Telling people to avoid the PE terminal, when trying to earth stuff to achieve safety, is... dangerous nonsense?
As to why not to use the PE terminal in a wall socket, to earth namely a rack cabinet, I can offer the following opinion:
The wall socket is intended for attachment of a single power-consuming device or appliance, generally something you can move about the place.
In contrast, a rack cabinet can be considered a piece of permanent installation, a part of the building, or some such. Something you don't generally move about the place, and are unlikely to detach from the wall/mains socket. Such metal objects, especially if relevant to electric installations, should probably have a permanent connection to the PE distribution system in the building - preferably a thick one, thicker than the cross-section of your mains live and neutral.
I mean to argue that for connecting a rack cabinet, the PE terminal in a 16A wall socket might be considered too feeble / finicky / at risk of developing faults due to the plug being pulled and re-inserted repeatedly.
So if you have e.g. 3x2.5 mm2 of a typical single-phase wall socket circuit to power the active elements inside your rack, I wouldn't be ashamed to install an extra yellow-green wire of about 4 or 6 mm2, from the "equipotential busbar" located in a relevant nearby mains wiring cabinet, to the defined PE terminal of your equipment rack. Ahem - 4mm2 on the condition that the existing mains cabinet has comparable cross-section available in its incoming yellow-green PE :-)
Note how the Protective Earth is distributed in a building. Here is a picture of my own:
At the base of the building, there is a point where several earths meet: the inner yellow-green PE, the mains neutral incoming into the building, the mains neutral spreading through the inner wiring of the house, a local "earth termination system", and an outside lightning down-conductor system.
So much for anyone touting a steel bar rammed into the ground, as an alternative to the PE in your wall socket. I mean: you should have both in the building, they should be safely interconnected, and a single bar in the earth likely won't do it :-)
The picture above comes from a longer howto that I've scribbled on the topic of antenna surge protection. Which is off topic here. In that howto, the picture is an animated GIF. The howto doesn't go into detail on the wiring of mains live and neutral, hence the "blue" neutral is missing (at least it used to be blue by definition in parts of Europe, not sure about elsewhere in the world).
From a safety standpoint, in electric wiring installations inside the building, the yellow-green earth should always exist first and foremost! :-) and be preferably rock solid. That is, if we stick to the TN-S system, which nowadays appears to be the canon for residential and commercial buildings. More precisely, in some examples around me, the mains backbone of multi-story buildings can be TN-C, with TN-S getting forked per floor or per apartment - not sure if this is the current recommended way, or a legacy of "TN-C everywhere".
The idea behind the comprehensive yellow-green TN-S grounding is, that any fault that results in live getting in touch with "accessible metal surfaces" is guaranteed to be localized, not spread further, and guaranteed to throw a circuit breaker. In modern installations, the level of protection should be improved even further, by correctly installed (topologically placed) Residual Current Circuit Breakers that register even relatively faint, non-lethal leaks from the live mains into the PE. The RCCB actually responds to a tiny disbalance between mains live and return current - 30 milliAmpers seems to be the nominal value for household installations.
If I should mention some applicable norms, I'd say:
- IEC 60364 for mains wiring
- IEC/EN 62305 for lightning protection
I am not a certified EE tech, so the opinion presented here may be naive / incorrect / incomplete. A certified EE inspection engineer should be able to tell you exactly, including a reference to the relevant paragraphs in the applicable norms. Contact an inspection engineer local to your country. Either he may be able to help you on the phone with this particular question, or you may want to invite him for a visit, if you're not sure about the state of the yellow-green wires or electric wiring in your building.
A correctly installed PE system in your building, including the PE terminal in a three-prong single-phase wall socket, should generally be better suited for any kind of protective earthing, than a DIY section of steel bar rammed into the ground. And, definitely better than "nothing at all".
So... if you feel that the wall socket PE "should be good enough" for your modest equipment rack, (10RU? in a household?) but you feel uneasy about the actual quality of your PE, by all means consult a local authorized inspection engineer. They tend to be surprised and flattered by this sort of active interest :-) as more conventionally they're regarded as the one who only ever comes to point out other people's mistakes = "blame innocent house-owners and cause trouble"...
As for "crystal radio": I assume that we're speaking long waves and medium waves (upper dozens of kHz to just over 1 MHz) - where an outdoor antenna is just a long (tall) piece of wire.
First and foremost, the likely culprit here in terms of safety is the antenna, because it's likely in the outdoor environment. See my howto to get a clue how that should be properly protected. I.e. the PE in your wall socket is better than nothing, and the inner PE in your building is generally the right place to earth surge arrestors on incoming signal wiring.
As for a suitable "radio earth" = counterweight to the signal output of your antenna: the PE in your wall socket should be OK in terms of safety, if the PE in your building is kept in good shape. Then again, speaking of "radio quality", especially in this day and age of Switch-Mode Power Supplies everywhere, the PE will be infested by EMI from the numerous SMPS around you. The switching frequency of the SMPS (and some higher-order parasitic glitching) leak RF AC current into the PE. And, households and offices are a notorious source of this type of EMI.
Thus, some earth node closer to the "natural earth potential" might theoretically be cleaner.
The optimal environment for reception of LW or MW transmissions would be deep in the nature, away from sources of urban EMI, possibly on a hill-top. Drive a metal rod (or a shovel) into wet earth, shoot a piece of wire into the canopy of a tree (for an antenna) and enjoy your primitive radio - just watch out for thunderstorms :-) And, you may want to form your antenna input in such a way that for DC, the antenna is attached to ground (as opposed to DC-isolated by a capacitor). I.e. consider inductive coupling by a signal transformer. Even on a sunny day, if the conditions are right, a long antenna on a hill-top can collect quite a bit of static, which would arc across your signal coupling capacitors...
EDIT based on further debate:
The PE System in a building is never perfectly silent, even if ohmically rock solid.
The SMPS leaks add noticeable RF chitchat and a mains 50Hz residue. Yes the power supplies are based around an isolated RF transformer, with a separate primary and secondary. But, at the RF switching frequency, there is some amount of capacitive coupling of the AC part. Common mode capacitive coupling from the primary to secondary winding. SMPS designers counter this RF leak by putting a so-called Y-capacitor or Y-cap between the cold ends of the primary and secondary windings, which shorts most of the RF leak - but instead, the Y-cap introduces a weak (< 1mA) leak of the rectified 50Hz residue, from live into PE. If the secondary DC ground gets earthed to PE (safety class I), that leak goes directly to the PE. If the secondary DC ground is left floating (safety class II), some part of the leak may get siphoned into PE by high-resistance paths (human body standing on a floor). In some cases the bare "DC ground" of such devices can faintly tickle your fingers... The leak from several SMPS "consumers" combined can trigger a 30mA RCCB. If you have a choice, i.e. you're shopping for an SMPS adaptor, look for models labeled "for medical use" - they feature a better isolation in the RF trafo and a lower leak (say 1/4th to 1/10th that of a "regular consumer grade" SMPS down-converter).
in buildings where the backbone is TN-C (probably for historical reasons) combined with TN-S peripheral sections, say from the local wiring cabinet with circuit breakers and RCCB's downstream, you get the voltage drop caused by mains return through the shared ground of the TN-C part of the system, appear on the yellow-green PE in your wall sockets (at the TN-S end). This "mains return residue" corresponds to the "Kirchhoffian node" where TN-C meets TN-S = where the yellow-green PE forks from the shared PEN (leaving a dedicated N for the working return).