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Unlike HF, VHF and UHF can be challenging for long distance contacts
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Last time, we talked about propagation of the high frequency radio wave and the layers in the ionosphere that help reflect those waves.
I mentioned in passing that VHF and UHF can be more challenging over long distances. So let's take a quick look at why.
Ionospheric reflection is very difficult at frequencies of 50 mhz and above. The D, E and F layers allow these higher frequencies to absorb rather than reflect.
But it is this difficulty which makes working the amateur bands above 50 mhz challenging and fun.
On most any day, an amateur can talk to European stations on 20 meters, around 14.2 mhz. And while that is cool, it is not unusual.
Talking to Europe on six meters, two meters, and 70 centimeters takes just the right conditions to work.
Sporadic E happens when parts of the E layer become ionized by the sun, normally about 65 miles above the earth. It normally happens in the summer.
These Sporadic E patches can come and go, but can often reflect a signal on the higher frequency bands that you could never hear during normal propagation.
The other big high frequency propagator is Tropospheric Ducting. The best explanation of tropo is a temperature inversion, or smog! The scientific explanation is "an inversion over a stable high pressure area ahead of a cold front." Whatever!
Amateurs who work tropo on VHF and UHF have amazing results. Legendary Amateur Gordon West works two meters (144 mhz) from his California home to Hawaii every summer.
There is one additional mode of VHF and UHF propagation. It's called Meteor Scatter, where the meteor trails (shooting stars) reflect higher frequency signals. These meteor showers peak around 6 a.m. local time and reach a low point at 6 p.m. Amateurs who use meteor scatter usually have unique equipment for transmitting and receiving.
If you'd like to learn more about VHF and UHF propagation, here is a great website. It talks more about TV and FM, but the basic idea is the same.
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