Slight frequence difference noted
hfeinstein at cox.nospam.net
Thu May 13 17:12:54 CDT 2004
A few months back I noted strange frequency differences of a few hertz on
the carriers of three different stations operating on TV channel 6. I use
to monitor meteor reflections, listening to the Doppler shift of signals
reflecting off the
ionized trails. After the trails are produced by the meteor they move and
are blown around by upper atmospheric winds. This results in a complex
signal received on the ground.
The carrier I use here in the Northern Virginia area are on channel 6 and
visible from three different TV stations. Although too weak to see on a
standard TV, on a narrow band spectrum display using Spectran, all three
are visible and a few hertz apart.
When I first saw these three carriers a few hertz apart I was surprised.
professional TV stations are supposed to be rock solid. I had not contended
with the fact
that they were actually on slightly different frequencies and all within
tolerance! In fact, they do
hold their channels well.
The June 2004 QST World Above 50Mhz column contains a discussion on early
detection of sporadic E openings on 2meter. The crux of the technique is to
monitor continuos carriers on a frequency near 2m to determine if "long
skip" is present. The author uses a FFT display, like spectran, for this
purpose and looks for sideband energy from a TV station that is a good
distance away. In column 4 of this article, on page 81 of QST, the author
mention that some of the TV stations he monitors, while on the same
channel, are a few hertz off! So, I now know my equipment is working OK
and is not seeing images, strange reflections or who knows what.
The other big surprise was the prolific amount of reflected signals from
aircraft. These signals are the TV carrier signal reflected off a moving
aircraft. On Spectran it appears as a carrier a few hertz off the main TV
carrier and moving slowly up or down in frequency. Those that maintain on
a steady fixed frequency a few hz away from the TV carrier frequency are
actually traveling on a leg of a nearly hyperbolic course between my
station and the TV transmitter. Watching these Doppler trails can be very
interesting in itself. Similar "airplane flutter" signals can also be heard
and seen from conventional 2m simplex operations. I have heard low powered
2m FM signals from Baltimore MD (about 80 miles away) appear as aircraft
lift off from one of the three major area airports (I don't know which one)
or aircraft that transit the airspace over this area. The signal starts
out fluttering followed by a useable conversation period and ends in a deep
flutter. The flutter has a very well defined structure that starts very
slowly, increases in speed, peaks to a useable steady period and slows
again, repeating as the path geometry changes. Might it be possible to
treat the flutter with an FFT to create a useable but short duration
communications signal? (I assume the flutter is caused by multipath).
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