OH6BG Elevation-Angle Analyzer for VOACAP


Updated: 13 November 2011

The OH6BG Elevation-Angle Analyzer (or EAA for short) reads VOACAP output files and generates a statistical elevation-angle data format, which can directly be used in the HFTA (HF Terrain Assessment) program, developed by Dean Straw N6BV. HFTA is available on the latest ARRL Antenna Book CD-ROM. HFTA shows visually how the elevation angles of a horizontal dipole or Yagi or stacked Yagis cover the statistical distribution of elevation angles that are necessary for communication via the ionosphere from one location to another.

We must always remember this simple truth: The ionosphere controls the elevation angles, not our transmitting antenna!

And thus:

  • VOACAP predicts which elevation angles are supported by the ionosphere for communication
  • HFTA graphically shows you how your antennas cover the predicted elevation angles
  • EAA brings these two programs together by generating an HFTA elevation-statistic file from a VOACAP output file

EAA (v1.0, 13 Nov 2011) is available for free downloading for non-commercial personal use. This software is still in beta which means that there can be bugs, and unexpected things can happen. There is no warranty whatsoever. So be warned that you use it at your own risk! Nevertheless, I have used it extensively under Windows XP, and so far, without any major problems.

How does EAA work?

The EAA basically reads VOACAP's textual output files and assumes that the calculations have been made using either Method 25 or Method 30, or both. Other Methods are not supported at the moment. I strongly urge that if you choose to try this software for serious analysis, please use the 081227 (Dec 27, 2008) version of VOACAP.

  • This particular VOACAP version writes the output in a manner that EAA can read all propagation modes (of Method 25) correctly. If you use an earlier version of VOACAP, then EAA fails to read all propagation modes if there are more than 10 of them. This is a bug in EAA but it will not be fixed because the latest version of VOACAP has the fix!

Please note that EAA does not care which input values you have chosen for creating the VOACAP output file. So it is your responsibility to make a good model of your case in VOACAP if you want to get some sensible predictions. However, there are some restrictions on the use of frequencies and the SSN values if you want to make full use of EAA:

  • Only the following frequencies are considered when the HFTA elevation-angle data files are being generated: 3.6, 7.1, 10.1, 14.1, 18.1, 21.2, 24.9 and 28.4 MHz. All other frequencies are ignored for the HFTA files, although the frequencies are read into the database.
  • Use the SSN values of 5, 25, 50, 75, 100 and 150 (the entire solar cycle) if you want to make use of the special filtering options in EAA. Otherwise, all given SSN values are being considered in EAA.

The slow part: Creating the database with REL values > 0

When EAA processes the VOACAP output and builds its database, it only picks those propagation modes (and frequencies) from the output where the predicted REL value is greater than zero. The REL value is in direct relation to the Required Signal-to-Noise ratio (SNR) which is a user-settable value in VOACAP. Typically, I set the REQ.SNR value to 24 dB/Hz which can be considered as the lowest value for the CW mode.

  • To open a VOACAP output file, select "Open VOACAP output..." from the File menu, or use Control-O.
  • In effect, only those propagation modes and their elevation angles are considered where there is at least some hope or probability of achieving the Required SNR.

Reading an output file can take a lot of time, I mean - a lot. While reading a huge output file (say, tens of millions of lines), the user interface of EAA may seem unresponsive for minutes. Please wait patiently, though. Processing such files can take up to 10-15 minutes, depending on your CPU.

The user interface

So, this is EAA:

Let's start by examining the column headings in the upper part of the user interface. These are the bread and butter of your EAA database:

Column heading Explanation
# the record number in the EAA database
TX the name of the transmitter location
RX the name of the receiver location
FREQ the frequency
SSN the Smoothed Sunspot Number (use 5, 25, 50, 75, 100 and 150 only, for filtering purposes)
MONTH the month of the year (please note that the year itself is irrelevant in VOACAP)
HOUR the UTC time (e.g. "1" = 01 UTC, provided UTC was used in VOACAP...)
SDBW the Signal Power at the receive antenna (can be translated to S-Meter values)
TANGLE the predicted take-off angle of the transmit antenna (in degrees) for the predicted MODE, see below.
REL the reliability value i.e. the probability of achieving the user-defined Required SNR (REQ.SNR in VOACAP).
"0.1" = 10%; "1" = 100%
MODE the predicted propagation mode for the given hour, month, SSN, and frequency.
E.g. "2.F2" means two hops via the F2 layer, "3. E" means three hops via the E layer.
TO the minimum take-off angle of the antenna (in degrees, a system parameter in VOACAP).
Also in this cell, the VOACAP Method, either "25" or "30". Not visible in the screenshot.

All columns can be sorted in ascending or descending order, just click on the column heading. This feature makes it easy for you to view the database e.g. by frequency or by Signal Power (SDBW). However, if there are more than, say, 50,000 records in your database, sorting the data takes a lot of time, and there is the freezing effect as described above.

The HFTA Elevation-Angle Data

The lower part of the user interface, named "Elevation angles by band", consists of a textual data window, some special filters (REL, SSN and Month) and a plotting function. The data in the window is in the format that is directly understood by HFTA. With the special filters you can fine-tune your HFTA data file, and there is also an option to graphically view the elevation-angle distribution data by band. The "Plot chart" button and the Band and Scale selector (defaults to "35", the number of elevation-angle markers in the plot) go together.

Let's start with the interesting stuff first by plotting an elevation-angle chart for one band. For instance, when the textual elevation-angle data is visible and you are interested in viewing the elevation-angle data for the circuit on 10M (28.4 MHz), select "10m" on the Band selector, leave Scale selector to "35", and then click on the "Plot chart" button. The following chart will appear in a separate window:

This chart demonstrates the predicted percentage - or statistical distribution - of the elevation angles that the ionosphere can support for the circuit on the 10M band. We can see in the header section of the chart that the solar minimum and maximum (SSN 5-150) on all months of the year have been considered. The REL value used in this chart was 0.1 (10%) which means that all database records where the REL was greater than or equal to 0.1 was taken into consideration.

  • To run an elevation-angle analysis of the EAA database, select "Analysis" from the Run menu, or use Control-R.
  • A new analysis is always run automatically when you choose a different REL (reliability value), a different SSN or a different Month.
  • In practice, in our example, the "REL >= 0.1" means that all propagation modes where there is a probability of at least 10% to achieve the Required SNR on the given circuit have been considered. This probability can also be expressed in the number of days: 10% of one month is 3 days. Therefore, we can express this in other words, "Show me the elevation angles which are supported by the ionosphere for at least 3 days a month at the Required SNR on the given circuit."
  • Try also other REL threshold values, such as "0.5", "0.7" or "0.9". "0.5" means that only those elevation angles are extracted from the database where the probability of achieving the Required SNR is equal to or greater than 50%, or 15 days of the month. "0.7" means a probability of 70%, or 21 days per month. Similarly, "0.9" means 90%, or 27 days of the month.

The Y-axis of the chart show the predicted percentage of the elevation angles. The figures in the bars have been rounded. The X-axis shows the elevation-angle markers from 1 to 34 (degrees). For instance, the elevation-angle marker "1" covers the predicted angles from 1.00 to 1.99 degrees, the marker "2" covers the angles from 2.00 to 2.99 degrees, and so forth.

  • So, this chart says, for instance, that 26% of all predicted elevation angles for this circuit (using the given filters) are between 3.00 and 3.99 degrees (the angle marker "3"). Therefore, looking at the entire chart, if you want to ensure good communication on this circuit, you should build an antenna system that radiates and receives as efficiently as possible at those low angles from 3 up to 7 degrees. In this particular case, these angles cover almost 75% of all predicted elevation angles.

Solar minimum vs. the total solar cycle

By default, the total solar cycle, or all sunspot numbers from 5 to 150 are considered while the HFTA elevation-angle data is being run. This may give you an overall picture of the elevation-angle distribution in general but it is of little help when you happen to be in the sunspot minimum, as we are at the time of writing. Therefore, the SSN pop-up menu offers some helpful preset values to choose from:

  • All (5-150)
  • Low (5-25)
  • Medium (50-75)
  • High (100-150)
  • 5
  • 25
  • 50
  • 75
  • 100
  • 150

You can select an individual SSN number such as "25" which is close to the current predicted SSN value. You can also select a group of SSN values such as "Low" which then considers the SSN values of 5 and 25 in the final analysis.

If you choose an option from the SSN pop-up menu, the analysis is re-run automatically. Please wait until the analysis is completed. Typically it will take only a few seconds.

Specific month(s) vs. all months

By default, all months of the year are taken into the analysis. If your database does not cover all months, EAA will nevertheless consider all available months. For the sake of comparison, it can be useful to run the analysis against just a specific month, or a group of months.

  • The Month pop-up menu allows the selection of either all months or any individual month of the year. It also has two special options: March and May ("Mar and May"), and October and November ("Oct and Nov"). These two special options are offered for the elevation-angle analysis on the months when two major world-wide contests are taking place.
  • If you choose an option from the Month pop-up menu, the analysis is re-run automatically. Please wait until the analysis is completed. Typically it will take only a few seconds.

Saving the elevation data

As soon as you launch EAA for the first time, a small 2KB database, named "ElevationAngleDatabase.rbd", will be created into the directory where EAA is located. This is your elevation-angle database, and if you, after a long EAA run, want to back up your data, just make a copy of that database file, and give it a more meaningful name. Remember to keep the extension of ".rbd".

You can always open any old database you have created with EAA. Just select the "Open database..." option from the File menu. Huge databases may take up to 15-30 seconds to open. Most of the time is spent in populating the database listbox view in the upper part of the user interface.

Saving your data as an HFTA elevation-statistics file

The main purpose of EAA is to create elevation-statistic files for use with HFTA. Choose the option "Save elevation data..." from the File menu, and give your file a meaningful name. Remember that all HFTA statistic files ends with the extension of ".prn". So, don't forget to add it!

Exporting your elevation data into a spreadsheet

The elevation-angle data you have created is not the sole monopoly of EAA. You can export this data into a tab-separated file format which can then later be opened e.g. in Microsoft Excel or OpenOffice Calc. Of course, you can open the file with your favorite text editor as well.

  • To export your data to a text file, choose the option "Export database as text..." from the File menu. The suggested file suffices are ".txt" and ".prn". Just save it with the default extension of ".txt". Change the suffix to ".csv" later if you want to open this file in a spreadsheet program.

Thank you

I wish to thank Dean Straw N6BV for our discussions during the development of this software. He was a great inspiration.

Also a special thank-you to Greg Hand, the maintainer of the VOACAP code, who made changes to VOACAP even on his birthday!