Setting VOACAP Input Parameters
After you have launched VOACAP, the first and most important thing is to select and enter right parameter values in the program. Here we discuss the most important steps.
Below you will see the main screen with a set of reasonable input values for a high-power broadcasting station with transmissions to Southern Africa:
Figure 1. The VOACAP Main Screen
These are the basic settings which you can initially set up as follows:
Parameter Recommendation Method Select Method Number 20 (Complete System Performance). If you are running circuits over 7000 km, use Method 30 (Short/Long Path Smoothing). Year This is a user-specified value. It has no effect on calculations. Coefficients Select CCIR (Oslo). This is the default for using VOACAP in long-term predictions. Do not use URSI88 (Australian). Time Select UT (Universal Coordinated Time). Local Mean Time does not mean your local time but the local sun time at the receiver. Transmitter Select the transmitter location from the pre-defined lists, sorted by city (worldwide), by nation (worldwide) and by state (USA). You can also enter the coordinates manually. Receiver Select the receive location from the pre-defined lists, sorted by city (worldwide), by nation (worldwide) and by state (USA). You can also enter the coordinates manually. Path Select Short or Long Path. The default is Short Path. Freq(MHz) Enter the frequencies to be used for calculation. Use the zero (0) value to remove a frequency from the set. A set of 11 user-specified frequencies can be defined. Also three pre-defined frequency sets are available which can be modified if needed. Fprob Do not use all default values. Set foE to 1, foF1 to 1, foF2 to 1 and foEs to 0 (zero, instead of 0.7).
A Note about CoefficientsAs noted earlier, while using VOACAP or VOAAREA, be sure to select the CCIR coefficients. George Lane explains,"Do the URSI coefficients provide more accurate predictions? We do not know. What we do know is that original IONCAP using the CCIR data base, corrects the predicted values by using the Transmission Loss tables and determines the variability of the MUF values over the month using the F-tables. These are huge corrections. They are based on taking the predicted ionospheric calculations using the CCIR data and subtracting them from actual system performance data taken over the 1958 to 1963 time period. The differences are collected into the two sets of tables I just mentioned. The corrections are applied to any path prediction you make using IONCAP in order to predict signal power, SNR, reliability and required power gain. If you change the ionospheric maps for only the F-region and do not recalibrate the predictions against measurement, you do not know if you made an improvement or not. Chances are you have ruined the correlation."
The following are critical user-specified parameters.
Parameter Recommendation Months Enter the number of the month (e.g. December = 12.00). You can specify a total of 10 months at a time. The format is MM.DD (month.day). VOACAP is not, however, suitable for daily predictions. Therefore, do not specify any day since if you do, the value of Coefficients will automatically be set to URSI88. SSNs Enter the monthly smoothed sunspot number for the given month.
Choosing the correct sunspot number (SSNs)There are a number of sunspot resources available on the Net. However, be sure to use the values from the following resource:
- Predicted Monthly Sunspot Numbers (from National Geophysical Data Center)
These are the current values for the year 2003 as of writing:
JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 82.9 79.4 76.9 74.3 71.2 68.7 66.7 64.8 62.6 59.8 57.8 56.7
Parameter Recommendation Man-made noise This is a critical setting. You can accept the default value of 145 (residential environment). If your receive location is in a quiet environment, you can use 155 or higher. There are still other values available, see the input help. Min. take-off angle If you are a radio amateur or SWL, enter the take-off value of 3. For better antenna installations, you can use the value of 1. Req'd circuit reliability Accept the default value of 90%. Req'd SNR This is a critical setting. For a reasonable BC listening quality, use 67. For a reasonable CW reception quality, use 24 (or 27) and for SSB, 45. Multipath power tolerance Accept the default value of 3 dB. Max. tolerable time delay Accept the default value of 0.10 milliseconds.
Selecting the transmitter antenna
Parameter Recommendation TxAnt= Select the transmitter antenna from the pre-defined lists. You must familiarise yourself with the available antenna patterns using HFant program, included in the VOACAP software package. This window is the place where you can define different antennas for different frequency ranges. In the example above, we have defined one antenna for the frequency range of 2 MHz (Min) to 30 MHz (Max). MainBeam Enter the main beam of the antenna (in degrees). at Rx Sets the main beam towards the receive location. TxPower The transmitter power which is fed to the antenna. Usually, we need to assume some power losses in transmission lines, etc. so you may set this value, for instance, to 65% of the transmitter power.
Selecting the receive antenna
Parameter Recommendation Receive Antenna= Select the receive antenna from the pre-defined lists. You must familiarise yourself with the available antenna patterns using HFant program, included in the VOACAP software package. The default antenna SWWHIP.VOA represents a typical shortwave whip antenna. Receiver bearing If it is a directional antenna, enter the main beam in degrees. For non-directional antennas, enter the value of 0. at Tx Sets the main beam towards the transmitter location. Gain This is valid only for isotropic antennas. The gain value is ignored if used with other antenna types.