Minimum Angle in VOACAP
Q: Which Minimum Take-off Angle should we use for the antennas? 0.1 degrees or 3 degrees? Something else?
George Lane: 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. If you are using a realistic radiation pattern taking into account any horizon obstructions, then the low angles below 3 degrees will disappear anyhow. The reason for this is that gain at angles below 3 degrees for antennas over real ground is very low and also because John Lloyd programmed IONCAP (now VOACAP) to favor angles of 3 degrees or higher.
So Lucas is right (use 0.1 degrees for minimum angle) when the radiation patterns for the antenna have very low gains at near-horizon angles. But if you don't know the low angle gain and/or are using isotropes, then Lloyd is right (use 3.0 degrees for minimum angle). If you see many cases where TANGLE is below 3 degrees, you should suspect you have used erroneous input somewhere.
Q: Is there something inherent to VOACAP to make the model always pick the lower takeoff and arrival angles?
George Lane: The simple answer is: Yes, the model likes the lower angles because the path length is less and the transmission losses are less.
The more complicated answer: Yes, VOACAP can miss the most reliable mode which is at a higher angle. How can this be? We have to remember that VOACAP only considers 3 modes for each ionospheric layer. So if VOACAP starts with 2F2 mode, then only the 3F2 mode and the 4F2 mode will be considered for the most reliable mode. But let's say that the antenna patterns have their maximum gain at higher angles, will the 5F2 mode be considered. No, it will not.
What can we do to prevent VOACAP from finding the 'best' mode for the antennas we are using? The answer is fairly simple. If both the transmit and receive antenna have positive gain in dBi at angles below 3 degrees and there are no horizon obstructions, then one sets the minimum angle to 0.1 degrees in the input to VOACAP. If the transmit and receive antennas have patterns that combine to give maximum gain at angles above 3 degrees, then set the minimum angle to 3.0 degrees.
Let's say I have a site which has an obstruction at the horizon of 9 degrees, should I use a minimum angle of 9 degrees? No! When you specify minimum angles above 3 degrees, you can make the program reduce the MUF. This causes the program to make all kinds of stupid predictions. Many people think that the minimum angle is used to account for horizon obstructions and that is not true. The program lets you decide whether you can use angles of less than 3 degrees but should not be used to block higher angles above 3 degrees as that modifies the ionosphere. The best solution is to modify the antenna pattern for horizon obstructions using a program such as NEC.
Using a Minimum Angle of 0.1 vs 3 degrees
This is getting confusing I am sure. Let's take an example of a path from London to Raleigh NC. If I set the minimum angle at 0.1 degrees for antennas having maximum gain at angle well above 3 degrees, then I find that the most reliable mode is the 4F2 mode with takeoff and arrival angles of 15 degrees with a median SNR of 19 dB*Hz. The MUF is 16.4 MHz and the operating frequency is 7 MHz. One would suspect that this circuit should not be very reliable. But actual experience has shown that 7 MHz at 01 UT from EU to NC is very good.
So I set the minimum angle to 3.0 degrees in VOACAP. Now for these antennas with higher takeoff angles, I find the program selects the 5F2 mode at 21 degrees as the most reliable mode. Why did this happen? Before the program had found the 2F2 mode at 2.4 degrees as the lowest order mode. Then it applied the 3 hop rule and looked at the 3F2 and the 4F2 modes and stopped. When I used the 3 degree minimum angle, the 2F2 mode was blocked, so VOACAP started with the 3F2 mode at 9.4 degrees and then found the 4F2 mode at 15.2 degrees and the 5F2 mode at 20.7 degrees. The 5F2 mode has a median SNR of 23 dB*Hz. This is getting close to a SNR that can be used for reception of manual CW between trained operators. Also the MUF remained at 16.4 MHz for the 2F2 mode.
I then set the minimum angle to 9.4 degrees to notch out the 3F2 mode. This caused VOACAP to reduce the MUF from 16 MHz to 13 MHz. That is not good as we have now modified the ionosphere with an obstruction angle. THAT IS A NO-NO!
Conclusion
For most amateur applications, one should use a minimum angle of 3.0 degrees. The default value in VOACAP is 0.1 degrees. The reason for this is that the Voice of America spends a lot of money to build antennas that do produce considerable gain at angles below 3 degrees so we changed the IONCAP default from 3 degrees to 0.1 degrees for that reason.
A note for VOA: If you use one of your variable beam arrays in the high angle mode, you should change the minimum angle from 0.1 to 3 degrees!
A word of caution about isotropic patterns: Never use an isotrope with the minimum angle set to 0.1 degrees. The reason is that a gain of 0 dBi at 0.1 degrees is almost infinite gain from an antenna over a ground plane. VOACAP will lock on to that impossible gain and make predictions for modes that are really below the horizon. The program will ignore higher angle modes which are above the three hops which the program will consider.