Antonio wrote:
Today while driving, I was listening to WDOD 96.5 near Chattanooga,
somehow they were off the air..and instead there was a Mexican station
taking over the signal, very strong signal, fading and coming back,
then there was another Mexican station in the same channel.By the city
and frequency I found it to be XHRN-FM in Veracruz.It was the same on
other frequencies, like 94.5FM, there were 2 or 3 Spanish stations(One
of those was from Mexico City, Insituto Mexicano de la Radio
XHIMER,playing classical music). Then I moved to 101.7 and there were
another 2 spanish stations there.
Why does that happens and how often? I think it has to do with the
weather being hot and humid.
Any comments about that?

Sporadic-E. (see R.J. Carpenter's links)

On occasion, one of the atmospheric layers responsible for long-distance
reception on shortwave becomes briefly capable of supporting MUCH higher
frequencies, like FM broadcasts. There are many theories as to why this
happens but none are proven.

Weather is not a cause. Sporadic-E is more prevalent from late May
through mid-July (and again around Christmas). It's also more prevalent
in mid-morning and early evening.

The tropospheric ducting mentioned by other posters is almost certainly
NOT responsible for this DX. Tropospheric propagation is almost never
able to cross mountains, and the 1400-mile distance to Mexico City would
be VERY, VERY unlikely. Sporadic-E, on the other hand, doesn't have
much grief with mountains. 1400 miles is on the long side, but
certainly possible.

(I had several Mexico City stations in just this morning at my place in
Cheatham Co. about 30mi. northwest of Nashville)

Keep an eye on channels 5 and 6 on a TV with an antenna. (also 2 and 4
if you're far enough out that WRCB doesn't spill over too much) The
skip will hit TV first.

Take a look at http://www.anarc.org/wtfda for more information.


--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com