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UHF propagation question

I was just wondering about this and perhaps someone knows the answer. We're about 60 miles North of Boston, which is where most of our OTA TV comes fr om. I use an old Channel Master 5 foot parabolic, (unfortunately NLA) along with a Winegard low noise mast mounted Preamplifier. The parabolic beats t he pants off the yagi that I was previously using. There is also a rotor. Our elevation is rather low, however regardless of the season we can usuall y get the networks, PBS and a few old movie channels, however every once in awhile we might get a day where the signal will break up, and then after that it's good again for weeks. We're not big TV watchers so it's nothing w e can't live with.

There is a strange phenomenon though that seems to occur at times, generall y when its very cold and clear. During some of those periods, but not all t he time it seems like the band just opens up and I get all these additional stations, both low and high frequency ones, and some with 75 to 85% relati ve signal strength indicated as well. Sometimes these additional stations w ill be gone the next day, and other times they might hang around for a week or so and then, just as quickly as they appeared they're gone again until the next time they mysteriously reappear.

Sometimes I have thought that snow might be a factor, that is not when it's snowing but after it's on the ground and the temperature is cold. I used t o DX VHF TV with just a Vbeam when I was a kid in New York City but It's be en my experience that UHF doesn't usually skip. Could this be ground wave, but on UHF?

If anyone has any thoughts on this and how and why it seems to relate to we ather I would be very interested in hearing them. Lenny

Reply to
captainvideo462009
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Its called "ducting". Signals are reflected between layers of the ionosphere, traveling long distances. It is sort of a "waveguide in the sky".

I understand it is a kind of weather phenomenon, sometimes called a temperature inversion. I found one article about it at: .

Fred

Reply to
Fred McKenzie

It sounds as if you may be experiencing "tropospheric ducting".

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I've heard of people being able to go up a thousand feet or so in the Santa Cruz mountains, aim an antenna southwest, and work a VHF radio repeater system (2-meter, 145 MHz) located at a similar altitude on one of the islands in Hawaii, when the ducting conditions are right.

Tropospheric ducting is similar in some respects to the "skip" which affects lower-frequency transmissions... both are due to the RF signal being refracted, and thus "bent" out of its usual line-of-sight path. HF "skip" is commonly an ionospheric phenomenon, while tropospheric propagation/ducting occurs much lower in the atmosphere and does not (I believe) require that the air layers in question be ionized.

You could think of tropospheric ducting as sort of a high-altitude version of the refractive effect that causes the illusion of water on the ground, when you look out over a hot patch of desert or asphalt.

Clear, cold air probably helps in another way... by reducing the amount of moisture in the atmosphere it can decrease signal losses.

Reply to
Dave Platt

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