[SOLVED] calculating real portion of impedance of a small loop antenna

[krazyfencer]

Hello,

I am trying to calculate the impedance of a small-loop receiving antenna for AM broadcast. I've seen sites that show how to calculate the inductive reactance for a given design, and I'm trying to figure out the real portion of the impedance. I've heard that for this kind of antenna, the R(radiation) is much less than the R(loss), and that should drive the real portion of the impedance. Does this mean that I can just hook a multimeter to the thing and the resistance detected should be pretty much the real component of the impedance? Are there other resistive factors that aren't going to show up on the multimeter, but will end up being important when trying to match the impedance (besides the inductive reactance, which I plan on cancelling out with a capacitor across the antenna)?

Thanks!

 

[chiques]

What frequency?

Have you connected it to a VNA?

 

[FvM]

I've heard that for this kind of antenna, the R(radiation) is much less than the R(loss), and that should drive the real portion of the impedance.

This might be the case for a particular antenna design, but radiation and loss resistance have quite different nature. Both are frequency dependent and surely can't be measured with a multimeter. Radiation resistance can be calculated from loop geometry, empirical formulas can be found in antenna theory text books. Loss resistance is dominated by skin and proximity effect (the latter for multi-turn loops) and can be calculated as well.

 

[krazyfencer]

Thanks FvM, that was what I was missing. So the loss resistance is going to be the sum of the dc resistance of wire, the skin effect resistance and the proximity effect resistance. And all a multimeter would give me would be the dc resistance (which is probably dwarfed by the other two). Thanks, I'll look into the equations and see if I can figure it out.