Understanding VOACAP predictions

Generic notes

Ten Common Mistakes in Using VOACAP. This story is supposed to be taken in a light note although these are the potential risk areas that can turn your prediction upside down.

Field Strength vs. Signal-to-Noise in VOACAP. Field strength or signal power is only half of the story. A poorly designed receive antenna will deny reception of a signal just as though the transmitter had been shut off. A receive antenna in a high noise environment will mask the signal such that no intelligence can be detected.

Choosing the correct Sunspot Number in VOACAP. The National Geophysical Data Center maintains the (predicted) SSN figures based on the Lincoln-McNish smoothing function. These are the sunspot numbers used in the database reduction for the worldwide ionospheric maps used in IONCAP and now VOACAP.

VOACAP Predictions at Low Frequencies. There was very little data below 4 MHz when IONCAP was designed. Still, it has proven to give good results for Near Vertical Incidence Skywave (NVIS) situations.

Comparing VOACAP, ICEPAC and REC533. There was a recent comment about ICEPAC giving better predictions at long distance on high latitude paths than VOACAP. I have not found that to be true but have had only limited experience.

VOACAP Method 25: Ionospheric Parameters and Semi-thickness. The definitions of the Method 25 ionospheric parameters.


The MUF and SNR Distribution: Choosing the Best Frequency in VOACAP. You have now run the prediction and are anxious to operate between the chosen locations on the frequencies you entered. There are two things to discuss in our analysis: What is the best of our frequencies? What is the predicted SNR (Signal-to-Noise) distribution on that frequency?

Understanding Above-the-MUF Predictions in VOACAP. The old rule was that if you went above the MUF, no signal propagated back to earth. Measurements have shown that if the signal power is strong enough, you can get a return signal on the earth at frequencies above the MUF.

Calculating the MUFdays in VOACAP. The practical meaning of FOT and HPF [As obtained from Method 9 in VOACAP] is that they determine the frequency range within which we should find 80% of the measured MOFs.

Maintaining the Required Grade of Service: Calculating the Circuit Reliability for a Given Hour in VOACAP. Let us assume you wish to know on how many days in a month a certain grade of service can be maintained on a given hour between two locations. The grade of service is expressed in VOACAP by the parameter REQ.SNR (the required signal-to-noise ratio in dB-Hz). This is a critical setting that must be considered carefully. The correct value to select is dependent on the transmission mode, and the reception quality we like to maintain.

Signal Power & Noise Power

Summing Signal Power from Multiple Modes in VOACAP. VOACAP is attempting to compute the hourly median signal power values for the 30 days in the month at that hour and SSN. It does this for up to 21 different possible modes [ e.g. 2F2, 3F2, 4F2, 2F1, 3F1, 4F1, 4E, 5E and 6E for the high ray and the low ray modes plus the 4Es, 5Es and 6Es for a total of 21 ].

Calculating the Received RF Noise Power in VOACAP. The gain of your receive antenna is indeed ignored in the calculation of the noise power. The noise power is computed from the CCIR radio noise model where the noise power is assumed to arrive from all directions equally. The use of the theoretical short lossless whip (1/32nd of a wavelength) was to give slightly higher noise for the lower arrival angles.

Antennas & Radiation Angles

VOACAP Method 15, or Transmitter & Receiver Antenna Patterns. It is not necessary to run Method 15 in order to obtain valid circuit predictions. However, it is necessary to input the antenna data correctly in order to obtain valid circuit predictions. It is very easy to generate a wrong antenna pattern and it is very difficult to determine that the antenna pattern is wrong when you are just looking at the circuit predictions.

VOACAP Angle Predictions. The arrival angle predictions made by VOACAP are not so well founded empirically as, for example, the predictions of signal strength. They are fixed values for simple ray hops. Exotic modes such as N and M are ignored, too. The reason is that there was no measured data which would provide a statistical representation of the angle distribution.

Minimum Angle in VOACAP. According to Donald Lucas, using 0.1 degrees gives you a better calculation of the frequency dependence of the ionosphere (i.e. the MUF for the lowest order mode for the circuit). John Lloyd told me the value of 3 degrees was better from a practical standpoint. It depends on the actual horizon clearance at both the transmit and receive site.

Predicting the Takeoff Angle Parameter TANGLE in VOACAP. For path lengths less than 7000 km, the angle prediction takes into account the gain of both the TX and RX antennas. For longer paths (over 7000 km) where the Method 21 can become the dominant propagation mechanism, the best takeoff angle and best arrival angle are computed and printed out based on the ionospheric conditions between 1000 and 2000 km of each end of the circuit.


S DBW to S-Meter Conversion Table in VOACAP. This table shows how S DBW (median signal power expected at the receiver input terminals) values can be mapped to S-meter readings. There are a few assumptions: the receiver input impedance is 50 Ohms and the transmission lineloss is minimal. Also we assume that the S-meter is correctly calibrated. In practice, S-meters are mostly imprecise and can only give relative readings.

The Z Tables in VOACAP. Converting Z values to percentage values and corresponding number of days.