I've been doing this radio and TV stuff for over 40 years and I've heard a lot of myths and misinformation over the years.
The most popular myth these days is that rain attenuates VHF/UHF signals. It does not. This myth seems to have come about because of the well known rain fade affect on satellite TV signals. Rain fade occurs when the size of the raindrops becomes a significant percentage of a 1/4 wavelength of the frequency. This starts to occur more frequently around 10 GHz. Raindrops cannot reach the size of a 1/4 wave on UHF.
Anyone who has signals that disappear when it rains is just unaware that they're relying on some amount of atmospheric bending of the signals to receive them in the first place.
Temperature inversions are what bends the signals. It doesn't take a very strong inversion to have some affect. Just the normal day/night cycle is enough to bring in a station that needs a little help. Solar heating tends to break up the inversions in the daytime, especially in the afternoon, which is why so many people complain about poor reception in the afternoon. Around sunset inversions reform as the atmosphere stabilizes and many report better reception.
Each situation is different though and the exact location of the transmitter, your location, the elevations at each end, and what's in between make a lot of difference. These factors can account for nearly all of the vast array of experiences that people have with reception.
I can receive stations from more than 10 different transmitter sites so there isn't much I haven't experienced here. Compared to most people's experiences with DTV reception, I have the opposite experience with my local stations. My local antenna farm is Walnut Grove which is 54 miles away over a 2 edge path. Those antennas are about 2000' above sea level. My antennas are 2650' above sea level. Not many people live at a higher altitude than the transmitters. My best reception is in the afternoon with no inversions and can be quite poor on some nights depending on the height of the inversions.
A typical evening cycle here is good reception in the late afternoon, then a decrease in the signals around and after sunset, and then a return to normal later in the evening. I believe what is happening here is that an inversion forms below me but above the transmit antennas causing the signals to be bent down thus decreasing my signal strength. Later on the inversion lowers below the transmit antennas, there is no bending, and the signals return to daytime levels.
As to the affect of inversions on VHF versus UHF, from what I've read and what I've experienced would indicate that UHF is affected more, and I understand microwaves are affected even more. This is not to say that high VHF is unaffected. I've seen 30 db increases on high VHF on some paths but 40 dB increases on UHF on the same path. I've seen some pretty dramatic increases even on channel 2.
The issue of tropospheric ducting and the affects of temperature inversions is a little more confusing since people mean different things when they talk about this. Often tropospheric ducting is used to describe the more typical extended bending of a signal that allows it to be received a couple hundred miles away. Other times it means an actual duct in the atmosphere where a signal is trapped between 2 layers in the atmosphere and can be propagated for thousands of miles like the one that occurs between the west coast and Hawaii in the summer. Stations at each end of the duct have to be at the right altitude to get signals in and out of the duct. I suspect that most of what we experience with DTV is just signal bending to extend the range and not true ducting.
Yesterday morning I experienced an unusual path to the south. I received strong signals from KEYT and KPMR from Broadcast Peak north of Santa Barbara which is 257 miles from here and my most distant DTV reception. KEYT had to override KEXT-CA analog on 27 and KPMR had to override KMAX on 21 which is a local 1 MW station.
Chuck
The most popular myth these days is that rain attenuates VHF/UHF signals. It does not. This myth seems to have come about because of the well known rain fade affect on satellite TV signals. Rain fade occurs when the size of the raindrops becomes a significant percentage of a 1/4 wavelength of the frequency. This starts to occur more frequently around 10 GHz. Raindrops cannot reach the size of a 1/4 wave on UHF.
Anyone who has signals that disappear when it rains is just unaware that they're relying on some amount of atmospheric bending of the signals to receive them in the first place.
Temperature inversions are what bends the signals. It doesn't take a very strong inversion to have some affect. Just the normal day/night cycle is enough to bring in a station that needs a little help. Solar heating tends to break up the inversions in the daytime, especially in the afternoon, which is why so many people complain about poor reception in the afternoon. Around sunset inversions reform as the atmosphere stabilizes and many report better reception.
Each situation is different though and the exact location of the transmitter, your location, the elevations at each end, and what's in between make a lot of difference. These factors can account for nearly all of the vast array of experiences that people have with reception.
I can receive stations from more than 10 different transmitter sites so there isn't much I haven't experienced here. Compared to most people's experiences with DTV reception, I have the opposite experience with my local stations. My local antenna farm is Walnut Grove which is 54 miles away over a 2 edge path. Those antennas are about 2000' above sea level. My antennas are 2650' above sea level. Not many people live at a higher altitude than the transmitters. My best reception is in the afternoon with no inversions and can be quite poor on some nights depending on the height of the inversions.
A typical evening cycle here is good reception in the late afternoon, then a decrease in the signals around and after sunset, and then a return to normal later in the evening. I believe what is happening here is that an inversion forms below me but above the transmit antennas causing the signals to be bent down thus decreasing my signal strength. Later on the inversion lowers below the transmit antennas, there is no bending, and the signals return to daytime levels.
As to the affect of inversions on VHF versus UHF, from what I've read and what I've experienced would indicate that UHF is affected more, and I understand microwaves are affected even more. This is not to say that high VHF is unaffected. I've seen 30 db increases on high VHF on some paths but 40 dB increases on UHF on the same path. I've seen some pretty dramatic increases even on channel 2.
The issue of tropospheric ducting and the affects of temperature inversions is a little more confusing since people mean different things when they talk about this. Often tropospheric ducting is used to describe the more typical extended bending of a signal that allows it to be received a couple hundred miles away. Other times it means an actual duct in the atmosphere where a signal is trapped between 2 layers in the atmosphere and can be propagated for thousands of miles like the one that occurs between the west coast and Hawaii in the summer. Stations at each end of the duct have to be at the right altitude to get signals in and out of the duct. I suspect that most of what we experience with DTV is just signal bending to extend the range and not true ducting.
Yesterday morning I experienced an unusual path to the south. I received strong signals from KEYT and KPMR from Broadcast Peak north of Santa Barbara which is 257 miles from here and my most distant DTV reception. KEYT had to override KEXT-CA analog on 27 and KPMR had to override KMAX on 21 which is a local 1 MW station.
Chuck
FM stations have been logged via ducting on the Big Island from coastal (southern) California and over to the (mainland - not just Baja) west coast of Mexico.
