UHF-for the broadcaster-its more expensive to use-You need more transmitter power to cover the same area.UHF broadcast is "line of sight" as UHF frequencies are.UHF TV broadcast antennas use both mechanical and electrical "Beam tilt" so listeners close to the broadcast station tower will get reception.TV broadcast antennas are made up of "bays" individual antenna elements made up into one unit.For the UHF ones-the lower bays are actually tilted out from the antenna mast-physical beam tilt.VHF doesn't have this problem.When I lived in Wash DC area-the lowest power UHF station was running 55Kw visual transmitter power.(one 55Kw klystron stage)this was the standard output power for that type of Klystron tube.Becuase of circuit losses-you have to run up to three times more input power to the stage-since it has to be linear-broadband-the tube is conducting all of the time.The largest UHF station in that area had a 220Kw 5 Klystron monster-the first Harris 220U they built.This thing pulled over a MEGAWATT at dark picture.Their cheif engineer(CH#20)described how all of the streetlights on River Road Bethesda would go out when they went dark picture.PEPCO put in a separate substation for them--69Kv to 13.8Kv and at Ch20's building they have a VERY large 13.8Kv to 480V transmformer.For VHF-esp low band-less transmitter power is needed-these run 25-35Kw and use conventional triode or tetrode transmitting tubes.These transmittrers are more efficient.Smaller size and less power.Hi band VHF runs 40-60Kw.the losses start going up here.But much less than UHF.And to top it off-VHF is easier for you to receive and tune.But today-----Low band VHF frequencies are to be auctiuoned off by the FCC to other services.Modern digital TV uses Hi Band VHF and UHF.Digital transmitters can be lower power than analog ones.But they still have to use the linear stages to pass the RF modulated by the digital pulse signals.22Kw serves us OK here in place of the 220Kw monster-to see these HUGE transmitters is quite something!Knew some folks that used to run Wash DC Ch20's transmitter.and most TV transmitters have a built in spectrum analyser for tuning their visual stages-its like an oscilloscope-but shows a bandwidth curve instead of an impression of an electrical waveform as an oscilloscope does.TV service techs-when they used to fix TV's used a spectrum analyser to tune the TV's video IF stages-just like tuning the RF stages in the TV transmitter.I have had to do these things-can be sort of fun if all goes well."Shape the curve" as its said-the ideal visual bandwidth curve is sort of like the shape of a loaf of bread!






