Predicting the Takeoff Angle Parameter TANGLE in VOACAP
Q: Are both the TX and RX antennas considered while predicting the takeoff angle (TANGLE) of the transmit antenna?
George Lane: For path lengths less than 7000 km, the angle prediction takes into account the gain of both the TX and RX antennas. Remember that VOACAP develops a reflectrix table of possible takeoff and arrival angles which will be supported on that path hour and operating frequency. The program finds the most reliable mode and its associated angle based on combining the path loss for the possible angles and the combined gain of the TX and RX antenna at each possible angle. For longer paths (over 7000 km) where the long-path calculation model (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.
Q: So, from a modelling point of view, is it better to use an isotropic RX antenna with +6 dBi gain, for instance, when we actually do not know all RX antennas, or should we just choose an antenna that could be perhaps most likely?
George Lane: Good question. There are several factors. First, we don't know what type of antenna is being used at the distant station. The directivity pattern is probably similar to a dipole but we do not know how well the antenna performs at the low angles nor do we know if the antenna is pointed at our location. It is the similar problem when you are dealing with ground-to-air HF communications. I have found engineers spend great sums of money to obtain accurate models of the aircraft antenna radiation pattern. But in practice, you don't know the orientation of the aircraft with respect to the ground station. So when in doubt about the distant station radiation pattern, the isotropic pattern is a reasonable first assumption. Any antenna placed over earth produces an average gain of 3 dBi (half of the power is radiated into the hemisphere and the other half is reflected from the ground for a net gain of 3 dB over that of an isotrope in free space). Therefore, I would recommend using up to a 3 dBi omni-directional pattern (isotrope +3 dBi).
One must be careful with isotropes, however. Zero dBi or +3 dBi at 0.1 degrees elevation angle is a tremendous amount of gain compared to what is possible over real earth (-20 to -30 dBi is more realistic). With such gain at low angles, one will force VOACAP into erroneously predicting low angle modes with a very high signal power. To minimize the effect of this high gain at low angles, one should set the Minimum Angle at 3.0 degrees. Even a gain of 3 dBi at 3 degrees above the horizon requires a very good antenna.
The second part of your question is whether or not there is a better antenna assumption for the distant station. If you are only dealing with a point-to-point circuit, one could assume, for example, a half wave dipole at 30 feet in height as a typical antenna. However, if you are using VOAAREA, one must specify a bearing angle for the receive antenna. With the assumed dipole, this bearing will be used at each grid point for the receive dipole. That means the radiation pattern of the assumed receive antenna will vary from one grid point to the next. One could use Inverse VOAAREA which would allow one to point the dipole at the grid points toward your receive location. But then again, one is faced with the problem of what bearing you should use for your antenna. I still prefer VOAAREA using an isotrope with up to 3 dB additional gain and a Minimum Angle set at 3.0 degrees for the general case when you don't know the details of the distant stations.