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VHF

"VHF frequencies' propagation characteristics are ideal for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter (see formula below). Unlike high frequencies (HF), the ionosphere does not usually reflect VHF radio and thus transmissions are restricted to the local area (and don't interfere with transmissions thousands of kilometers away). VHF is also less affected by atmospheric noise and interference from electrical equipment than low frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less bothered by buildings and other less substantial objects than higher frequencies."

UHF

"The transmission of radio waves from one point to another is affected by many variables such as atmospheric moisture, the stream of particles from the sun called solar wind, and time of day. All radio waves are somewhat absorbed by atmospheric moisture. This reduces, or attenuates, the strength of radio signals over long distances. However, this effect increases according to the frequency: UHF signals are generally more degraded by moisture than lower bands such as VHF. As well, the layer of the Earth's atmosphere called the ionosphere is filled with charged particles that can reflect radio waves. This can be helpful in transmitting a radio signal, since the wave bounces from the sky to the ground over and over, covering long distances. However, UHF benefits less from this effect than lower (VHF, etc.) frequencies. As the atmosphere warms and cools throughout the day, UHF transmissions may be enhanced by tropospheric ducting."

I live eight miles from Mt. Wilson with a clear line of sight. I use a VHF/UHF indoor antenna. There are times that I have to change the configuration of the VHF "rabbit ears" and/or UHF blade to receive the OTA desired channel clearly. I'm even thinking of using some aluminum foil sometimes ;-)

I'm happy that Santa brought me two PBS HD stations! Happy Holidays!
 

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Éminence grise
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The antenna pattern of the transmitters may be narrow enough vertically that you are well below the center of the beam. That can make reception difficult, even though you may be able to see the towers (a 6000' mast on your roof may help!)
 
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