Pot Unconnected Lug acting as antenna ?

[LaloFP]

Hi guys!

I know that we usually connect the "unused" pot lug to the center pin, to help if the wiper don't function properly (so the value goes to Pot's Max and not infinity).

   But, in addition to that, is it correct to assume that an unused lug, if not connected, can act as an antenna for RF Noise?

Looked up the web and didnt find a final response for that hypothesis.

Thanks and love!!

[GibsonGM]

I wouldn't think it would act any more like an antenna than a simple piece of wire/low value resistor, Lalo. Especially when it's inside a grounded enclosure!  Just my opinion.

[R.G.]

Strictly speaking, yes, it's an antenna. But the math of antennas and RF says that no radio gets in noticeably for wavelengths more than four times the antenna length. An unconnected pot lug is - what, about 6mm or 1/4" distance from the other lugs. Doing a quick wavelength to frequency calc, I get 49+ GHz for that being 1/4 wavelength. Anything much lower than 49GHz will have very poor coupling to the "antenna". I would not sweat it.

[Rob Strand]

  But, in addition to that, is it correct to assume that an unused lug, if not connected, can act as an antenna for RF Noise?

Looked up the web and didnt find a final response for that hypothesis.

A lot of audio people use the word antenna but in many cases in audio the way buzzy noise gets into a circuit is by capacitive coupling.   Yes it's all electromagnetics but the capacitive coupling view gives you more insight into the mechanisms.  We don't think of a signal through a capacitor going from one side of a capacitor's plates as a wave transmitted and then being received on the other plate.

Think about capacitive coupling as a small capacitor linking a noisy source in the outside world (sometimes actually in the same box) to a point in circuit.   The capacitance depends on the area of the plates and distance between the plates.   If you have an exposed plate then noise can couple to that plate.   Inside a shielded box the external noise/field is going to be small as the shield box has removed the coupling to the outside world.  However with something like a plastic PC speaker the act of putting your hand near a volume pot to adjust the level can inject noise into the audio (external noise couples onto your large body area then putting your finger nears the pot couples the noise into the circuit.; two capacitors in series with you in between)

Like most things grounding the unconnected pot lug is a precautionary act.   IMHO it's more a precaution against the pot going infinite than capacitive coupling.

Think about a 100nF film cap.   The outside layer has a partially exposed plate.  Encasing it in plastic prevents our eyes seeing it by it it still there.  If you touch the cap you an inject noise into the circuit.  For some circuits one lead is more sensitive than the other (it also depends on which way around the cap is orientated!).

You can say the same thing about PCB traces.   If you put a large copper plane on the input terminal you are asking for noise to get coupled in.   On the other hand putting ground wires and planes around input wires diverts the capacitively coupled noise away from the signal wires and down the ground wires thus reducing capacitively coupled noise.

[LaloFP]

Awesome guys! Thanks

I'm glad that my intuition was right about the nature, and I'm glad I asked to correct for the disproportionate concern 

Thanks!!