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Many Antennas are grounded (eg for lightning protection), but that has no relevance for their effectiveness as an antenna. Because the size is much less than a
by Johnythree 8y ago
Many Antennas are grounded (eg for lightning protection), but that has no relevance for their effectiveness as an antenna.
Because the size is much less than a wavelength, the body can be at ground, while the active parts can be at RF potential.
Example: Your phone is grounded, but it still works.
- Uberphallus 8y agoThey are grounded with a precise length of coax (multiples of 1/4 or 1/2 wave, IIRC) to keep the VSWR ratio low for the intended frequency. In a pole you can get to do that by probing the signal at certain height, but the active part of a patch antenna (which I assume a manhole works as) is at the extremes of it (as such, probes are always off center for circular polarization, and at the extreme for linear), and those are grounded. Patch antennas are normally grounded at the center, as the voltage/current there should be close enough to 0 anyway unless there's an electrostatic discharge.