# Real resistance at AC vs DC frequency?

I am using an LCR meter to measure the characteristics and impedances of various coil antennas. This LCR meter can measure both real and imaginary components of impedance at a certain frequency (R and X) as well as DCR, which I assume is mostly just the pure ohmic resistance of the copper conductor.

My question: what is the difference between the real component of impedance measured (R) and the resistance measured at DC current (DCR)?

Also why does R increase drastically when the antenna is in close proximity to metal? I understand why the inductance of the coil decreases due to eddy current losses, however I am lost on the physical reason behind the increase of the real component, R.

• When you have metal close to an antenna, it's said to be in the near field. When metal is in the near field it has a strong coupling to the radiator and will detune it from its intended impedance. Your intuition is correct, any excited inductor will change inductance when placed near metal, especially ferrous metal. Jan 30 at 21:27

Also why does R increase drastically when the antenna is in close proximity to metal?

Eddy currents circulating in the nearby conductive metal will look like the equivalent impedance of an inductor in series with a resistor. This equivalent impedance is, in-effect, connected to the secondary of a transformer. The metal acts like a shorted secondary coil.

The primary is your "antenna" but, practically, it's just a coil in that produces flux and, the nearby metal acts like an impedance close to a short circuit (small L and small R).

Now you have a step-down transformer that, when analysed from the electrical connections of the primary (your antenna), has the secondary load (inductor in series with a resistor) impedance transformed to the primary and clearly, a small value of resistance on the secondary is "stepped-up" to a moderate value resistance in the primary.

This is how we analyse transformers by the way.

So the secondary resistance transformed to a higher value in the primary adds to the already-present loss resistance in the primary winding and, you get a significant increase in resistance.

It also depends how your LCR bridge is set-up. For instance, if it is set-up to deliver an equivalent parallel LR reading, then it will be a lot different to the value if set-up to deliver a series LR reading.

My question: what is the difference between the real component of impedance measured (R) and the resistance measured at DC current (DCR)?

Without the influence of the nearby metal, the resistance of the antenna will get higher due to skin and proximity effect. These are well-know effects and, a lot is written on the matter.

Skin-effect basically boils down to current preferring to take the lower impedance route through the wire at higher frequencies. That lower impedance route is near the wire surface and, because of this, current density is unequal and this increases resistive losses.