Where Does The High Voltage Come From In Mag Loops?

For a long time I have wondered WHY do people always scream a warning at you if you plan on using a small magnetic loop.  “Be careful, there is a lot of voltage across the capacitor!”  yeah, yeah. . . OK, whatever. . . I’ll be careful.

But Why?!?

I asked a number of friends.  I heard a lot of explanations.  The one that stood out to me was that a mag loop acts like a auto transformer.  uhh. . . I’m sure someone reading this is going, “yeah!”  You could have told me that it has to do with a wind storm on Mars and particle entanglement.  This seemed just as plausible to me.

Recently I was helping my wife study for her Extra exam.  That is right OMs, My YL is studying for her Extra exam.  Although I am becoming convinced that she is studying for it just so I can learn all the stuff I didn’t understand when I took it!  It’s amazing how 3 years of playing with stuff can cause you to learn so much.

In section 4 of the ARRL extra class manual they begin really digging into Inductance and Capacitance.  You calculate frequency, R, X_L and X_C, amount of and sign of X, power factor, tuned circuits and Q.

Then it came together for me!

****WARNING:  I am about to attempt to convey to you a very technical concept in cave man terms.  This is not an attempt to be scientific or scientific in my language.  Read beyond at your own risk.  ****  /WARNING

We know from our study materials that capacitors store energy in a static field in the dielectric between the plates.  We know that inductors store energy in a magnetic field.  We also learn that at resonance, circuits have a peak in total energy.  Dave Casler has a great video on this concept.

Now these fields that are storing energy influence the frequency at which an oscillator will oscillate.  You learn in the Extra class studies about “back EMF” in inductors and about a capacitors ability to block DC,  and about the phase differential (ELI the ICE man).  But the amount of energy used for RF purposes in each cycle is well timed, very small, and as the circuit hits resonance, the fields become quite large (full of potential energy) and will continue the oscillation for some time after the signal generation is removed.

I’m guessing you skipped the video. . . from earlier in the post.  Let me attempt to give the illustration.

It works similar to pushing a child in a swing.  Once the swinging motion is started it requires a little less effort to keep the swinging motion if you time it right.  If your push is in resonance with the swing.  As radio waves enter the L/C circuit it is the same effect.  When you hit resonance, virtually no energy is required at all to keep the waves going, but there is tremendous potential energy stored up in the capacitor ready to “push the swing” if needed.

Now short the capacitor plates to ground while in this peak resonance phase and the field in the capacitor will collapse and deposit all its stored energy into the path to ground all at once.  This friends is where all the potential energy (Voltage) comes from. . .  It is sitting in the dielectric between the capacitors.  Shorting this out is the equivalent of walking in front of your child’s path while swinging.  Its a wrecking ball!  Now as the AC in the circuit stops flowing through the inductor (because you shorted it to ground through yourself in our example), the inductor field will collapse.  The inductor will want to resist the change in AC and will use it’s field to put current through the circuit.  (Read:  Potentially into you, who have the shorted the circuit to ground. . . this depends on how the circuit is constructed and exactly how you shorted things out.)

Now. . . in a magnetic loop or for that matter, any kind of antenna matching network, you are using a radiator that is technically not long enough for resonance.  So you build a circuit that will compliment the radiator and create a resonant point for what conductor you do have.  You do this with inductance and capacitance.  The smaller your radiator, the greater the amount of inductance and capacitance needed and the greater the magnetic/static field you will be creating.  Magnetic loops are small, ergo. . . high voltage.

By the way. . . this all happens in a regular 1/2 wave dipole as well.  But the energy is spread out over the entire length of the antenna and doesn’t store the energy in any single point that you can short to ground.

I hope my cave man level description helped you understand the concept.

KF5RY