The boys over at Belden are going to be really disappointed when they read this and realize that none of the cables made using copper or aluminum for shielding have done any good. Since the dawn of time, electronically speaking. Faraday will also be unhappy because his cages are universally made of copper mesh.

Aluminum is crummy at shielding or stopping a constant magnetic field, for instance from a magnet. Once the magnetic field is varying or flopping back and forth as does the field associated with any sine wave or other signal going plus and minus at some rate, for instance 60 Hz or a radio carrier, aluminum works well because the varying field generates a voltage in the aluminum, which is then shorted in the aluminum, effectively stopping the field.

Think about it: how many models of chassis, from, say, LMB and Bud, are NOT steel? MOST of them, by far. And they all shield. Faraday cages are typically made of copper mesh because the resistance of the copper is lower than that of steel, so it will shield better than steel. But only from waves, not from constant magnetic fields.


Mu metal is used to lessen constant magnetic fields because the standard shielding process -- a piece of metal looking like a "shorted turn" * -- will not attenuate constant fields. Mu metal does NOT have to be grounded unless it is being used to drain off electrostatic fields. RF fields will be stopped by a shield, but grounding is only necessary to drain electrostatic fields. There is almost always an electrostatic field on a shield, however, because there is always a difference in distance from the source from one end of the shield to the other, allowing such a field to arise...as the RF is shorted out.


Otherwise...
I have never thought of receiver desense as described above, and it is a really helpful concept.

Another thing, though: digitally modulated signals could be present at the receiver, not show up on the LED, but still muck things up. An analog TV will not show anything at all on UHF channels, but if you add an analog signal on one of those channels, say by modulating a camera signal, the analog signal will look like hell. The receiver shows nothing without an analog signal but will not let the analog signal come through properly.

* a "shorted turn" is an analogy; if a turn of a transformer is shorted, the energy that should go to the secondary will be shorted through this winding, and the transformer will be much less effective, i.e. the shorted turn will take a device that depends on no shielding of its AC signal and turn it into a pretty good shield between the primary and secondary. You can also see this effect if you short one secondary winding of a transformer; the others, although not connected to that shorted secondary, will be tremendously reduced in output. It is best to learn about this with line level audio, or with RF transformers, because they will tolerate shorting of turns due to the power level being low. High level audio or power circuits generally are destroyed when turns are shorted, so you can experiment with shorting turns with these, but be ready for destruction and/or fire.

This message was edited by Ernie Bornn-Gilman on 08/16/05 11:30 ET.