saps the Q, then any impact on the Q by the ferrite will not be noticed.

The equations I used include Q in the formula for received signal strength. So if the ferrite impacts the Q it won't work as well as expected by the equations (unless you measure the Q and include that).

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Rick C
Reply to
rickman
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Exactly. Rather than make an RFID chip that works at LN2 temperatures, it's a mechanical resonator string with a small coil antenna, in a strong magnetic field, pinged and sensed by a (not cold) sensing coil. Some of the resonators are damped in production -> 52 bit code-word containing ECC codes.

Reply to
Clifford Heath

It does in my situation, high Q coil and very good cap. I have a high input impedance amp with a gain of one. I drive ch 2 and get an amplitude of 250mv pp. If I then connect a X10 scope probe to the LC then retune for the added capacitance, I only get 70mv on ch 2. The probe loads my LC heavily. Unloaded Q is 1200 plus or minus 200 :-)

If your serious, I built the Kleijer amp, 2nd version from this page.

It got a little flack from here, everyone could do better, but know one did. Your High Q LC doesn't know it's connected.

All ferrite has losses and usually the higher you go in frequency the more lossy it becomes. Here's a test I did yesterday, I put a rod in the center of a 6" dia. coil. I reduced turns to get approx. the same inductance.

Best Ferrite Poorer ferrite 236uh 232uh 216uh 33 turn 30 turn 30 turn Air core coil Ferrite Coil Ferrite Coil

590kHz 59 mv 43 mv 59 mv

1290kHz 15.5 mv 6.7 mv 7.1 mv

1430kHz 10.5 mv 4.2mv 3.2 mv

You can see how the ferrite causes additional losses in the upper frequencies. If this is not understandable, let me know I'll post the whole experiment. Also let me know if you do decide to build a Kleijer amp, I might save you some hassle. Mikek

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Reply to
amdx

** Again you have built a frame antenna, not too different from what was fitted to many radios prior to the arrival of ferrite rods.

Having a ferrite rod sited in the middle is ineffectual, the reduction in turns is not offset by any benefit.

Is it that hard to do what the OP asked for?

With a 6 inch LONG tube that a 6 inch LONG ferrite rod can neatly fit inside.

It's guaranteed the ferrite version will then win by a large margin.

..... Phil

Reply to
Phil Allison

It's not, If the OP had answer my questions about what size diameter PVC he wanted I would have done it today. Yes, when the OP said 6" length, I got diameter, probably because I have five 6" diameter coils completed on the shelf.

A 3/8" diameter fit neatly inside my 1/2" OD styrene tubing, not so neatly inside a 1/2" pvc pipe.

Yes, I agree.

Mikek

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Reply to
amdx

Neat!

I wonder if that would be any easier by, say, punching a strip of magnetostrictive material into various length fingers. Like this,

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Maybe the coupling between fingers would be too much to achieve Q like that.

Tim

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Reply to
Tim Williams

  • Err...something radically wrong. You seem to be saying that adding ferrite (in the coil) does NOT change the inductance. There would be a major change (increase) of inductance if ferite was added (inside) coil.
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Reply to
Robert Baer

It is hard to find any info on a Kleijer amp through google. I found a thread where you were discussing building one with Fred Abse. One other discussion by OErjan and kiwi_steve refered to usign a Kleijer amp with links to Dick Kleijer's pages on his work.

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I'm sure you've seen this. How does your amp vary much from this?

Interesting stuff. I might want to build one of these.

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Rick C
Reply to
rickman

What is wrong is that the ferrite rod is only 3/8 inch diameter and the coil is 6 inches. So overall the ferrite has little effect.

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Rick C
Reply to
rickman

As I like to say, the world's smallest electric guitar :) It survives autoclaving and strong gamma radiation. I wrote chunks of the decode software, amongst other things. Developed and patented by .

Interesting, I don't know. I suspect someone would have thought of that during the >10years of research that went into this. Would radiation kill the bias magnet? (of course, for this purpose it wouldn't need to be built-in).

Reply to
Clifford Heath

Please do not use PVC as RF insulation or support pieces. It is lossy to extremely lossy at RF.

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-TV
Reply to
Tauno Voipio

I posted that link above. I built mine from his schematic, and pictures. Mine is pretty much the same except I had a pcb made.

PM me if you want to buy a stuffed and tested pcb. I have two left.

Here's the inside of mine.

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I have found mine very useful. I recently put a battery pack together so I can get out in the yard away from noise sources in the house, while testing ferrite antennas.

btw here's Kleijers home page, page to the bottom to see links to some of his well detailed tests and experiments. Good Stuff! >

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Mikek

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Reply to
amdx

Aha! Caught 'ya!

Grins, James Arthur

Reply to
dagmargoodboat

Nice. You can improve the front-end with a trivial mod by returning the cold-end of the 20 megohm input resistor to T1's source, instead of ground. That bootstraps (cancels) the input resistor's capacitance.

Original: Vdd -+- | T1 |--' BF256C | 100nF >----+----->|--+--||---// | | [10M] | | [470] | | [10M] | | | === ===

Modified: Vdd -+- | T1 |--' BF256C | 100nF >----+----->|--+--||---// | | [10M] | | .-----+ | | | [10M] | [470] | | | '---' ===

It's a small improvement, but it's free. If you want to get fancier and even lower input capacitance we can bootstrap the drain, too. A bootstrapped shield for your pass-through (where you bring the input through your metal box) would help, too.

Thanks for the cool measurements Mike.

Cheers, James Arthur

Reply to
dagmargoodboat

Very nice on the bootstraping, I'll do that, but then I'll need to build a second one to verify that I don't need to retune when I connect it to the LC. :-)

btw, my bookmark is two pages past bootstraping in "The Art of Electronics" Third edition, by Winfield Hill, available on Amazon, in case anyone is unaware. Thanks Win.

Mikek

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Reply to
amdx

If you will look at my picture,

you will notice I put the 470 ohm (turquoise resistor) on the other side of the shield, (I don't know why). What affect would that have?

OK, I'd like to get fancy, but I don't understand "A bootstrapped shield" for my pass through. I would like to understand physically what I need to do.

Here's another picture showing the input, there is 5/8" hole in the case and I glued polystyrene sheet over it with the input wire coming out the center.

Ah, here's a picture of a previous input cap, I don't use anymore, but it shows the input better.

Mikek

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Reply to
amdx

The idea is to shield your signal with coax, then to drive the coax's shield to reduce the effect of the coax's capacitance.

In this case it was because I thought you were using a high-capacitance (for the situation) signal feed-through, but it looks like you've already done a very nice job providing low capacitance.

The idea of bootstrapping is to provide a unity-gain buffer, then make all the circuit nodes swing with the input voltage, so that the input signal doesn't have to charge any of the nodes' capacitances.

This illustrates the ideas--

+12V +12V -+- -+- | | | [22k] R4 \| | Q1 |---+----------. .
Reply to
dagmargoodboat

I have seen this done for electrometers where the concern is leakage current. There they call it a "guard" ring. As James says, it will also reduce input capacitance.

I don't understand how you can bootstrap the drain though. Since it is connected to the positive rail, I'm not even sure what that means.

This latter image doesn't look like what is used in this image.

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Was this just a temporary cap in place until you got the detail work done?

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Rick C
Reply to
rickman

Ok, I get it now. So the BF256C ends up being operated with a constant voltage across the D-S? Essentially all three terminals run at the same AC voltage.

Is there any reason to limit the input resistors to 20 Meg? From what I've seen on Kleijer's web site 20 Meg would be significant. But I am forgetting about the bootstrap. That also bootstraps the resistance making it *much* higher, yes?

Why use the BF256C as opposed to any other part? What characteristic makes it a good choice over other devices?

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Rick C
Reply to
rickman

Ok, so if I was using short coax to connect to my LC, Q2 would drive the shield to null it's capacitance.

I now use two 6" wires to clip to the LC. the two wire run about 2" apart. I have wondered how much capacitance the leads add. For a permanent test fixture with a tuning cap and a signal strength meter, ready to plunk any coil into for testing this would be great use of a fixed coax input.

Still talking a about coax capacitance? Right

Now is that from the 20meg moving to the T1 source, or some more from Q1 or both?

Note I added the missing 0.3pf cap to the schematic.

Can I tell this is working if my 1X gain increases? The 17 to 1 divider of the input cap and the gate capacitance and the 17 times gain of the amplifier equals 1X.

Say I get 80% T1 gate cancellation (by moving the 20Meg), now we have effectively 1pf.

1pf/0.3pf = 3.33 and the amp gain 17 / 3.33 = 5.1 So I would think my total circuit gain would increase to 5.1. Or do I not get it? Mikek
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Reply to
amdx

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