AreaVOACAPWinCAPI ndustrial-140.4dBW-125dBWResidential-144.7dBW-136dBWRural-150.0dBW-148dBWR emote-163.6dBW-164dBWDefaultQRM level associated to the receive location. Note that sincethe release of VOACAP ten years ago, WinCAP Wizard found that it wasmore realistic to increase the QRM level in the most inhabitedareas, a sign of times. Note that -125 dBW is a good S5.Theyare of course theoretical values. Most of the time you even don't know what DXstation will answer to your call and thus neither in what type of area livesyour correspondent or what is his or her QRM level. The QRM can also vanish asfast as it appeared. These approximations canhowever 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 fromcities 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 of20 dB, or 2, a power ratio of 100times! If the band is not in good shape, and whatever the SSN, that means thatit could be close under these conditions.
Thus better to use the appropriatevalue to get here also an accurate prediction.RequiredCircuit Reliability (%), aka SNRxxThis field isprobably the most important and requests some a detailledexplanation. As we just explained, a forecast is a prediction, a probability estimationto work in specified conditions (the 'circuit'). The term 'Reliability' means'time availability'. When you set the required reliability at 50%, it means that theresulting predictions will be as stated or better during the half ofthe 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 characterizesthe expected signal quality that you consider as acceptable. It is maybe validfor you and today, but tomorrow you will maybe prefer working in 'Hi-Ficonditions' as far as possible.Butthen, why to not set SNRxx to 100%, do you wonder?
Well, if you set requiredreliability at, say 90% as suggest VOA engineers, then predictions will be as stated orbetter during 27 days over 30. Theoretically, in average, amateur working conditions match with a90% reliability. In the field, that means that the concerned frequency is highly reliable, likethe 'BUF'.This is however a high valuemainly suited to broadcasters that use 100 kW emitters, or bigcontest teams and so big DX-peditions who work in rocking-chair, Hi! A casual amateur might satisfy with only50%, but of course the higher the best, all the more that the advanced amateurwill probably no more work like a casual amateur. Indeed, the beginner worksoften whatever propagation conditions (even in close bands!), where the OldTimer, experimented, will only switch his or her transceiver when thepropagation will be good to excellent to avoid useless QRN and poor propagationconditions.
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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 seethe change on the signal strength and pwoer at the target location.Inaddition we can say from experimental trials that the reliability reaches90% 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 outputparameter SNRxx. This function is only supported in ICEPAC, included in the samepackage. As we will see later, ICEPAC takes also into account disturbedconditions near polar caps.Required S/N ratio (dB), aka SNRThisis the second very important parameter that must be set correctly. SNR is ranging between -30 and 99dB.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 thespecified bandwidth relative to the hourly median noise in a 1 Hertz bandwidthwhich is necessary to achieve the type and quality of service (mode) required,hence its name.Saysimpler, 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)Forexample, in SSB a signal offering a 3 kHz bandwidth gives a SNR of 45 dB. A 500Hz CW tone is close to 38 dB. A digital mode, using a larger bandwidth, will give a still higher ratio.VOACAPuses a default value of 73 dB what is qualified as 'Good'.
Hereare 'milestone' values associated to the SNR. Service(mode)BadPoorFairGoodExcellentBCL10 kHz.
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 requiredcircuit reliability SNRxx of 90%, thus valid 27 days per month in average.VOACAP is uncomplete and provides not much infomation contrarily toICEPAC. 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 on40 meters. At right same conditions but for a SNRxx of 10% only,thus optimistic but valid only 3 days per month in average.
This timeSNR reaches 42 dB and bands are open up to 15 meters. Who to believe?Stricto senso, more than probably the highest reliability, thus thesecond chart, what in fact was confirmed in the field.Minimumtakeoff angle, aka MTAItis the minimum takeoff angle of the main lobe of the transmit antenna expressedin degrees. Values accepted are ranging between 0.1 and 40° of elevation, andfor simple ray hops only.Unfortunately,almost each word of this definition request a comment, and not the least becausethe takeoff and arrival angles of your signal affect much its strength at targetlocation. Under VOACAP two parameters are badly defined and are subject tointerpretation too: the elevation angle considered as fixed, and the angles ofarrival that are very inadequate.Firstlet's see the elevation angle. We explained in the first page that VOACAPinherited some settings from IONCAP, and among them there is the takeoff angle,or rather its updated value. IONCAP used by default a MTA of 3°.
Butfor their broadcasting activities, VOA engineers designed special variable beamarrays showing considerable gain below 3°. Therefore developers changed thedefault from 3° to 0.1° of elevation.
In theory this is an advantage becauseusing so low angle VOACAP will predict less transmission losses with a shorterpath and stronger signal at target location. But is it really realistic?InVOACAP the default MTA is set to 0.1° of elevation. What does it mean? Bydefinition only an antenna displaysin free space the same constant gain of 0 dBi between 0-90° of elevationas shows the hyperlinked graph.
But in the field, even an isotropic antenna willbe affected by the ground and will more than probably display at 0.1° ofelevation a gain almost infinite over a ground plane. This is for this reasonthat in WinCAP the elevation is set by defaut to a more realistic value of 3.0°, which isalready an excellent minimum takeoff angle. Radiationpattern of a 3-element Yagi placed 7m high for the 20m bandcalculated with HFANT. The maximum gain is 11 dBi (8.9 dBd) at29° of elevation and drops to -30 dB, thus to a power ratio 1000 times lower atground level.
For VOACAP, in all cases where your maximum gain isat angles above 3° of elevation, you must set the minimum takeoffangle at 3° only.Whatis the right elevation to set in VOACAP? According George Lane, if your antennamain lobe is over 3°, even at 10 or 20° of elevation you must set the minimumtakeoff angle at 3° of elevation only, not more. If you enter the value foryour antenna maximum gain, for example 29° fo elevation as display above,VOACAP will reduce the MUF with all consequences on your hops and signalstrength.Forexample, as show well the two next predictions established for a circuit between Argentinaand Belgium in September 2004 (SSN = 27), a MTA of 30° of elevationreduces 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 ofyour input power reaches the DX. Your signal strength at targetlocation is only.233 dBW on 20m, you are in the background hash!
On the samecircuit, another antenna showing a MTA of 5° only will show a signalstrength of -133 dBW or S4. Knowing that you reduce your power by 50% each timeyou lost 3 dB, working at 30° of elevation you must expect a signal lost of 41dB, 4 S-units or a power ratio over 10000 times lower! VOACAPprediction for a low (3°) and high (30°) minimum takeoff angle for acircuit between Hawaii and Belgium in September 2004 (SSN =27). Where the 20-m band is open in the first case, if you workwith a takeoff angle over 30° of elevation, your chance to workDX stations in good conditions are almost null, especially duringperiods of low solar activities. You signal will be near thebackground hash and your correspondent will probably lost yoursignal from time to time as if the band was close.Onemust 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 lowerthan expected and only a kW-class amplifier might help you to work this DX inthese very bad conditions.Atlast, there is the problem of the angles of arrival.
VOACAP doesn't work withmultiple ray hops. One can thus consider that it is almost blind to predict theadequate pattern of arrival angles at target location. Without PropLab Proof a similar ray tracing application, you don't know either at what angle skywaves will reach the distant station (all depend on the status of ionospheric layers, yoursignal power, takeoff angle, multipath, delay, azimuthal spread, among other variables), what will be theabsorption level and thus the S/N ratio at receive location, and you ignore whatmode are used (ordinary, N, M, etc). These are as many parameters that arebypassed in VOACAP and that will false the prediction to some extents. But thisis not all.Addto this long list that without the Skyloc interferometer system (aka Andrewangle of arrival system) you cannot measure angles of arrival, phase differenceand amplitudes of sky waves. For VOACAP, and many other models using single rayhops, the propagation of sky waves is quite simple, using ordinary modes, andangles 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 intervalstrongest signals can spread up to 30° of elevation and 3° in azimuth althoughVOACAP predicts a clustering or rays within a few degrees.MultipathPower Tolerance (dB)Itis expressed in percent.
Its meaning is simpler that what it seems tostate. But here also take a deep breath, Hi! With wordsborrowed to physicists, it is the maximum difference in delayed signal powerbetween 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 variousmodes on the strength of your signal.
This probability gives you an estimationwhether or not multiples sky-wave modes exist within the specified power tolerance and outside the time delaytolerance (see below). A very large tolerance spreads your signal in many pathesand affect thus your signal strength at the target location.Ifyour working frequency is close to the MUF, there will have always more than onesignal path and several hops depending on the distance to travel. In theseconditions you can experiment deep QSB (10-15 dB).MultipathPower Tolerance is ranging between 0 and 40 dB, with a default set at 3 dB. Iftolerance is 0 dB, multipath is not considered.
This parameter only affectsthe output parameter MPROB that displays the probability of additional mode inmultipath tolerances. So don't expect a change in your SNR graph, HI!Maximumtolerable time delayItis also called the multipath delay and is expressed in milliseconds.This parameter is associated to the previous one.
It is the maximum differencein delay time between sky-wave propagation modes to permit satisfactory systemperformance 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 largetime delay (say over 5 ms) produces a perceptible decreasing of the signalstrength. This parameter can be simulated in a program likefrom Michael Keller, DL6IAK. This parameter also affects and only the output parameter MPROB.Thisachieves the System group set up.FProbThisgroup includes four fields that characterize the critical frequency height of each ionospheric layer: foE, foF1, foF2 and foEs.Bydefault the limit of each layer is already set (a multiplier factor of 1 perdefault for the three first layers and 0.7 for foEs).Recallthat the critical frequency of a layer is the higher frequency that can bereflected from it at vertical incidence.
Sky waves of higher frequencies passsimply right through the layer without be reflected. Note that in this conceptwe ignored the effect of the geomagnetic field and don't speak at all of skywaves arriving under a low takeoff angle. The MUF depends directly on thecritical frequency of the layer (E or F) and of course on the geometry of thecircuit (angle of incidence of the sky wave).