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[aniccame]

Good day,

The setup:
I have built a 1/2 wave dipole antenna that is a few hundred turns on an oatmeal box. It is centerfed by a dual mosfet driver, one half of the driver being phase inverted. So the mosfet driver has two positive outputs that are 180 degrees out of phase. These outputs are attached to the inputs of the antenna.

I have placed a 100 kOhm potentiometer across the output of the antenna.

The effect in question
I have noticed that when the resistance across the output of the antenna is lower, the input current is lower and the voltage between the two output terminals is also lower. When the resistance is removed, the input current is highest.

So by allowing more current to flow through the potentiometer, it is increasing the input impedence of the antenna.

What I don't understand is: Why?

Possible Explanation
The theory I have is that the reason impedence is so much lower at resonance is that the Potential between the input and the output of the antenna is highest at resonance. By adding the potentiometer, I'm reducing the potential (amount of positive or negative charge) on each end of the antenna. This reduced potential means less current will flow from the input of the antenna to the output of the antenna, which means the impedence will be higher.

Any thoughts?

[bigSteve]

I'm not sure if you do a few hundred turns of wire on something you can really call it a dipole antenna. It seems like more of an inductor. What frequency and distance are you trying to communicate through?

Also, what do you mean by the input and the output of the antenna? I assume the input is where you feed it, but this would also be where the power is output as well. It sounds like you may actually have a transformer here - this may not be directly an antenna question.

Look up voltage and current ratios in an idea transformer, this would probably make what you are trying to demonstrate make more sense.

[aniccame]

Thanks for your reply and your time!

I'm not actually communicating with the antenna/coil, just experimenting to learn more about them.

You're right on. I did a simpler test, with the same results.

It is just a simple series inductor / capacitor circuit driven by a 12V square wave. The 300pF capacitor is between the 850 uH inductor and ground. The AC voltage on the capacitor peaks at 350V at resonance.

Then, with a 100k pot across the capacitor, the input current goes down when the value of the pot is decreased. (Of course when the resistor value gets too low the current increases rather rapidly). At the same time, the peak output voltage across the capacitor goes lower.

This seemed counter-intuitive since usually lower resistance means higher current.

I'm pretty sure this same thing would happen in an antenna, and the physics of it makes sense despite the initial counter-intuitiveness.

For me, it's makes sense that equalizing out opposite charges on the tips of a dipole antenna would increase the input impedence to an antenna by decreasing the potential (and thus decreasing the amount of current) between the antenna center-feed and the open end of the antenna.

I will heed your advice and do some research into transformers.