Ian Roberts ZS6BTE,
December 2012
This
article was published in Radio ZS, February 2013
Solar cycle 24 is due to peak in 2013 but “we'll see a very weak solar
maximum in 2013, if at all…this could be the last solar maximum we'll see for a
few decades.” If you are interested in VHF DX get going –
time is running out!
Ray
Cracknell, ZE2JV (SK) working out of
More than
50 years later VHF amateurs in
The
ionization centre is the geomagnetic equator sloping slightly southwards east
to west from the Horn of Africa to
It might be
a surprise to learn that a transmission from the better sited northern ZS area
travels nearly 7000 km before making landfall in the Mediterranean, or further,
up to about 8000 km (Austria, Hungary) if conditions are better. Therefore it
is not possible to work countries over the greater part of the African
continent under typical TEP conditions and logging these countries is
challenging for ZS ops.
Fig 1: TEP geometry
after ZE2JV
ZE2JV noted
on 50 MHz a few
According
to TV DXer Roger Bunney in
his booklet Long Distance Television 1976, at sunset the two daytime F layers break up and merge into the F2 layer
approximately 250 miles (400 km) high. This breaks up into small clouds and
multiple reflections occur as signals are scattered through the cloud region.
Signals, as for conventional F2 layer propagation, suffer multiple images,
smearing and have a characteristic flutter effect. In winter the cloud is less
expanded by heat and the ions thus more compacted in unit volume, increasing
the MUF.
During solar cycle 21, Costas (SV1DH) and ZS6PW (SK)
conducted timing pulse tests to learn more about the TEP path using the
frequency and time references available at the time. SV1DH writes “Back on cycle 21 (1982), we conducted E-TEP propagation
delay measurements from
Solar activity peaks every 11 years and the peak
lasts 2-3 years. The peak months are around the
equinox with the sun over the equator, i.e. March and September. The worst
period for ZS is December and January even during active solar conditions when
the sun is far south and a long way from the geomagnetic equator. During our
mid-winter TEP coupling to Sporadic E in the northern hemisphere, where it is
summer, helps the propagation to northern
It was noticed in traveling around,
operating portable from various locations in southern
From the above,
TEP is obviously a complex mechanism and in modern terminology has two main
modes, to which a sub-mode may be added.
Under
current solar cycle 24 conditions (rather poor), from about
This mode
lasts until after sunset and provides the best DX conditions. The amplitude
flutter effect is limited compared to eTEP and there
is little frequency shifting of a RF carrier. The frequency stability was
confirmed over years by monitoring the German, Swiss, and Austrian 48.25 and
49.75 MHz TV carrier frequencies (now QRT) which were locked to rubidium
quality oscillators and generally frequencies were within 1 Hz or so of the
nominal value. In other words, there is little Doppler shifting of the carrier,
the Doppler “look angle” is also just about zero because of the great distance.
This mode may fade out slowly, or transform over the next period to eTEP,
typically after about 7.00pm local time.
It would be
easy to confuse aTEP with F2 propagation. The difference is the propagation
distance. If it were F2, TV carriers, etc from central
Fig 2
illustrates multipath as viewed on a TV picture. A TV line scan takes 64 µs,
several ghostly images of inverted phase (white instead of black) are visible
each side of the main picture information and at the speed of light, 0.3 km per
µs, one can calculate the respective times of arrival and glean some
information regarding timing of the various picture elements.
It is
apparent multipath picture information of the announcer’s face arrives at the
beginning and end of a scan line representing distance differences of 0 to 64
µs, i.e. 64 x .3km = 19.2 km and this is maintained for the update rate of 25
Hz (0.04 seconds). It would be interesting to lengthen the scan time if
possible to several hundred µs to see just how far in time this multipath
exists. Under eTEP propagation (described below) the
picture breaks up completely into vertical bands.
Fig 2:
Occasionally
there would be no aTEP and conditions manifest as straight eTEP after dark.
This always indicates poor TEP conditions.
Usually
aTEP would change to eTEP with or without a fade-out during the transition and
this mode is strongest around 7.30 to 8.30pm or so local time. It provides by
far the highest MUF, over 144 MHz from
This cycle
ZS6 ops have worked to
The
tumultuous nature of the propagation is even more diverse than one would think
and this is readily observed on TV pictures received. All synchronization
information is destroyed and it is impossible to lock a DX TV picture on eTEP.
To counter this a method of turning off sync processing in the Philips
demodulation chip in PC TV cards is used so that the picture is displayed
stripped of sync (i.e. with a manually adjusted free running time base). Of
course the in-band selective flutter-fading caused by multipath reception
(group delay) cannot be compensated for and the picture takes on a
characteristic appearance reminiscent of water boiling in a pot with fast
out-of-context transitions from black to peak white caused by all the phase
cancellation and phase summation and placing video AGC processing under
pressure.
This
multipath has implications for SSB operation and at times copy is difficult
with cranky squeaks accompanying drop-out of audio information. CW should not
be sent too fast as complete “dahs” and “dits” will be lost, losing the copy.
In Fig 3 captured in the time domain, there are frequent dips to noise level in
the slow CW caused by lack of reflection and anti-phase combination. And
various enhancements, well above the average carrier level, resulting from
in-phase combination. Due to all this, eTEP signals
might need a higher signal-to-noise ratio to copy them properly. The average
signal-to-noise ratio shown is about 10 dB with peaks around 15 dB. The ZS6DN
beacon used 4 long Yagis pointed north and 100W apparently.
Fig 3: Morse letter
“N” from a sound recording by SV1DH in
Sound file processed
for display by ZS6BTE
This mode
has not been described in any easy-to-read literature I can find. It occurs
often enough (possibly around 10-20% of the occurrence of eTEP) in the evenings
after 8.00pm, peaks around 9.00pm and may last until 10.00pm local time before
slowly fading. It always follows eTEP. It is characterized by a dramatic
shortening of the earlier eTEP path, to the extent the Mediterranean region is
received very weakly, if at all. It offers VHF communications with central
In Fig 4 a
“peak hold” function of a few seconds on a spectrum analyzer was used to
capture the peak-to-peak Doppler shift of 26 Hz on the
Alternatively,
an immense vertical zone of ionization might occur over the geomagnetic equator
partially isolating ZS from the northern hemisphere and allowing ready
reception of the central African area by back-scatter from this sheet of
ionization. This requires intense ionization and compaction of the zone
otherwise the critical frequency would be exceeded, yielding only weak
back-scatter signals in the south. If this latter possibility is in play, it
needs high velocities of reflecting components to produce the measured Doppler
shifts illustrated in Fig 4. The vertical sheet possibility becomes attractive
when examining amateur stations received under these conditions – African stations
on or just south of the geomagnetic equator (Equatorial Guinea, Gabon,
Cameroon, etc) are readily received, while stations just north of it (Senegal,
Chad, etc), and further north, remain illusive under leTEP.
Fig 4: Peak-to-peak
Doppler shift of 26 Hz on the
The
frequency shifting phenomenon requires the bistatic range L in Fig 5 to change
at a rate fast enough to generate measurable Doppler shift. Bistatic Doppler
shift is proportional to the rate of change of bistatic range in period “t1-
t” seconds. Thus, two bistatic ranges
are calculated, firstly at time “t”, then at time “t1” seconds. During this
time period the target has moved and increased or decreased the bistatic range.
The two values are subtracted to provide the change in bistatic range as at
time t1.
Change in
bistatic range: ΔR
= (RTX RRX – L)t – (RTX RRX – L)t1
If the range has increased the
Doppler shift will be negative, and positive if the range has decreased. The
shift can be negative even if the “target” (the ionized zone) is moving closer
to RX. As shown in Fig 4 both shifts can happen at the same time indicating
different ionized zones moving at different speeds and directions as viewed by
the receiver. Around the ellipse bistatic range does not change or when the
target moves along L, so bistatic Doppler shift then is = 0.
Fig 5: Doppler shift
is explained by changes in the bistatic range L (Wikipedia)
This works
when both antennas point to the north. Intense ionization of the TEP zone is
required, such as occurs under ideal eTEP or late evening TEP conditions.
Amateur backscattered signals are weak and may not be readable from the
This mode
is an east-west phenomenon and in my experience takes place in the local day
time hours commencing at
It accounts
for a few 50 MHz contacts with
Ops new to
50 MHz, this is understandable, and many experienced ops, can be clueless
regarding the many indicators available. For instance, the importance of IDing and using TV transmissions as indicators is
relatively new to most amateurs. But the whole of
Table 1: A few important VHF
propagation indicators for ZS ops
Item |
Frequency |
Information
– use USB to hear properly |
Woodpecker RADAR |
34.262 + 15 others! |
Southern Russia 7.5 Hz PRF Russian early warning |
RADAR |
36.924 |
|
SNOTEL RADAR |
41.700 |
|
RADAR |
41.424 |
|
|
48.249 |
Central-south, broadband hash no AM sidebands |
Kenyan TV 15 kW |
48.249952-978 |
South west |
Ukrainian TV 50 kW |
49.739594 |
Buky (central) and |
Russian TV 200 kW |
49.747383-411 |
|
Moldavian TV 12 kW |
49.748812-9.749 |
Cahul |
|
49.749987-91 |
|
|
49.750000 |
|
Armenian TV 45 kW |
49.760420-423 |
Amasia |
SV1SIX beacon 25 W |
50.040 |
|
5B4CY beacon 20 W |
50.0185 |
|
VK6RSX beacon 50 W |
50.304 |
|
This Table has been updated
There are
dozens of 50 MHz beacons listed on the i-net and it’s
a good idea to enter some into scan memory, particularly for countries where
low VHF TV is now longer in use.
Satellite
technology has considerably enhanced our knowledge of the TEP mechanism,
referred to in scientific circles as equatorial plasma bubbles, plumes or
Equatorial Spread F (ESF) at heights around 400 km. Gravity waves at low
latitudes are thought to act as seeds for equatorial plasma bubble formation.
Various satellites have been used including military and the GPS range and
their beacons are monitored for amplitude and frequency scintillations.
Equatorial plasma bubbles are intervals of depleted and irregular plasma
densities that degrade communication signals i.e. exactly what is wanted for
successful TEP operation from the amateur standpoint. By measuring wave
propagation through these plasma bubbles, movements and densities can be derived
and used in explanatory modeling. In Fig 6 it is clear the vertical plasma
velocity is around 45 m/s (162 km/h) at 20.00 hours (the peak time for eTEP) at equinox March and September. In other words, the
plasma bubble has about two hours in the period from sunset to
These high
horizontal and vertical velocities of the plasma bubble explain the Doppler
frequency shifts measured in Fig 4.
Fig 6: Local time
variation of the equatorial vertical plasma drift at longitude 0°E as predicted
by the ROCSAT-1 plasma drift model for December solstice (blue), equinox (green),
and June solstice (red) for a solar flux level of F10.7 = 150 (Illustration from Stolle,
Lühr, Fejer, 2008 in
reference section)
In the
references below is a link to an opinion and model concerning solar cycles 24
through 25-26, etc.
At the time
of writing this model is proving depressingly accurate. Don’t move QTH in the
hope of working great TEP this solar cycle or in the next few..
So far
cycle 24 looks likely to top out in 2013 about SFI 180 at best or well down on
the last cycle, 23 (SFI 273), which was disappointing enough. By comparison,
cycle 21 in 1982 topped out at SFI 290. ZS ops hoping to achieve 100 DX
entities on 50 MHz in cycle 24 are in for a hard time and the next cycles are
not predicted to add much to the tally and may be restricted to n-s operation
to the Mediterranean area, if at all. The decreasing level of solar activity
from cycle 21-24 can be seen at http://www.solen.info/solar/images/comparison_recent_cycles.png
The sun is
in a strange mode regarding solar radiation. Currently only one face is
partially active, the opposite face is largely inactive. This causes the 10.7cm
Solar Flux Index to alternate with each face between an average of ~145 and
only 95 with little variation. The trend is readily displayed at http://www.solen.info/solar/ and has
occurred for the last 11 solar rotations and there is no reason to expect
changes and it impacts directly on TEP working. From this the table below has
been compiled for another 8 solar rotations to June 2013 using the synodic rotation period of 26.24 days (same solar face
visible). It looks as though early April 2013 (just after equinox) will produce
peak TEP conditions of cycle 24 for ZS ops.
Table 2: Date centres for SFI maxima to June 2013.
Date |
Anticipated average SFI peak value |
|
145 |
|
145 |
|
145 |
|
145 |
|
145 |
|
145 |
|
145 |
|
145 |
Major Drop In
Solar Activity Predicted by Staff Writers Boulder CO (SPX)
Long Distance Television Roger Bunney
1976
Relation between the occurrence rate
of ESF and the equatorial vertical plasma drift velocity at sunset derived from
global observations
Stolle, Lühr, Fejer Dec 2008 www.ann-geophysics.net/26/3979/2008/
Solar Terrestrial Activity Report http://www.solen.info/solar/
Bistatic range:
http://en.wikipedia.org/wiki/Bistatic_range