UHF propagation question

[captainvi...@gmail.com]

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 from. I use an old Channel Master 5 foot parabolic, (unfortunately NLA) along with a Winegard low noise mast mounted Preamplifier. The parabolic beats the 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 usually 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 we can't live with.

There is a strange phenomenon though that seems to occur at times, generally when its very cold and clear. During some of those periods, but not all the 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% relative signal strength indicated as well. Sometimes these additional stations will 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 to DX VHF TV with just a Vbeam when I was a kid in New York City but It's been 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 weather I would be very interested in hearing them. Lenny

[Fred McKenzie]

In article <0571e2b0-15aa-4c50...@googlegroups.com>,

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:

<http://en.wikipedia.org/wiki/Tropospheric_propagation#Tropospheric_ducti
ng>.

Fred

[Dave Platt]

>There is a strange phenomenon though that seems to occur at times,
>generally when its very cold and clear. During some of those periods,
>but not all the 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% relative signal strength indicated as well. Sometimes
>these additional stations will 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 to DX VHF TV with just a Vbeam when I was a kid in New York City
>but It's been my experience that UHF doesn't usually skip. Could this be
>ground wave, but on UHF?

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

http://en.wikipedia.org/wiki/Tropospheric_propagation#Tropospheric_ducting

http://www.dxinfocentre.com/tropo.html

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.