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United Airlines oceanic radio question

Old 10-23-2012, 08:45 AM
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Default United Airlines oceanic radio question

Recently I rode a flight from SFO - KOA on a United Airlines 757, at the same time the San Francisco Giants were playing game 6 of the NLCS. About an hour into the flight, after multiple requests from other pax about the score of the game, the pilots came on and told everyone that the Giants game was available on channel 9 (usually the live ATC channel). I tuned in and sure enough we were getting the local San Francisco radio broadcast for the game. Coverage was a little fuzzy, but very listenable and continued pretty much until the game was over, at which point we were at least 1000 miles from land.

My question is how did they do this? I know nothing about oceanic or HF, it sounded to me like an NDB signal (local AM radio station) but was very suprised that the signal carried as far as it did. Thanks!
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Old 10-23-2012, 09:24 AM
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Using the ADF to tune in the AM frequency is most likely how they did it.

AM radio waves will travel further distances and are not limited by line-of-sight limitations that you are familiar with in the "modern" world.

Take an AM radio into the mountains, do you ever wonder why you can still receive a signal and enjoy Paul Harvey reruns but you can't tune in your favorite rock/country station?
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Old 10-23-2012, 10:17 AM
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Default The ionosphere

DIRECT WAVES
These are contained within the troposphere, and are
otherwise known as tropospheric waves. They are also line-ofsight,
meaning that anything in the way, like hills or
buildings, will have a detrimental effect (direct waves will
not bounce like those used with HF, below). You will get
best reception if the transmitter and receiver are in sight of
each other, but, in practice, you can expect a little more
than that, actually to just beyond the horizon, due to
effects like refraction within the troposphere (the
calculations are under the VOR section). The actual figure
is greater by a factor of around 4/3. Air-ground
transmissions are limited to 25 nm in the UK, up to 4,000
feet for tower frequencies and 10,000 feet for approach.
SKY WAVES
A space wave may leave the antenna at an upward angle, or
be bounced off the ground. Otherwise it will be a sky wave
when headed for the ionosphere, where it might be
refracted downwards again, if the angle is right, and reach
further distances (on HF).

THE IONOSPHERE
This is a region surrounding the Earth where the Sun's
UV rays dislodge electrons from the gas molecules,
making them positively ionized (and therefore charged)
and creating several conductive layers a couple of miles
thick around the Earth, starting about 60 miles up, but
lower during the day, and varying with the seasons (they
are not spheres, but change their shape constantly).
The ionisation makes the gases (nitrogen and oxygen)
conductive, like a fluorescent light. The nitrogen is ionised
at the higher levels - lower down it is the oxygen. This
happens mostly during daytime and is at its minimum just
before sunrise. In other words, air is a good insulator in
the lower parts of the atmosphere, but ionisation makes it
more conductive as you go up. Recombination is the process

where electrons and atoms get back together again,
starting during late afternoon and early evening, and
continuing overnight.
The ionosphere depends directly on the Sun’s radiation, so
the way the Earth moves around the Sun affects the
ionosphere’s characteristics (the angle of elevation of the
Sun changes frequently, which accounts for variations in
its height and thickness). Some of these changes are
predictable, and some are not, but all of them affect the
propagation of radio waves.
The regular variations can be 27-day, daily and 11-yearly
(from sunspots), but the daily ones have most effect on
aviation operations. As the atmosphere is bombarded by
waves with different frequencies, they produce 4 cloudlike
layers of electrically charged gas atoms, between 50-
300 km above the Earth, called the D, E (Heaviside), F1
and F2 layers (Appleton).

The first was discovered by James Van Allen in 1957, a
NASA engineer, hence the naming of one of the layers as
the Van Allen belt. UV rays with higher frequencies can
penetrate deeper into the atmosphere so they create the
lowest ionised layers.
• The D layer sits between 50-100 km high during
the day. Ionisation is low because fewer UV waves
penetrate to this level. The D layer refracts VLF, as
long as large antennae and high power transmitters
are used, but it absorbs LF and MF waves, so the
range during daytime is about 200 miles


• The E layer is higher, between 100-150 km, and
almost disappears by midnight. It allows mediumrange
communication on LF and HF bands
• In daylight, the F layer splits into the F1 and F2
layers. It is responsible for most HF long-distance
communication. During maximum sunspot
activity, F layer atoms can stay ionised all night. For
horizontal waves, the single-hop capability can be
up to 3000 miles, and more with multiple hops
REFRACTION
Anyhow, any wave that hits the ionosphere is bent, as the
side of the wave that hits one of the layers of the
ionosphere first starts to speed up (because of the reduced
dielectric constant from ionisation), which makes it turn.
The effect is similar to that of light refraction in water
which makes an object appear to be displaced.
Eventually, if the angle is increased, the bending will be
enough to bounce the wave back to Earth (we won't get
into Moon bouncing here!) The angle at which this first
happens is called the critical angle., which is the smallest
angle that will allow a wave to be refracted and still return
to Earth. Any rays more vertical than this angle are called
escape rays. The critical frequency depends on the density of
the layer concerned. If a wave passes through a layer, it
can still be refracted from a higher layer if its frequency is
lower than that layer’s critical frequency.
The lower the frequency of a wave, the more rapidly it is
refracted, and the larger will be its critical angle, but the
less the distance it will travel. A 20 MHz wave will be
detected further from the transmitter than a 5 MHz one.
The first wave to reach the ground after being refracted is
called the First Returning Sky Wave, until the maximum
range is reached. When a wave leaves an antenna, the
ground wave will be detected until it fades, or attenuates.
Between that point, and where the first sky wave comes
from the ionosphere, is an area where nothing is heard, a
skip zone, or dead space.
The skip distance is the Earth distance taken by a signal
after each refraction, or the distance covered by the first
sky wave. 30 MHz signals do not return because they are
too high in frequency, being at the bottom of the VHF
band (15-25 MHz is more typical for bouncing). You can

refract off the ionosphere and back off the ground several
times for multiple hops
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Old 10-23-2012, 10:22 AM
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Originally Posted by fatmike69 View Post
Recently I rode a flight from SFO - KOA on a United Airlines 757, at the same time the San Francisco Giants were playing game 6 of the NLCS. About an hour into the flight, after multiple requests from other pax about the score of the game, the pilots came on and told everyone that the Giants game was available on channel 9 (usually the live ATC channel). I tuned in and sure enough we were getting the local San Francisco radio broadcast for the game. Coverage was a little fuzzy, but very listenable and continued pretty much until the game was over, at which point we were at least 1000 miles from land.

My question is how did they do this? I know nothing about oceanic or HF, it sounded to me like an NDB signal (local AM radio station) but was very suprised that the signal carried as far as it did. Thanks!
Not just any AM station, particularly this one:
KNBR - Wikipedia, the free encyclopedia

The main station, KNBR (680 kHz), licensed to San Francisco, broadcasts on a clear channel from transmitting facilities in Belmont, California. KNBR's non-directional 50 kilowatt (or 50,000 watt) signal can be heard throughout much of the western United States and as far west as the Hawaiian Islands at night. Because of its extensive range, it is sometimes called by the nicknames "The 50,000 Watt Flamethrower", "The Blowtorch", and "The Mighty 680". Prior to adopting a sports format, KNBR enjoyed a long history as the flagship of NBC's West Coast radio operations.
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Old 10-23-2012, 11:54 AM
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HF? Google 'hf frequencies' are there are several stations that are HF news/sports/etc frequencies.

Listened to Bush's announcement of war a decade ago on HF over the Atlantic.
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Old 10-23-2012, 11:57 AM
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I can't speak for UAL 757's, but none of the 4 Boeing models I've flown had ADF. I guess I'm too young.
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Old 10-23-2012, 01:13 PM
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Originally Posted by Ottopilot View Post
I can't speak for UAL 757's, but none of the 4 Boeing models I've flown had ADF. I guess I'm too young.
We do have ADF's.

I was flying the other day and had a passenger command us to put the Purdue game on channel 9.

I guess he had a crew put a game on once, so he thought it was part of the onboard product.

We decided that, if he was a real Purdue fan, he wouldn't be working during the game. No dice.
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Old 10-23-2012, 01:27 PM
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Originally Posted by Ottopilot View Post
I can't speak for UAL 757's, but none of the 4 Boeing models I've flown had ADF. I guess I'm too young.
I can and they do.

I'm sooo glad I'm not that young and can look back on the "good years" !

G'Luck Youngster
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Old 10-23-2012, 01:54 PM
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Originally Posted by Ottopilot View Post
I can't speak for UAL 757's, but none of the 4 Boeing models I've flown had ADF. I guess I'm too young.
What 4 models would that be? Or did your company remove them to save wt?
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Old 10-23-2012, 01:59 PM
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Originally Posted by Timbo View Post
What 4 models would that be? Or did your company remove them to save wt?
So here is a question. How does Chanel 9 work? More specifically how is it plumbed? Is it based on the captains audio panel or acms etc?
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