We have 2 Antenna suppliers, aside from the antennas we design ourselves.
Chang Hong has not replied about the dc-short yet.
Beyondoor has the dc-short present in most of their outdoor antennas, and has advised that it is usually required for lightning protection..... not giving away much info here.
For what it's worth, my understanding of the "DC-short" design goal is as follows:
- If the antenna elements connected to the "centre pin" and "outer shell" terminals are completely insulated from one another (ie an open circuit for both AC and DC), then the possibility exists for one conductor - usually the non-grounded centre pin - to develop large potentials (voltages) relative to the other one, for example due to a build-up of static electricity. Since this (centre) terminal is usually connected to high-impedance input circuitry with limited voltage withstand, large potentials can arise and lead to parts of the modem's front-end circuitry breaking down.
Providing DC continuity (in some way) between the conductors prevents any significant DC potential from arising in the first place.
- If there are lightning strikes in the vicinity of the antenna, large surge potentials can be induced into one or both of the antenna conductors (more usually the non-grounded side). In a similar way to that described above, providing DC continuity between the antenna conductors can help limit the surge potential. My own view, however, is that the benefit gained from this is rather limited, ie it doesn't provide very robust protection.
- In the worst case, if there is a direct strike to the antenna itself, to the antenna mast or even just the same building, then
massive surge potentials will be induced into the antenna wires. In this case, my view is that DC continuity provided by means of small-gauge internal wires, passive components or small-signal semiconductors will not help at all - the available surge energy is simply too large and will fry them. Earthing the mast will help (by providing a path for most of the surge current to flow directly to earth), but the connected equipment is still likely to be damaged.
The only thing that will help prevent this kind of damage are dedicated surge-arrestors in the antenna cable(s).
It's worth noting that DC continuity (as a means of limiting dangerous potentials) could be provided either in the antenna itself or in the connected equipment, and there are different methods of doing so. Also, commercial equipment like a "home" modem/router usually has very limited voltage withstand and does not include any specific protection devices (to minimise cost). More "industrial" devices typically have much better inherent withstand and may even include their own protective devices, like gas discharge tubes and semiconductor surge-suppression devices, like the venerable "TransZorb".
So, my own view is that while a "DC short" is
desirable in an antenna, it is by no means an absolute requirement, nor a total solution, for surge protection. If the antenna you use doesn't have one, it just means that you have to worry that little bit more about providing surge protection.
I think
Azimuth had a good reference somewhere on the "myth" of a "DC short" providing surge protection - hopefully he will post it here, if it's not been linked already.
Nevertheless, we are looking at the Beyondoor range of outdoor antennas a bit closer now, as this dc-short is clearly a requirement for some routers, whatever the reason.
It's good that the issue has profile now, but for reasons outlined
elsewhere, I would still be wary of drawing the conclusion that this is an absolute requirement for external antenna detection (on the B593, at least). I do understand that you've done several tests with Otto's own antennas which seem to indicate otherwise.
To illustrate : I use a B593s-601 with Poynting LPDA-0020-V1 antennas, which do not have any form of DC continuity that I can detect. When my B593 is set to "Auto antenna detect", the LPDA-0020's are detected and used just fine, provided of course they are in good signal and set up right.
