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Software review

Close up on the System group. 

VOACAP propagation analysis and prediction program (III)

System group

This is a set of six fields which importance appears often at a second reading if you are not used to VOACAP and haven't take the time to read additional documentation to master the program.

The System group is one of the most important because its settings (of course in addition to the others) affect the prediction accuracy and reliability, essentially output parameters like SNR and REL. Caricaturing, we can say that a wrong value inserted in half of these fields, can transform your forecast in anything but a prediction that matches your real working conditions ! So let's see each field in detail and how to adjust them.

Man-Made Noise level

To not confuse with natural noise (QRN). This is the expected QRM at the receive location expressed in dBW.Hz (or dBW, decibels below 1W). This value is calculated by reference to a noise of 1 Hz bandwidth at 3 MHz. The default value is -145 dBW or S2. Not all applications use the same threshold. Here are for example the receive noise levels set in VOACAP vs. WinCAP Wizard :





-140.4 dBW

-125 dBW


-144.7 dBW

-136 dBW


-150.0 dBW

-148 dBW


-163.6 dBW

-164 dBW

Default QRM level associated to the receive location. Note that since the release of VOACAP ten years ago, WinCAP Wizard found that it was more realistic to increase the QRM level in the most inhabited areas, a sign of times... Note that -125 dBW is a good S5.

They are of course theoretical values. Most of the time you even don't know what DX station will answer to your call and thus neither in what type of area lives your correspondent or what is his or her QRM level. The QRM can also vanish as fast as it appeared. These approximations can however be specified during the QSO or earlier, when you planned your activity. 

On remote islands, in the country or in the bush at tens of kilometers away from cities where the QRM strength is unreadable on the S-meter, you can easily use the lowest value of -164 dBW.  The difference between extremes (Remote-Industrial) can make the S/N ratio at receiver drop of 20 dB, or 2 S-units, a power ratio of 100 times ! If the band is not in good shape, and whatever the SSN, that means that it could be close under these conditions. Thus better to use the appropriate value to get here also an accurate prediction.

Required Circuit Reliability (%), aka SNRxx

This field is probably the most important and requests some a detailled explanation. As we just explained, a forecast is a prediction, a probability estimation to work in specified conditions (the "circuit"). The term "Reliability" means "time availability". When you set the required reliability at 50%, it means that the resulting predictions will be as stated or better during the half of the month, or 15 days over a 30-day month. That means also that during the other 15 days, the actual conditions may be worse. So, SNRxx characterizes the expected signal quality that you consider as acceptable. It is maybe valid for you and today, but tomorrow you will maybe prefer working in "Hi-Fi conditions" as far as possible...

But then, why to not set SNRxx to 100%, do you wonder ? Well, if you set required reliability at, say 90% as suggest VOA engineers, then predictions will be as stated or better during 27 days over 30. Theoretically, in average, amateur working conditions match with a 90% reliability. In the field, that means that the concerned frequency is highly reliable, like the "BUF".

This is however a high value mainly suited to broadcasters that use 100 kW emitters, or big contest teams and so big DX-peditions who work in rocking-chair, Hi ! A casual amateur might satisfy with only 50%, but of course the higher the best, all the more that the advanced amateur will probably no more work like a casual amateur. Indeed, the beginner works often whatever propagation conditions (even in close bands !), where the Old Timer, experimented, will only switch his or her transceiver when the propagation will be good to excellent to avoid useless QRN and poor propagation conditions. Thus setting SNRxx depends also on your habits and your know-how. But in all cases it is interesting to set this parameter to 50% and 90% to see the change on the signal strength and pwoer at the target location.

In addition we can say from experimental trials that the reliability reaches 90% when the geomagnetic index Ap £ 27 and Kp index £ 4, so during phases of quiet sun or so.

This entry will of course affects the calculation of the output parameter "SNRxx". Note that VOACAP does not display iso-contour map when you request the output parameter SNRxx. This function is only supported in ICEPAC, included in the same package. As we will see later, ICEPAC takes also into account disturbed conditions near polar caps.

Required S/N ratio (dB), aka SNR

This is the second very important parameter that must be set correctly. SNR is ranging between -30 and 99 dB.Hz, a very wide power spectrum, hence all the importance to set it correctly too. Technically speaking, take a deep breath, SNR characterizes the ratio of the hourly median signal power in the specified bandwidth relative to the hourly median noise in a 1 Hertz bandwidth which is necessary to achieve the type and quality of service (mode) required, hence its name...

Say simpler, Hi !, this is the amplitude of your signal over the noise at target location. This signal-to-noise ratio is valid 50% of days in a month. It is set for a receiver bandwidth of 1 Hz (for noise only) anddepends on the mode, what VOACAP engineers called the "service" used according to the next relation :

SNR (dB) = 10 + 10 log (bandwidth in Hz)

For example, in SSB a signal offering a 3 kHz bandwidth gives a SNR of 45 dB. A 500 Hz CW tone is close to 38 dB. A digital mode, using a larger bandwidth, will give a still higher ratio.

VOACAP uses a default value of 73 dB what is qualified as "Good". Here are "milestone" values associated to the SNR : 

Service (mode)






BCL ~10 kHz

< 52 52 - 64

65 - 77

78 - 87

88 +

SSB 2.5 kHz

< 49 49 - 63

62 - 74

75 - 84

85 +

CW 250 Hz

< 40 40 - 47

48 - 54

55 - 61

62 +

SNR value and signal quality associated to usual modes (services). A digital mode will give a still higher ratio. Values include an allowance of 12 dB for fades.

Note that VOACAP engineers use the word "service" instead of "mode" that we use when we speak about CW, SSB or SSTV modulation mode for example to not make the confusion with propagation modes (2F, 3F2, etc). To avoid questions during this review, I will use both words together.

Although the SNR can vary, and varies from people to people depending on the signal quality you want to achieve for the concerned service (mode), it is recommended to use standards values, and for example to not set a too high SNR that seldom matches common working conditions of amateurs. We usualy state that over 75 dB in SSB, the signal quality is really very good, typical of a regional broadcast station; many amateur satisfy with poorer conditions close to 50 dB, even less in crowed bands or in presence of atmospherics. As soon as your hear QRN or QRM on your frequency (like a strong "white noise") you can already reduce the required SNR of about 10 or 20 dB. Idem in presence of fading. Therefore more than 80 dB is really exceptional, a quality that I would qualify of "Hi-Fi" or "FM grade", Hi !

In CW for example, many amateurs work in heavy QRM conditions or use a very narrow bandwidth in cascading several DSP filters. We can set the SNR for CW as low as 24 (Poor); in SSB in poor working conditions the SNR can be set as low as 44; at last in you listen to DX broadcast stations, the SRN can be set as low as 65. Of course any higher value will improve your signal but it will also mean that VOACAP will calculate a higher reliability, thus conditions more restrictive (pessimistic) too, but also probably valid more days per month.

Associated to the SNRxx, these both factors will amplify or decrease the "grade" of the expected S/N ratio. Incorrectly set they will emphasize extremes, with for consequence to false all your prediction, showing for example a wide opening with strong S/N where the band is in reality almost close, and the opposite.

Case study : reliability factor in VOACAP and ICEPAC

Let's see an example showing the influence of SNR and circuit reliability (SNRxx) on band conditions. Below are three predictions of the required circuit reliability calculated with VOACAP and ICEPAC for a circuit between Belgium and Arctic Bay in Far North Canada (73.03° N, 85.18° W) scheduled in September 2000 (solar maximum) for a SSN of 122 and 100 W PEP output. The QSO was not succeeded. Try to know why. 

To reach VY0 sky-waves pass over Iceland. Unfortunately, at that time the PCA exceeded 150 dB at 2200 UTC near Iceland, just mid-way to Arctic Bay, as confirms DXAtlas. This auroral oval was very active and affected thus high latitude propagation. As it, its intensity and its effects on propagation are not taken into account by the program, because VOACAP has no model to calculate them and no connexion with the outside world to update its data, but other VOACAP-based applications use near-real-time data to forecast these disturbances. Unlike VOACAP, ICEPAC is completed with a special IONED model that uses some functions dedicated to high latitudes of the northern hemisphere, taking into account several events related to aurora. But here also, working offline with median values, ICEPAC can only "see" the high latitude of the VY0 station, and takes only into account an average auroral activity. Its predictions are however more accurate than the ones of VOACAP as shows very well the next example.

Required circuit reliability prediction (Req.Rel.) calculated for September 2000, respectively with VOACAP (left) and ICEPAC (center) for a S/N reliability (SNR) of 38 dB (for a 3.2 kHz SSB signal) and a required circuit reliability SNRxx of 90%, thus valid 27 days per month in average. VOACAP is uncomplete and provides not much infomation contrarily to ICEPAC. We can only see that the SNR is close to 1 dB at 2001 UTC on 14.193 MHz, very bad. In ICEPAC (center) the S/N reliability reaches 5 dB. There are well some openings, but very weak, in the morning on 40 meters. At right same conditions but for a SNRxx of 10% only, thus optimistic but valid only 3 days per month in average. This time SNR reaches 42 dB and bands are open up to 15 meters. Who to believe ? Stricto senso, more than probably the highest reliability, thus the second chart, what in fact was confirmed in the field.

Minimum takeoff angle, aka MTA

It is the minimum takeoff angle of the main lobe of the transmit antenna expressed in degrees. Values accepted are ranging between 0.1 and 40° of elevation, and for simple ray hops only. 

Unfortunately, almost each word of this definition request a comment, and not the least because the takeoff and arrival angles of your signal affect much its strength at target location. Under VOACAP two parameters are badly defined and are subject to interpretation too : the elevation angle considered as fixed, and the angles of arrival that are very inadequate.

First let's see the elevation angle. We explained in the first page that VOACAP inherited some settings from IONCAP, and among them there is the takeoff angle, or rather its updated value. I explain. IONCAP used by default a MTA of 3°. But for their broadcasting activities, VOA engineers designed special variable beam arrays showing considerable gain below 3°. Therefore developers changed the default from 3° to 0.1° of elevation. In theory this is an advantage because using so low angle VOACAP will predict less transmission losses with a shorter path and stronger signal at target location. But is it really realistic ?

In VOACAP the default MTA is set to 0.1° of elevation. What does it mean ? By definition only an isotropic antenna displays in free space the same constant gain of 0 dBi between 0-90° of elevation as shows the hyperlinked graph. But in the field, even an isotropic antenna will be affected by the ground and will more than probably display at 0.1° of elevation a gain almost infinite over a ground plane. This is for this reason that in WinCAP the elevation is set by defaut to a more realistic value of 3.0°, which is already an excellent minimum takeoff angle.

Radiation pattern of a 3-element Yagi placed 7m high for the 20m band calculated with HFANT. The maximum gain is 11 dBi (8.9 dBd) at 29° of elevation and drops to -30 dB, thus to a power ratio 1000 times lower at ground level. For VOACAP, in all cases where your maximum gain is at angles above 3° of elevation, you must set the minimum takeoff angle at 3° only.

What is the right elevation to set in VOACAP ? According George Lane, if your antenna main lobe is over 3°, even at 10 or 20° of elevation you must set the minimum takeoff angle at 3° of elevation only, not more. If you enter the value for your antenna maximum gain, for example 29° fo elevation as display above, VOACAP will reduce the MUF with all consequences on your hops and signal strength.

For example, as show well the two next predictions established for a circuit between Argentina and Belgium in September 2004 (SSN = 27), a MTA of 30° of elevation reduces your chance to work a DX station to practically nothing; the takeoff angle is too high and your signal enters the F2-layer under a too steep angle. The result ? Your signal falls much closer than expected and a small amount of your input power reaches the DX. Your signal strength at target location is only... -233 dBW on 20m, you are in the background hash ! On the same circuit, another antenna showing a MTA of 5° only will show a signal strength of -133 dBW or S4. Knowing that you reduce your power by 50% each time you lost 3 dB, working at 30° of elevation you must expect a signal lost of 41 dB, 4 S-units or a power ratio over 10000 times lower ! 

VOACAP prediction for a low (3°) and high (30°) minimum takeoff angle for a circuit between Hawaii and Belgium in September 2004 (SSN = 27). Where the 20-m band is open in the first case, if you work with a takeoff angle over 30° of elevation, your chance to work DX stations in good conditions are almost null, especially during periods of low solar activities. You signal will be near the background hash and your correspondent will probably lost your signal from time to time as if the band was close.

One must not be clever to understand what that means for you as well as for VOACAP : the band is close, the estimated "MUF" is two or three bands lower than expected and only a kW-class amplifier might help you to work this DX in these very bad conditions. 

At last, there is the problem of the angles of arrival. VOACAP doesn't work with multiple ray hops. One can thus consider that it is almost blind to predict the adequate pattern of arrival angles at target location. Why ? Without PropLab Pro of a similar ray tracing application, you don't know either at what angle sky waves will reach the distant station (all depend on the status of ionospheric layers, your signal power, takeoff angle, multipath, delay, azimuthal spread, among other variables), what will be the absorption level and thus the S/N ratio at receive location, and you ignore what mode are used (ordinary, N, M, etc). These are as many parameters that are bypassed in VOACAP and that will false the prediction to some extents. But this is not all.

Add to this long list that without the Skyloc interferometer system (aka Andrew angle of arrival system) you cannot measure angles of arrival, phase difference and amplitudes of sky waves. For VOACAP, and many other models using single ray hops, the propagation of sky waves is quite simple, using ordinary modes, and angles are fixed in both elevation and azimuth. But reality is far different ! In the field, using the Skyloc system we can show that in a 5-minute interval strongest signals can spread up to 30° of elevation and 3° in azimuth although VOACAP predicts a clustering or rays within a few degrees. 

Multipath Power Tolerance (dB)

It is expressed in percent. Its meaning is simpler that what it seems to state... but here also take a deep breath, Hi ! With words borrowed to physicists, it is the maximum difference in delayed signal power between sky-wave modes to permit satisfactory system performance in the presence of multiple signals. Oops ! It concerns in fact effects of multiple sky-waves pathes using various modes on the strength of your signal. This probability gives you an estimation whether or not multiples sky-wave modes exist within the specified power tolerance and outside the time delay tolerance (see below). A very large tolerance spreads your signal in many pathes and affect thus your signal strength at the target location.

If your working frequency is close to the MUF, there will have always more than one signal path and several hops depending on the distance to travel. In these conditions you can experiment deep QSB (10-15 dB).

Multipath Power Tolerance is ranging between 0 and 40 dB, with a default set at 3 dB. If tolerance is 0 dB, multipath is not considered. This parameter only affects the output parameter MPROB that displays the probability of additional mode in multipath tolerances. So don't expect a change in your SNR graph, HI!

Maximum tolerable time delay

It is also called the multipath delay and is expressed in milliseconds. This parameter is associated to the previous one. It is the maximum difference in delay time between sky-wave propagation modes to permit satisfactory system performance in the presence of multiple signals. Oops ! It is ranging between 0 and 100 ms, with a default set at 0.85 ms. A very large time delay (say over 5 ms) produces a perceptible decreasing of the signal strength. This parameter can be simulated in a program like IONOS from Michael Keller, DL6IAK. This parameter also affects and only the output parameter MPROB.

This achieves the System group set up.


This group includes four fields that characterize the critical frequency height of each ionospheric layer : foE, foF1, foF2 and foEs. 

By default the limit of each layer is already set (a multiplier factor of 1 per default for the three first layers and 0.7 for foEs).

Recall that the critical frequency of a layer is the higher frequency that can be reflected from it at vertical incidence. Sky waves of higher frequencies pass simply right through the layer without be reflected. Note that in this concept we ignored the effect of the geomagnetic field and don't speak at all of sky waves arriving under a low takeoff angle. The MUF depends directly on the critical frequency of the layer (E or F) and of course on the geometry of the circuit (angle of incidence of the sky wave).

Side-view of the ionospheric layers boundaries like displayed in a digisonde plot.

The critical frequency is related to the electron density of the medium :

fc (MHz) = 9.10-6 x Ö N

where fc is the critical frequency and N the electron density expressed in electrons/m3. For example, the electron density at 100 km is roughly 8.1010 electrons/m3, yielding a critical frequency of 2.6 MHz.

The critical frequency of the E-layer is defined by the next relation :

foE (MHz) = 0.9 x [(180 + 1.44 x SSN) x cos(Z)]0.25

where Z is the solar zenith angle, and SSN the smoothed sunspot number.

foE shows the maximum value at the sub-solar point. At the time of the equinox for example, for a SSN of 75, foE = 4.1 MHz for local noon at the equator.

In the same way the critical frequency of the F1-layer at daytime is defined as follow :

foF1 (MHz) = [4.3 + 0.01 x SSN].[cos(Z)]0.2

According digisonde plots, if you estimate that limits of these layers move with an amplitude larger that usual, you might insert a correcting factor in your simulation. But if you are not an expert it is better to never modify this group. The possible consequence ? Adding for example only 10% to the height of the layer associated to the critical frequency foF2, all bands will open and your signals will increase between 1 and 9 S-points, maybe more...

Note. Do not confuse this multiplier with the MUF Boost factor which is a multiplier to apply at night to the F-layer critical frequency, the MUF, when you use an antenna displaying a low takeoff angle. 

Tx Antenna

It allows up to 4 frequency ranges to be chosen with a different antenna for each. By default there are 70 different antenna files available but many additional files are available. Very few are true dipoles or beams. However, the user can create as many custom antenna files that he wants using either the HFANT program described on the last page or any other VOACAP compatible application (I think to NEC-based applications). Usually each application creates its own subdirectory where it stores its antenna files.

The Tx Antenna group permits to define most of antenna properties : the antenna model, its gain, the beam main lobe heading (or select "at Rx" if this is a directional array directed just to the receive antenna), and the transmitter power expressed in kW (thus usually in decimal form, e.g. 0.1 for 100 W).

If your antenna is fixed like most dipoles and cannot be rotated, you must specify the antenna azimuth, the direction of its main radiation lobe. For example, for a dipole tight N-S, usually the main lobe is either at 90° (E) or 270° (W). Select one or another according your circuit (eastward or westward). To achieve this, either you know its radiation pattern and all its properties or run the additional program called HFANT (see next page) or any antenna modeling program.

Knowing the Tx Antenna azimuth, VOACAP will use data shared with HFANT to determine the azimuth difference between the great circle related to the True North (not the magnetic north) and the one you specified. This value is called the "off-azimuth". Then the pattern will be computed for each degree of this sector from the main lobe azimuth of the antenna. For omnidirectional antenna (e.g. vertical or discone), one can ignore the off-azimuth. In theory the same principle applies to Rx Antenna (see below).

The antenna gain input has also an impact of the way that VOACAP calculates its prediction. In the field, performance of an antenna depends, beside of its design, on the antenna height above ground because its radiation pattern is affected by ground properties up to 10l in front of the antenna (and up to 2000 km if we take into account the "control area") . In practically all amateur working conditions (thus not in free space), at 0° of elevation an antenna shows a gain by far less than the 0 dBi of an isotropic. For example, see what happens to the curve of the 5-element yagi displayed below left. At 0° the gain drops of 30 dB (- 30 dBi) ! In fact, in the field practically no antenna develops a 0 dBi at those very low angles. The 5-element Yagi displayed below shows its maximum gain (14.5 dBi) at 10° only (erected close to 31m high or 95')...

Performance of a 5-element Yagi calculated by Ham CAP.

The left chart calculated for a 5-element Yagi with Ham CAP, shows also very well that no sky wave or at least only the weakest (-30 dBi) will leave your antenna at 0.1° of elevation. Using this antenna model on the 20-m band, the value becomes positive (0 dBi) from 1.2° of elevation only, and increases then rapidly to reach 14.3 dBi at 10° as displayed.

But the VOACAP interface works does not request the same inputs parameters as it works in a different way. If you must enter 3° (most of the time) for the minimum takeoff angle, you must enter the maximum gain of the main lobe as calculated by HFANT or another application like Ham CAP, not the value at 3° of elevation. If you set the 14.3 dBi gain at 3° for example your result will be wrong because at this elevation the gain is usually close to -10 dBi, or 24 dB less. Such a difference mean that your power ratio (signal strength) can fluctuate of a 200-factor if you don't take care !

In other words, if your takeoff angle and antenna gain are wrongly set, the prediction will consider either that you use a 2 kW emitter or a QRP station ! Here also, better to set the right values to expect accurate forecasts, Hi !

Note at last, that using an antenna showing a too narrow beam, say below 5-10° wide only in either the vertical or horizontal plane (but very rare in amateur HF installations), the Skyloc interferometer system can show that the signal at receive location can display deep fading due to instantaneous variation in the sky wave arrival angle. The QSB can reach 16 dB or more, the beam spreading up to 8° on both side of the central angle per mode. This factor is not taken into account by VOACAP that consider only single ray hops and predict a clustering of angles arounf the median value or around several medians for multiple ray hop modes. These lack of accuracy during the development of VOACAP affect the prediction of signal strength as well.

Rx Antenna

Contrarily to the previous field, only one receive antenna can be specified. But we have not to forget that VOACAP was written for broadcast purposes first, where emitters usually work with several transmit antennas tuned on specific frequencies to reach far countries in the best conditions to avoid silent zones or signal losses.

In most cases, if we believe the rule that states that "what is good for receive is good for transmit", or "if you can hear him you can work him", then the Rx ant = Tx ant. But to be accurate, like with the Tx Antenna group, if the receive antenna is directional or displays a main lobe, it is recommended to specify its bearing (e.g. the azimuth of the main lobe or to select "at Tx"), the model, its gain, and its power gain expressed in kW also.

Here also, view from the transmitter, the antenna minimum takeoff angle (usually 3°) is taken into account using only single ray hops what affects the prediction of some errors in calculating the signal strength.

Like the Tx Antenna field, the antenna gain is the value of the receive antenna main lobe, not the value at 0.1 or 3° of elevation.


Voilą, that all folks, as should say a well-known animal ! Working top-down, this last and 30th field achieves our settings and ends the input process, at last ! ... Aaah ! (sigh).

As you see there are a lot of information taken into account by VOACAP to establish a forecast, and so much the better ! This is through all these parameters, added to the list of ones ready to be displayed (see next page), that VOACAP and other ICEPAC show all their power. No other amateur program (excepted those using the same engine with or without additional functions) provides such flexibility, synonymous obviously in this case, of complexity in taking into account so many factors that might affect a transmission. Only VOACAP and alike characterize the working conditions with so much details.

However, I have never written that this flexibility and power were synonymous of accuracy. This latter quality is related to algorithms used by the model, and not to the amount of parameters used. In this regard, despite its numerous and various input screens, VOACAP is perfectible as we explained earlier and will tell again in the last page.

But I feel that you have maybe an unresolved question that rises at the horizon : with so many uncertainties and limitations associated to some fields, input errors and other bias, you maybe wonder whether we don't have to give up your chance to get an accurate prediction ? Not at all. You must know that VOACAP works with median values and that you will get a prediction, which accuracy will depend on values assigned to your input parameters. You will never get a result sure with 100% confidence like "you have 100% reliability to reach this VY0 station S-9 with an S/N of 30 dB at 1830 UTC with angles of arrival close to 10° in elevation during 5 minutes". Don't dream, no application, even those running on supercomputers, can provide you such a solution... To caricature I 'd say that for each of these answers you need a dedicated application. In all other cases you will only get the big picture and a statistical answer. Therefore you must very well know limitations and the meaning of each field before to be able to interpret any result.

Now that all inputs are complete and accepted by the system, let's see how to run propagation calculations and display theses results.

Last chapter

Forecasts and outputs

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