VOACAP Method 15, or Transmitter & Receiver Antenna Patterns

Q: I am learning VOACAP and have questions about Method 15. It is said that we should always compute the antenna patterns with Method 15 when we are analyzing a circuit for the first time.

George Lane: 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. Therefore, I recommend that users always run Method 15 when beginning an analysis to make sure the pattern for the transmit and receive antennas look reasonable for the antenna you are trying to model.

The pattern is shown as a table of gain values in dB relative to an isotrope in free space for the great circle route azimuth defined by the geographic coordinates you specified for the transmit and receive locations. The pattern is shown by elevation angles with 0 degrees being along the horizon and 90 degrees being straight up (y-axis) and by frequencies from 2 to 30 MHz (x-axis). For example, if you are modeling a horizontal dipole antenna at one quarter wave length above ground, you would expect to see the maximum gain of 5 to 8 dBi at the high elevation angles and negative gain values at the low elevation angles. Also the pattern should be relatively constant from frequency to frequency across the table.

Q: I cannot understand the meaning of "off-azimuth".

George Lane: When you input the antenna data, you must specify an antenna azimuth or select "at" the transmit or receive site. If you specify an azimuth, then the program will determine the difference between the great circle route azimuth and the one you specified. Then the pattern will be computed for that many degrees from the mainbeam azimuth for that antenna. For some antennas which are omnidirectional, one can ignore the off-azimuth. The isotrope, whip and discone are some examples of omni-directional antennas.

The antennas do have a real azimuth and that should be entered into VOACAP. Remember that VOACAP uses azimuths relative to True North. Often the azimuth of the antenna as built is specified in terms of Magnetic North as obtained from a compass reading. You will need to correct magnetic azimuths to true north azimuths using the correct declination angle for that geographic location.

Q: It is difficult to visualize the antenna patterns, as Method 15 gives only textual, not graphical, results.

George Lane: When you want to generate an antenna pattern for the antenna of interest, you should use HFANT. With HFANT you can see the pattern plotted out on a graph. When you have obtained patterns that you feel represent the antenna in question, be sure to name the antenna and save it in your antenna folder. Then when you run VOACAP, you can pull that antenna design from your antenna file. I still recommend that on the first run for that circuit, you run Method 15 just to make sure the right patterns are being called.

I remember being called by an individual who was getting very poor predictions on a path that should have worked very well with a horizontal dipole antenna. After much work, I noticed the dipole was nearly 90 degrees off-azimuth! It seems he thought the mainbeam azimuth of the dipole was the direction of the wire and not broadside to it. If he had run Method 15, he would have soon discovered that the dipole was not working the way it should.

Q: I have calculated MUF, SNR, etc. without calculating the antenna pattern with Method 15. Is this fatal to the results?

George Lane: NO. The MUF is dependent on the circuit, time of day, month and sunspot number and is independent of the antenna, transmitter power, etc. But, SNR, REL, RPWRG, Sig PWR, and DBU are dependent on the system parameters which include the antenna patterns.