point-to-point transmission forecast set for a power of 100 W in August 2004. Only the
year and month are used (CCIR coefficients) with a SSN
of 85. The path is set between Brussels and San Francisco using a Yagi at the
transmitter side and a dipole at the receive site with a takeoff angle
of at least 3°. Working in SSB, the SNR is
set to 48%. Remain to run the simulation and display the result in a
propagation analysis and prediction program (II)
Main menu and settings
After have launched VOACAP that loads quite rapidly, a large screen pops up as
shown at right. No embellishment, but an input screen with push buttons where all data are
gathered at a glance.
kind of interface is also shared by the other programs provided with VOACAP like
ICEPAC. However, the number of fields and submenus available depends on
the model used.
main menu includes five dialogs : File, Run, View, Save as and Help.
menu permits to save all settings in a .VOA extension file in the subdirectory
\saved\default; "Run" menu
permits to start the calculations for your specified conditions and create five
different output files (Circuit, Graph, Batch, Distance and Time);
"View" menu permits to display charts for various output parameters supported by the model.
"Save as" menu permits to save either your transmit, receive or the whole circuit using
a generic name, and the "Help" opens a short Windows HLP file explaining
what represent input fields and their specifications. But this help is
neither very verbose nor extended. However, it should read at least
shown at the bottom of the screen in the "Input Help:" message box, a
red commment is displayed. Each input field is indeed associated to a contextual help
that activates when you move your mouse over the field. IMHO this comment replaces
advantageously the information available in the Help menu.
main input screen is your interface with the point-to-point model. That means
that all circuit data must be set first in this screen and associated subscreens
before expecting to launch the calculation or display a forecast. In some
working conditions, you will get an error message (yes, in a red box !) if you
don't respect this procedure.
is a circuit ? A "circuit" is defined as a complete
communication system including users on both ends of the communication channel,
both antenna systems, the transmitter, the receiver, the working frequencies,
30 propagation methods or set of algorithms (functions) are provided to establish a forecast, among which the
MUF-FOT-Es, MUF-LUF-FOT and FOT-MUF-HPF to name some well-known acronyms. Without
more detail, it is better to select the default "Method 20" that takes
advantage of all the system performance. But in some working conditions it is
advisable to select the appropriate method.
example, if you work with the longest paths over 10,000 km, "Method 21"
uses special functions that are worth to be tested. In between, for target
locations ranging between 7 and 10,000 km "Method 30" takes advantage of
statistical functions to smooth irregularities between both models. At last
if you only need to know MUF and FOT and display the result in a chart,
select "Method 8" or "Method 9". However not all
have been cross-checked with VOACAP results, and according to George Lane
(2001), among them there are Method 16 and... Method 9.
are ionospheric parameters that take into account additional effects like the
presence of oceans or the day of the month. The idea came after engineers
corrected predictions using CCIR coefficients applying Transmission Loss tables
to determine the variability of the MUF. Substracting data taken between
1958-63 to the current ones, they noted huge difference that were compiled in
these both coefficient tables.
The default is "CCIR/Oslo",
but they cannot be interpolated when using the date-of-the-month. They lack also
of data when the path goes over the oceans, a deficiency of CCIR coefficients.
When the date-of-the-day is used in the field "Month"
("Groups" group) URSI 88 (Australian) coefficients must be selected
(their name come after 1988, the date of release by Australian researchers).
They provide a linear fit between the SSN calculated during the quiet solar
cycle (minimum) and the paroxysm of its activity (maximum). And as we told,
data over the oceans are also considered.
under specific conditions (for example requesting a special ionospheric map
without recalibrating the prediction against measurement) URSI 88 coefficients introduce errors in VOACAP
calculations, mainly when the foF2 data do not match the epoch at which other critical parameters
are measured. This problem was "solved" in using only the month as
input. Indeed, VOACAP was initially designed for experimenting with prediction
algorithms, but the experimentation was never finished due to lack of funding.
Take a look at the list of available prediction methods and parameters, and you
will understand what I mean... So excepted a special reason, keep the default
CCIR/Oslo coefficients and do never enter the day-of-the-month because you have
all chance to ruin the correlation defined for the specified prediction.
Groups subset provides two field groups, a range of months/days and the
associated smoothed sunspot number (SSN), to not confuse with the sunspot number
takes into account a
12-month average of the SSN. The "Month" field can be filled manually
or automatically pressing on button "Seasons" (1,4,7,10) or "All
MONTHS" (1 to 12). It also accepts a decimal
integer which format is "MM.dd" (month.day). For example June 30 must be encoded
as "06.30". But as we explained just above, in this case you must select
the URSI 88 (Australian) coefficients, and this is not recommended.
"Groups" subset, used to set the month and SSN values. Do
never enter the day-of-the-month or the daily sunspot number or expect errors in
some applications using a simplified interface, the fact to enter
the day of the month, forces the application to automatically switch
to URSI 88 (Australian) coefficients without warning.
all cases, using the day-of-the-month, expect to get some
"unattended" errors in forecasts. Thus avoid to use it.
But there is another reason to never request a forecast for a
specified day of the month with VOACAP : this is SSN.
we must know that the average solar flux index (SFI) and the smoothed
sunspot number (SSN) are correlated by the next relation :
SFI = 63.7 + 0.727 SSN + 8.95x10-4
SSN refers to the F10.7-derived SSN using the Penticton Radio
Observatory noon value.
example, a SFI of 85 is equivalent to a SSN of 28.
you lost this formula, use the small G4ILO's HFProp
program that converts automatically these values.
if you check on the Internet you will find different SSN values. How
to get the correct SSN ? We must follow George Lane's advice, one of the main VOACAP
developer who recommends to use the SSN listed at the National Geophysical Data
Center at NGDC
FTP website (for users who cannot access that FTP site, here is the GIF
equivalent content) because other SSNs - like the temporary values maintained at
SEC - are supposed to give erroneous results. For
the small story, only the NGDC's SSN has been used to calibrate predictions
from VOACAP, hence George's recommendation. To well understand this problem, I
warmly suggest you to read on this site the next page dealing with the various
SSNs, why they exist, then to come back to resume this review.
read : The
short history of the Smoothed Sunspot Number
during periods of low sun activity the different SSN values (e.g. SSN, SSNf,
SSNe, etc) don't differ much from one site to another (e.g. NGDC vs. SEC) or in
relation to the real-time values, during the period of sun paroxysm, these averages can be very
different from the daily real values. So, whatever the improvement made on the
VOACAP engine of your application, if you want to estimate the confidence of your propagation
forecast, even if the circuit required reliability is set to the highest value
(SNRxx of 90% or higher),
always run the simulation with values up to 20% higher and lower than the SSN to see what happens.
In the field I have noted that your DX coverage can vary from 10 to 25% !
last, as you noted, VOACAP doesn't provide any connexion to the Internet to update
automatically these fields or to use the system date. Hopefully other interfaces
solved this problem to name WinCAP Wizard that calls
or DXAtlas that calls IonoProbe to retrieve the monthly update.
predictions for a circuit between Belgium and Brazil scheduled in
September with 100 W PEP output, a takeoff angle of at least
3°, a S/N reliability (SNR) of 50 dB
in SSB and a circuit required reliablity (SNRxx) of 90%. For left
to right results for an SSN of 122 in 2000 during the maximum of solar cycle
23, an SSN of 27 in 2004 and an SSN
of only 7 during the minimum that occured end 2006. In 2000 (left), at 2200
UTC on 14.131
MHz we had a signal strength in Brasilia of -134 dBW or about S4.
predicted a S/N ratio of 24 dB, thus good signals. In a few years we observed a gradual closing of upper
bands, beginning with the 10 and 12m bands during the first two
years. By 2004 (center), late in the evening the 15 and 17m
bands were harder to work or never open to DX in SSB. On 20m at 2200 UTC the signal strength was
already close to -160 dBW, in the noise, and the S/N dropped to
-1 dB ! End 2006 (right) VOACAP predicted
till worse conditions with a signal strength of -182 dBW, and a
SNR of... -40 dB ! Not that long distance QSO were
impossible to work, even though, but propagation conditions were deeply
affected by the low solar flux that did not more ionized ionospheric
layers as it did some years earlier. We had to rely on
lower bands like the 40m to recover a signal strength equivalent
to the one of 2000 but with a lower SNR close to 15 dB at best,
but often lower than 10 dB.
you have understood, the SSN or the solar flux at 10.7 cm
(SFI) is the main factor to consider when calculating
the propagation for a band. Its influence is as important as the one of your
antenna takeoff angle. In addition, if the sun activity (radiation pressure) is not without effects
on the ionization of ionospheric layers, it also affects the temperature and pressure of the high atmosphere of the Earth.
This second effect mainly
affects the lowest band propagation, a factor too
rarely considered in propagation programs that ignore the weather conditions as
well as other factors (e.g. Appleton's magneto-ionic theory), and VOACAP doesn't go against the rule.
example, there is no input in VOACAP for the geomagnetic field, A- or K-indices despite
their utility to detect a possible geomagnetic disturbance or a storm. This can be a problem if
you want to work via the north pole, during
auroral activities, or on the top band, to establish short-term forecasts, or
when working with vertical antennas at low latitudes. Here also, more recent
applications developed new methods to bypass this problem, using for example
real-time ionosondes (e.g. DXAtlas).
group contains coordinates (longitude/latitude in two digits) and the name of
the transmitter location. These data are extracted from several text files
sorted by cities, continent, state, DX entity or any other custom selection.
You can add as
many location files as you want. By default they are saved in the
\geocity, \geonatio and \geostate subdirectories.
standard format is :
CITY COUNTRY LATD
LATM NS LONGD LONGM EW
extracted from "Europe.geo" :
BRUSSELS(BRUXELLES) BELGIUM 50 50 N 04 20 E
you note coordinates are recorded in degrees and minutes. City names or
countries do not need to be sorted. This is VOACAP that converts coordinates
in decimal values and sort the location file before displaying data on
group works exactly the same way as the Transmitter and requires no
special comment. Other settings related to the noise level at the receive
location are taken into account in the "System " group that will be
reviewed on the next page.
Transmitter and Receiver settings can be saved in generic files
respectively called "transmit.def" and "receive.def"
located in the \userdb subdirectory that lists all locations
specified in the circuit(s). All are text files that can be modified by
the user as long as the format is strictly maintained.
on the button you can select either a short or a long path (long great circle via the
antipode) for the prediction. All depends on your target location, the time of the day and your
antenna gain. For VOACAP a long path means also any path longer than 10,000 km
and in this case it also uses a different algorithm, "Methode
21", to calculate
frequencies listed in these fields, if you request a graph the propagation calculations
are always performed for integer frequencies between 2 and 30 MHz. These
range of frequencies are only used to mark the scale
on the output graphs.
addition they can be used to select
the default frequencies or a set of two user-definable frequency lists.