Dr Ronald Stiffler SEC technology

[gyulasun]

Thanks for the report.  It looks like your crystal oscillator works and it works better with the choke coil than with the 1 kOhm resistor in the collector which is okay.  Now just use the choke coil and you can focus on making the air coil resonant at 13.56 MHz.
Member Lidmotor's above video shows a coil construction to approach resonance at 13.56 MHz when the coil is loaded and tuned with a ferrite rod.   Notice that with ferrite tuning you reduce the air core coil resonance frequency (it is like you would increase the number of turns),
Gyula

[Lidmotor]

Gyula--- I think your 'wiper coil' is a really good idea.  What you just mentioned about the ferrite rod basically increasing the number of turns on the coil was very helpful.  All we have to do is build a longer coil and use the wiper method to find the correct length.  The ferrite rod could still be used (if necessary) for fine tuning.  Thanks a lot for that idea.  Here is crystal radio coil like that.  It is too big but it is the same idea.  ---Lidmotor

[gyulasun]

Hi Lidmotor,

Regarding the "wiper style" coil tuning, perhaps the main thing to care for is not to short circuit adjacent turns with the wiper, this may involve the use of a little distance between the turns when winding the coil. This distance may be insured by using two wires while winding as the insurance for keeping the gap between the turns. The second wire may be left unused. OF course better methods can surely be followed, maybe making 3 or 4 short air coil section with say 15-20 turn for each and mount them on a common bobbin in a slide-able fashion.
While I was writing this you posted the above picture and that is ok too. 

Gyula

[Lidmotor]

Gyula -----I made one of those wiper coils for the crystal circuit and it worked.  It is very touchy because of how we are using it.  If you get your hand near it that changes the local capacitance which throw things off.  You have to adjust it in small increments then get away from it to see what happens.  Fine tuning can be done with a ferrite rod if you need to. 

https://www.youtube.com/watch?v=4ZjAKK38UIA

--Lidmotor

[TinselKoala]

There are all kinds of ways to tune a coil set. One of my favorite ways is the "variometer".
The inner coil is connected in series with the outer coil, through the pivots. The inner coil can be rotated all the way around on the conductive pivots. The effect is like adding or subtracting the number of turns of the inner coil, or anywhere in between.