Souped-up GDO!

[Al (astral highway)]

Hey folks, I'm a long way off being allowed to do anything that weights my operated leg but I am keeping my mind busy.

Before my op I built a stable VFO tuneable from around 1.3Mhz to 2.0 MHz. The application is slightly unusual - I have a physically very large, home-wound inductor (weighs over a kilogram!) that would swamp a normal GDO so I am building a higher -powered version to test the resonant frequency to a high degree of accuracy. The normal calcs proved not accurate enough as they do not compute permettivity of the (Lexan) coil former or varnish and pitting resin insulation over the windings.

I also started to build this PA and the bias circuit, power supply, power MOSFET and associated circuitry apart from the filter are all complete.

This PA will figure but there remains the issue of coupling the output to the test coil, loosely only. What mods can I make to include a giant couple of coupling loops under high RF power while correctly loading this PA stage ? I will obviously not be needing an aerial so the coupling loops alone have to present a 50R load.

Please don't worry about RF menace as the whole test will be over in 30 seconds maximum. Also there is no AM reception in my flat as previously mentioned . I will consider any replies in this direction as OT; hope you understand.

Thank you folks.

[Al (astral highway)]

I was thinking that one option would be to have a physically large coupling loop or two in series with a dummy load - however I'd like to pass more current through the inductor than through the bulb dummy load...

[Bazz4CQJ]

Originally Posted by astral highway

I have a physically very large, home-wound inductor (weighs over a kilogram!) that would swamp a normal GDO

Could you expand on the meaning of "swamp"?

B

[Al (astral highway)]

Just that the inductor under test is several thousand times bigger than the one in a GDO and is wound with heavy gauge wire (1mm) and is very high Q. The tiny current coupled into it from a GDO won't allow all its characteristics to come into play. The rationale is set; only this method is the challenge for me now

I am wondering which characteristics may change with applied power level. A good capacitor (for this application) won't change value with volts (well may be a very weeny insignificant bit) the coil resistance will change with temperature though, warm it up with a hair dryer. As to swamping, I use a very low power VNA (Vector Network Analyser) to tune my aerials and they can be fed with 100's of Watts, the result is the same, a tuned aerial.

The bigger the GDO to tuned circuit ratio (size wise, bigger tuned circuit) the easier it is to get a good reading. The errors caused by the GDO interfering with the measured thing will be less.

[Al (astral highway)]

You may be right. But I'm both curious and determined to run this course, and pretty committed now in terms of hours already put in!

Do compare the result with a low powered GDO and let us know the difference. I was talking from my engineering head, it would be good to have definite results to compare with.

[Al (astral highway)]

Sure will! It may take a couple more weeks or more as things are still very slow here...
But I can report that GDO results are frustrating. I can only get a fix within 200-300 KHz on the GDO - somewhere between 1.5 and 1.8 MHz for the big resonator that I'm mostly focussed on.

I will be using far more elaborate methods to get this down to a peak. It is a very high Q coil (DC resistance only 0.6R) so this does seem to be about the limitations of a tiny little GDO coupled to a gigantic inductor.

[russell_w_b]

Originally Posted by astral highway

It is a very high Q coil (DC resistance only 0.6R) so this does seem to be about the limitations of a tiny little GDO coupled to a gigantic inductor.

The Q of your coil will depend upon the a.c. resistance (skin effect) as well as the d.c. resistance (I^2R loss). Ideally, you'd need to bridge it at the frequency of operation to determine this.

[russell_w_b]

Is this the secondary winding (and toroidal top capacitor) of a Tesla coil you're trying to find the resonant frequency of?

Have you an oscilloscope and a signal generator? I made a modest Tesla coil (a traditional spark one) and found the lowest resonant frequency, quite sharply, by feeding the signal generator between the ground-end of the coil and earth, and connecting the 'scope to a pick-up wire hanging a few feet away. As I wound the generator frequency up I watched for a peak in waveform amplitude on the 'scope.

[Al (astral highway)]

[QUOTE=russell_w_b;935992]

Hey Russell, I don't have a signal generator, but I am familiar with this method and have used it before. Thank you, though.

This inductor does appear to have a very high Q compared to others I have sound and I am committed to testing it with my 'souped up GDO' once I've finished it.

[russell_w_b]

Originally Posted by astral highway

This inductor does appear to have a very high Q compared to others I have sound and I am committed to testing it with my 'souped up GDO' once I've finished it.

Oh, I dare say it will have. It's just that it the absolute figure will not be as you expect from a d.c. resistance point-of-view.

You don't have any extraneous ironwork in the vicinity when using your GDO, do you?

[Al (astral highway)]

Originally Posted by russell_w_b

It's just that it the absolute figure will not be as you expect from a d.c. resistance point-of-view.

Sure, I know this, but I'm comparing it to a previous edition that was wound on much finer wire and had a DC resistance 10 times higher. So it's relatively speaking, at the very least, ten times higher Q than the previous one.

There is no ironwork nearby...

One complication is that the inductor has a native 'top-load', which is a small spherical antinode. This lowers the resonant frequency.

I have calculated the self-capacitance of this antinode, but ideally I need the resonant frequency of the inductor including the antinode empirically rather than by calculation.

My 'souped up GDO' will work by loosely coupling a home-made, high-powered RF source to the coil and by testing resonance of the unloaded coil (method is not intuitive and I will do a separate post about this) as well as of the coil loaded with a high voltage gas discharge tube. This will emphatically prove the peak resonance.

I am still frustrated by not being weight-bearing on crutches so can't carry things around yet and hence can't do any making or construction or soldering whatever. Patience, patience!