The Twilight Zone
Steve Nichols G0KYA
7, Quebec Close
Cringleford
Norwich NR4 6XU
E-mail:
steve@infotechcomms.co.uk
Tel: 01603 503076
Intro: Steve Nichols
G0KYA, of the RSGB’s Propagation Studies Committee, believes that propagation
around sunrise and sunset is not fully understood. Here he outlines the
mechanisms behind grey line and other twilight propagation modes and a research
project to help us understand them.
Worldwide
communication using the HF bands is dependent on radiation coming from the sun.
In general, and to grossly oversimplify reality, at LF (160, 80 and 40m) we
need a night-time path between the two stations. At 28MHz, a daylight path is
generally needed. But twice a day, at sunrise and sunset, the ionosphere
undergoes dramatic changes, giving enhanced propagation in some directions.
In
terms of radio propagation, the D and E layers are responsible for most of the
absorption of radio waves that pass through them, but the absorption is
frequency dependent. The D layer can completely absorb signals on 160, 80 and
40 metres during the day, and can attenuate signals on 20m too. Hence the
reason you don’t hear much, if any, DX on the low bands during the day as
sky-wave signals are absorbed before they can reach the E and F layers.
The
ionosphere undergoes a dramatic change in ionisation at the transition from day
to night. The electron (and ion) density in the E-layer decreases by a factor
of 200 to 1 and the F1 by nearly 100 to 1 (see graph). At sunset, the D layer
disappears rapidly.
Around
the other side of the world other regions that are entering into daylight have
yet to form any significant D layer and the E layer has not built up from its
night-time low. Therefore, for a short period propagation between two regions
simultaneously experiencing sunrise and sunset can be highly efficient. Signals
on the lower bands can theoretically travel over great distances with little
attenuation.
This
is well documented with many examples of grey line propagation being logged on
160 and 80m over the years.
Many
amateurs will be familiar with this so-called grey line propagation (the term
was coined in 1975 - see Ref 1) – propagation that occurs along a line
separating night from day. The line is called the terminator but it is diffuse,
due largely to the earth's atmosphere that scatters the light over a large
area. In radio terms, the radio terminator is not the same as the visual one.
The latter refers to the point when we see the sunrise or sunset at ground
level on the earth and the period of visual twilight that either precedes or
follows. The former refers to the way the sun illuminates the ionospheric D, E
and F layers.
For
example, the PC program Geoclock defines the point at which the sun
starts/stops illuminating the D-layer as being offset from the visual
sunrise/sunset by 6.596 degrees longitude. As the earth rotates 15 degrees per
hour this could be as much as 24 minutes before or after sunrise or sunset,
although the actual figure will depend upon the time of year and latitude (see
diagram).
The
HF “twilight” zone –the region on earth between the loss of the D layer and
where the sun starts/stops illuminating the F layer (roughly defined as being
offset from sunset by 14.165 degrees longitude) can therefore be almost one
hour before and after sunrise and sunset.
E
layer illumination starts/finishes somewhere in between these two, but the
average height is much closer to that of the D layer.
To
confuse matters, these values are based on average D- and F- layer heights and
the apparent heights of these can change too. So it is no good looking for grey
line DX exactly at your visual sunrise/sunset – you could be out by up to an
hour depending on the band, your respective locations, and the time of year
(see diagram).
And
even worse, for signals at an angle to the terminator we are interested in
where the first ionospheric refraction or hop actually occurs once you radiate
a signal, which is likely to be many hundreds of miles to the east or west of
you – where the sun may still be illuminating the F layer. This is well
illustrated on page 93 in the book “HF Antenna Collection” by Erwin David,
G4LQI – see refererence 7.
Most
books relating to HF propagation give a brief description of grey line
propagation, and how and why it works. What they don't tell you is the actual
frequencies affected, other than a vague idea that 80/160m are definite bands
for grey line, and "some" HF bands also exhibit grey line
enhancements.
Either
way, all these books tell you that grey line enhancements occur along the terminator.
That is, when both stations are at the sunrise/sunset condition.
John
Devoldere’s book "ON4UN's Low-Band DXing". suggests that his own
experience shows paths perpendicular to the terminator may enjoy the greatest
signal enhancement. That is, on the low bands, as sunset occurs at the
receiving station, you may get grey line enhancements at right angles to the
terminator in the direction towards the dark side of the earth - not along the
terminator.
He
also points out that the width of the terminator will vary according to the
season and your position on the earth, and cannot be thought of as a fixed
entity - the grey line will be narrower at the equator and wider at the poles.
So the time-span available for grey line conditions will also vary depending
upon the time of year, and the locations of the two stations, which is what I
proved earlier.
Likewise,
the width of the grey line will depend upon frequency as D layer absorption is
frequency dependent - you may still be able to work DX on 40m 24hours a day in
mid-winter, while DX on 160m will fade out quite quickly after sunrise to the
greater D layer absorption.
But
what of HF? There appears to be little hard research of grey line propagation
at higher frequencies. The vague suggestion in most books appears to be that
grey enhancements can and do occur on 20m. Ten metres is theoretically too high
for the effect to appear as D layer absorption is virtually non-existent
normally at these high frequencies, although I have read more than one article
about how to work grey line on 10m! See the graph of frequency -dependent
D-layer absorption predictions at http://www.sec.noaa.gov/rt_plots/dregion.html.
My
own studies show that enhancements on 10m do occur. On many occasions I have
heard signals from Indian, Indonesian and other stations on 10m just after
their local sunset - these stations were not audible before. I have also worked
a Brazilian (PT2GTI) station on 10m just after his local sunrise, receiving a
59+ report using just 10 Watts into an indoor dipole. I have also heard a
station in Puerto Rico (KP4NU) at 59, one hour after his local sunrise in
November. Both stations were still audible later that morning but at reduced
signal strengths – down 10-15db.
These
are not grey line paths, but there were definite enhancements.
Reports
of sunset/sunrise enhancements at 50MHz over long distances have also been
logged, notably between the UK and USA. One suggestion (see Ref 6) is that that
this is due to E or Es enhancements as the E layer increases in altitude at
sunset.
The
increase in altitude of the E layer needs further explanation. As the sun sets
the lower regions of the E layer are not illuminated so the effective height of
the reflecting layer appears to increase. Likewise, at this time we can imagine
the radio ionosphere as being tilted as it is being illuminated at an angle.
This is probably the vehicle for the enhanced propagation at 28MHz and 50MHz –
D layer absorption probably has nothing to do with it.
If
the theory holds, look for enhanced signals during local daytime in G from
stations along their terminator – from the west at their local sunset and from
the east at their local sunrise. The signals should be strongest at roughly
right angles to the terminator – the same as ON4UN’s prediction of propagation
on LF, but from the illuminated parts of the globe, not dark.
There
is an alternative way of looking at grey line conditions on 7MHz and 10MHz
connected with the critical frequency (fof2). At frequencies above fof2 a radio
wave travelling vertically upwards would pass through the f2 layer into outer
space. Below f0f2 it would be reflected back to earth. Now imagine a radio wave
hitting the ionosphere at about 75-85 degrees to the earth - a near vertical
incidence wave (NVIS). Below the critical frequency it would be returned. If it
is some way above fof2 it will pass into space. At some frequency close to fof2
it could be refracted through a large angle and could end up travelling almost
parallel to the earth, giving a very long first skip distance. This is the
condition for the Pedersen (see Ref 2) or critical ray, discovered in 1927,
characterised as being high angle, long distance and close to and probably
above the fof2 frequency. As there would be no intermediate ground hops the
signal strength could be very high indeed.
It
is likely that these conditions exist around local sunset/sunrise as fof2
passes through the two bands and could account for long distance communications
under grey line conditions on 7MHz and 10MHz – see http://www.spacew.com/www/fof2.html for predictions of fof2.
Either
way, there is more to grey line and twilight propagation than meets the eye.
The effects are different on every band, and the mechanism behind the
propagation is probably different too. What we can say is that twilight
propagation is not always best along the terminator and there may not be any
enhancement at all on some bands. Some books would have you believe that you
can just tune up on 20m at sunset and work ZL 59+20dB every day – if you can I
would like to hear about it!
I
am currently doing some research into twilight propagation on many of the
amateur bands, starting with 10m. The early results show that we can and do see
enhancements from signals originating from areas experiencing sunrise/sunset.
The
graph of the beacon SV3AQR on 28.182MHz is typical. This was produced using
SpectrumLab software connected to the audio output from a Yaesu FT-920. With
the AGC turned off, the vertical scale indicates signal strength while the
horizontal scale shows time. You can quite clearly see a 10db increase in
signal strength around the time of sunset at the beacon’s Greek location. The
effect has been seen on other beacons, but like all ionospheric effects, it
doesn’t occur every day and is virtually impossible to forecast.
More
monitoring work needs to be done before we can write the definitive guide to
grey line and twilight propagation and this is where I need readers’ help. If
you have a PC with a soundcard, can run the SpectrumLab software (www.qsl.net/dl4yhf/) have a very stable receiver (the
software needs stability in the order of a few Hz), and can leave your system
monitoring for an hour or more at a time then I would like to hear from you. As
part of the Propagation Studies Committee’s work I plan to systematically look
at twilight propagation on all the HF and LF bands using known, quantifiable
signal sources such as beacons and broadcast stations. This is not a
five-minute job though, but is essential if we are to finally clear up what has
been a grey area of propagation research for a long time – every pun intended!
Steve Nichols G0KYA
QTHR
Useful Software
Beacon Time Wizard by
Taborsoft (www.taborsoft.com)
GeoClock
(www.geoclock.com)
Grayline 1.2 by PA3CGR
References:
1. Dale Hoppe, K6UA et
al, "They Grayline Method of Dxing," CQ, Sept 1975, P27.
2. J. G. Lee, "An
Introduction to Radio Wave Propagation", Babani BP293, 1991.
3. Ian Poole, G3YWX,
"Your Guide to Propagation," RSGB, 1998.
4. Jacobs, Cohen and
Rose, "The New Shortwave Propagation Handbook", CQ Communications,
1995.
5. John Devoldere,
ON4UN, "Low-Band Dxing", ARRL, 1999.
6. Ken G4IGO, "E
layer and Sporadic E - Two Modes of Propagation at 50MHz", UK SMG web site
(www.uksmg.org).
7. E David, “HF
Antenna Collection”, RSGB, 1991
ENDS