Regarding the UHF and VHF power issue:
(This is not meant to be a rigorous engineering explaination, but meant to help the non-techies to have an idea as to why UHF TV broadcast stuff needs so much ERP.) There are reasons other than antenna physics for higher power, like the more 'straight' and more mirror like propagation characteristics at UHF, but I assume that everyone already understands that UHF is GENERALLY more limited to line of sight than VHF.
The energy being transmitted through the air can be measured in terms equivalent to 'volts per meter.' This 'volts per meter' represents conceptually an energy density because the impedance of free space is constant. Also, the 'volts per meter' can be sensed by a given length of antenna wire, where a longer antenna can sense more voltage -- EXCEPT: when transitioning between free space and an antenna wire, the impedance match isn't always very good. If you have an antenna that senses 1volt, but the impedance is 1Mohm, then the available signal power is much lower than if the antenna impedance is somewhere between 75 and 300 ohms or so. When 'matching' is practical, some of the effects of mismatch can be mitigated, but then there are also losses and sometimes frequency response variations (not all impedance is resistive, and dealing with resistance, reactance across wide frequency ranges is fun, challenging, interesting, and best avoided!!!)
Magically, at a specific frequency, certain lengths of wire in a dipole antenna (for example) will maximize this impedance match, and provide more energy transfer at the interface. Certain wire lengths will tend to maximize the available current for a given voltage sensed at a given frequency, and this impedance is often between 50 and 300 ohms depending upon the antenna architecture. (AFAIR, the impedance of free space is about 377 ohms (120 * pi), so when you have a certain
field strength measured in volts per meter in space or air, you have a representation of power also -- IN THIS CASE.)
Some antenna lengths will maximize the transfer, and some lengths will minimize the current (therefore act poorly.) For UHF, the ideal wire lengths for a dipole antenna are very short when compared to lower frequencies. That small antenna will capture less voltage for a matched antenna, because it is just physically shorter. You can use a longer antenna, or an antenna with larger capture area (essentially a different architecture that usually has gain and directivity), but the side-effect is greater directivity, sometimes in the wrong directions.
For point to point applications, well designed antennas that have alot of directivity at UHF or higher have little disadvantage, and in fact, have significant advantages in certain cases. For 'broadcast' applications like UHF TV, the receiving antenna that has a large signal collection area will also be directional. For the transmitting antenna, the directivity is often in the VERTICAL direction (generally towards the horizon), but the practical limitations to the antenna gain are associated with the usual desire for coverage in all horizontal directions. It is likely that even with transmitting antennas with as much gain as practical, the transmitters at UHF need more power to compensate for less antenna 'efficiency' (terminology misused, but conceptually correct.)
Similar to there being practical limitations for transmitter antenna gain (e.g. you don't want to use a 2 degree dish that provides huge gain, but very constrained signal delivery region), there are also practical constraints on receiving antenna designs (e.g. you can theoretically use a VHF dipole on UHF frequencies, and it will indeed capture more signal, but the directional characteristics for that multi-wavelength dipole are suboptimal in most cases.) A better 'large capture area' antenna for UHF TV reception would likely be a Yagi, log periodic or bowtie (depending on application and installation.)
In a way, the 'antenna gain' for a UHF antenna in some cases is necessary to overcome less signal capture from the dipole (or isotopic) at that frequency. Conceptual example (but maybe not totally correct), a dipole antenna at 10MHz with 0dB gain would capture a similar strength signal as a moderate gain antenna of 16dB at 400Mhz.
Again, this is 'conceptual', but the signal capture ability of an antenna is very much influenced by its size, no matter the frequency.
A most exaggerated example might be a 'dipole' for 10GHz (it is very small), which would be nicely matched at its design frequency -- providing the typical antenna impedances that we are familiar witih. From an intuitive standpoint (and this time, intuition is correct), that 10GHz antenna, however well matched, will provide a relatively small signal output for a broadcast-type signal, when compared to a 10MHz dipole being used at the front end of a 10MHz receiver.
For UHF and higher, the ability to use 20dB gain antennas on both the transmitter and receiver, for point to point applications, allows transmitter
power to be concentrated, and the 'gain' (directivity) on the receiver side have significantly different characteristics than using 0dB gain on both transmitter and receiver side at perhaps 10MHz.
In the case of 'point to point', where BOTH antennas can be very directional and have high gain, UHF communications can be implemented with rather nice, low powers. For essentially omni-directional broadcast applications, one should expect to need higher ERPs at UHF than at VHF,
even if not in challenging propagation conditions.
John
