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

In article ,
wrote:

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

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/Troposp...ospheric_ducti
ng.

Fred

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/Troposp...pheric_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.