I understand many omni-directional antennas utilize a ground (grounding?) plane to deflect the energy wasted (i.e., being projected downward) and utilize the energy more efficiently projecting upward and outward. However, I do not find ground planes on radio or TV transmission towers. In fact, the ground planes seem to be very present on home antennas, perhaps a ham operator, or such. Why are there usually no ground planes found on the towers the pros use? (NOTE: I am not an EE, just one with enough knowledge to do some serious damage! :eek: )
Most TV transmitters will be dipole antennas, which will have a current in a standing wave along the dipole, with two elements.
Ground plane antenna is essentially one half of a dipole mounted vertically, using the ground plane as a load on the other side. The ground or conducting surface needs to be at least λ/4 in radius or a pattern of λ/4 conductors as radials. but this type of transmitting antenna is typically a compromise for a unidirectional antenna.
A ground plane antenna typically requires twice the current to produce the same field strength at a distant location.
" Compared to the dipole’s vertical radiation pattern, the ground plane has a lower angle of radiation, giving it a longer-range propagation advantage at frequencies below about 50 MHz." , perhaps why the grounded monopole is used for long wave… not for broadcast radio and tv…
AM radio stations often use mast antennas, where the tower mast itself is the actual antenna. They tend to use the earth itself as the ground plane, so they are constructed in areas where the ground is flat and has good conductivity.
AM radio stations also often use groups of antennas in a phased array pattern, so that the signal can be directional. WWVA in West Virginia for example used to beam their signal northward towards Canada at night, for example (they might still do it, I dunno). Because AM signals are vertically polarized, and because of the frequencies used, the radio signal can bounce through the atmosphere and travel a very long distance.
FM radio stations, at least in my limited experience, tend to use dipole antennas. These are often mounted as high as possible, like on building tops, so that the station can have the farthest range possible given their broadcast power. FM signals are horizontally polarized and therefore only travel as far as line of sight.
I’m not sure what types of antennas are used for broadcast television.
They used to be. But it became a problem in the 1960s when FM radios were installed in cars. A vertical antenna was installed on the cars, which was only sensitive to a vertically-polarized signal. So the FCC changed its rules to allow FM broadcasters the option of using RHC circular polarization. As a result I believe most FM broadcast antennas today use circular polarization.
Horizontal polarization, I believe.