Antenna ferrite loopsticks verses air core?

Plasticized PVC ain't stellar but not horrid.

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Or do they add a lot of vispipuuro into the mix in Finland? :-)

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
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Are you reading the same page? It says PVC Tan? is 0.04 - 0.14 at 1 MHz. That is in no way acceptable for the sort of high Q circuits that are being discussed. That's comparable to wood, 0.059.

This page even lists PVC in the "Lossy" group as defined by Tan? ? 0.01.

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Rick C
Reply to
rickman

I'll need to read through this another time or two, as of yet, I don't understand where I got that math messed up, But I will figure it out. I need to complete my enclosure and get power and input/outputs installed. Should I test this with the BNC connector in place and connected or just a wire connected to my 0.4pf input cap? I need to finish up a honey do, the bedroom fan got slow and hummed loudly. I'm replacing it. Mikek

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Reply to
amdx

No, but there are many different mixes, none really good. Maybe

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-Tauno
Reply to
Tauno Voipio

I thought you might say something about the way I connected the scope, this makes a huge difference over running a 6" ground lead. Ringing your circuit is often caused by the ground lead.

Probably.

As the gate voltage varies the

Post a link to that graph. Thanks, Mikek

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Reply to
amdx

The losses in ferrite rods are nothing to sneeze at in comparison. I have used both in ham radio a lot when I was young. I built kilowatt level RF power amps, impedance matching boxes and similar gear. Ferrite rods in transmitters sometimes became so hot that you could barely touch them while inductors wound on some random piece of PVC pipe remained cool. I don't remember the PVC type other than that it was remnants they sold for pennies in the plumbing department so that was very likely well plasticized.

My comeuppance happened when I made a wideband and thus tapered-wound inductor on PVC pipe that was very long and I was concerned that it might snap off during transport. Fielddays, contests and such often required transport. So I took some allthread all the way to the top, big washer, cinched it down good and called it a day. HUGE mistake. It started to glow and melt the PVC. Luckily next to an open window as this stuff can potentially kill.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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You divided a ratio by 94pF -- that's the goof. You should've divided 94pF by the ratio.

If you have two caps and the voltage is evenly divided, the caps are equal. If you have two caps and one has 2/3rds the voltage and the other has

1/3rd, then one of the capacitances is twice as large as the other. And so on.

You applied 16V and got out 0.068V. That means one capacitance had 0.068V across it, and the other had (16V - 0.68V). So the ratio of the capacitanc es is, to be technically accurate, 0.068V / (16V - 0.068V) = 1:234.

That means the 94pF test capacitance is 234 times larger than your coupling cap.

It's the reactance that divides the voltage here. The larger the capacitance, the *lower* its reactance.

ed.

You mean you want to hang a 2 or 3 pF BNC on the .2pF node we're measuring? Don't add the BNC yet! Test this circuit like you use the other unit so we can get a decent comparison first.

After we get all that working, THEN we'll adding the BNC, and try to cancel its extra capacitance by driving its shield.

One step at a time.

Cheers, James Arthur

Reply to
dagmargoodboat

Yes, I've used similar probes on high speed digital circuits because of the ringing caused by long ground leads. But long I mean 3". Back in the day we used ground leads that plugged into the scope rather than the probe, lol.

My bad. This is for the BF256A which may or may not be the same part with a selected zero gate voltage current.

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Rick C
Reply to
rickman

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Cheers, James

Reply to
dagmargoodboat

AC Voltage at B just slightly less than AC Voltage at A. I had to look several times just to make sure there was a difference.

Preliminary data. I have just a wire input like the original. I have added my measured DC voltages to the schematic below. With 1Vpp input, T1s has 0.25Vpp and the output is 0.24Vpp. So, I think that means the bootstrap is working, I don't know how much change to expect, but that seems good. In the morning I will compare this to the original. Thanks, Mikek

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Reply to
amdx

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Okay, that means the drain bootstrap should be working.

:

The voltage gain, input-to-output, is about 1/4, or roughly 4x better than Kleijer's 1/17. But I was expecting to do a little better still and would like to figure out what's up, if you're game for poking and prodding this thing a bit more.

Your measurements mean we're dropping .75V across the 0.4pF coupling cap, and 0.25V across our input capacitance at the FET gate. Our input capacitance is thus about three times the coupling cap, or about 1.2pF.

You can compare to the original easily--temporarily connect R3 to ground instead of to R4. That disables all of the bootstrapping, which makes the new circuit operate as the original. Measure, and compare output voltage without bootstrapping to the value obtained with bootstrapping. The ratio tells us how much better we're doing than the original.

Also interesting: for the full circuit, 1V p-p input, o Output voltage w/C2 connected vs. disconnected? (Tells us how effectively the drain bootstrap is working.) o Output voltage with 100pF temporarily shorting the 0.4pF? (So we can meas ure the FET-follower voltage gain.)

Cheers, James Arthur

Reply to
dagmargoodboat

Everything checks as expect. With bootstraping T1s is 0.225 Ratio about 4 When R3 is grounded T1s is 0.60 Ratio about 17 (Just to be complete, I also lift the base of Q2)

This also is identical to the first circuit I built. ie, Ratio 17. 1 / 0.6 = 16.6666 I measured it this morning.

w/C2 coonected Vpp = 0.245 w/C2 disconnected Vpp = 0.13

I think you want me to put 100pf in parallel with my 0.4pf cap. When I do that the T1s voltage .98 Vpp When I remove parallel 100pf, I get 0.25 Vpp.

(T1s seems to be ever so slightly increasing, I'm not sure if that is the sig/gen or the scope warming up, or some other change. I'll ignore it for now.)

Thanks, Mikek

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Reply to
amdx

Maybe I don't recall the context of the ferrite, but the receiving antennas in much of this thread and described in detail at Kleijer's web site showed measurable losses with better plastics than PVC. Even cardboard was a poor support relatively speaking. Seems he would use a foam type plastic to get some rigidity with a minimum of loss. Foam PVC was not bad, but his Q measurements increased significantly with foam polypropylene. Obviously foam is better because there is less actual material. Solid PVC would show more pronounced losses.

He does some amazingly detailed work. You might find it interesting. The LC experiments were reported in a series of web pages. Page 10 doesn't have a link to page 11, but otherwise they are all chained.

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I found it very illuminating his tests with the variable capacitors. Even parts like bushings were found to make a difference.

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Rick C
Reply to
rickman

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Okay, nice work on the measurements.

Summary: o The FET-as-voltage follower has a gain of 0.98, which is quite decent for our purposes. o Input capacitance is 1.2pF. o We've reduced the input capacitance by a factor of about 4 compared to Kleijer, but it's still not as low as expected.

The shield driver + BNC isn't going to work, not until we get the gain clos er to 1, rather than 0.25.

Based on your report I think the culprit limiting our improvement is likely the drain driver, Q1 and related components. If you could 'scope (A) and (B), a.c.-coupled, with the 'scope set to subtract the two channels, (A)-(B) would tell us if our drain bootstrap is up to snuff.

I might have to cobble one of these together so I can probe it myself...

Nice work Mike. You've made something that's 4x better, even as it is.

Cheers, James Arthur

Reply to
dagmargoodboat

The gain of 0.25 is not because of the voltage follower, it's from the series cap. Are you suggesting you can get the amp input capacitance down to 10x less than the series cap?

Another approach would be to put the series cap outside the box in the probe tip as is done in o'scopes. Then the gain of the amp would be sufficient to tie the coax shield to the output of the amp.

What level do you think the amp input capacitance can reach?

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Rick C
Reply to
rickman

Looks like there isn't a page 11.

He gets good Q with PVC. Worse with cardboard but who knows what kind of stuff he used. It looks like carton material. That is often post-consumer recycled and you never know what's in it. Water content is another factor, got to bake it out and then lacquer it immediately afterwards.

The huge jump in Q in part 3 shows that he used the wrong measurement setup until then. You can't do this sort of stuff with a 100:1 scope probe, it has to be at least a high-end FET probe. However, hat off, he achieved amazing Q values in his experiments.

Look at the Q values he gets in part 7. Those are all regular plumbing PVC pipes. L16 has lower Q but that PVC tube isn't even particularly clean. It is important to keep or make them pristine when building high-Q stuff.

I remember that from my ham radio days. Dirty or oxidized rotor contacts ... phssssst ... PHUT ... *BANG* and worst case that could take the tubes into the abyss along with it. I also spent part of a Wenol polishing paste tube keeping the final Pi-filter inductor shiny so it wouldn't heat up too badly and unsolder a connection to it (which could also result in a loud bang).

BTW, while such high Q isn't all that useful in practice if you ever needed that there are simpler means to achieve it. An old method is the "Q-multiplier". In essence the resonance circuit sits inside an amplifier stage that is deliberately pulled very close to oscillation. That shrinks the bandwidth big time.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

I adjusted both probes to be flat, then I connected both probes to the same point (T1d) and adjusted *Var. for exact overlay of the two waveforms. Then I move one probe to (T1s), I can not see any difference in the signals.They still overlay, maybe a little thicker trace. I have the brightness turned way down to make a thin trace, focus adjusted. Looking for any small difference.

I'm a little confused by my scope. There is not a clear subtract (A-B).

The manual says ADD then INVERT to get a difference**. See page 3-5 in the manual.

Following the method in the manual, I get about 4mV of difference signal. If this is not satisfactory, I have another Tektronix scope (475), I can put on the bench. I think that has subtract.

  • this was a very minor tweak
** this seems a poor method if your wave form is slightly asymmetrical. But I don't see that. Mikek
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Reply to
amdx

ADD with INVERT should be exactly the same as subtract. I'm not sure what you mean about the asymmetry.

4 mV sounds good, but what it the context? What is the amplitude of the signal at this point? Is this the 0.25 Vpp measurement?
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Rick C
Reply to
rickman

After further review, that is nonsense.

Yes, still the 0.25Vpp measurement. Drain and Source AC voltage are very close. I'm using a magnifier to look at a signal that is about one small division. I just made a second measurement, it is about 2.5mvpp. Looking at the circuit, I think I might get by use a 1X scope probes. That would get me a factor of ten increase in size of my difference waveform. thoughts?

Mikek

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Reply to
amdx

Yeah, it will get you 10x, but add a few pF to the circuit. Certainly it's worth giving a try, but I think knowing 2.5 mV is good enough. That's 100:1 which is about what I would expect. Maybe drive a larger signal to the input? I guess you don't want too large of a signal.

I don't really have a feel for how low this voltage can go. It would be driven by the gain of the FET, but with the added drive of the bipolar devices the voltage can go even lower. Right now I would say 100:1 is pretty good as a goal. It means the effective capacitance of the input is 100 times lower. But your capacitance measurements don't agree with that.

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Rick C
Reply to
rickman

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