VOACAP Type 13 Files

L. B. Cebik, W4RNL

Between antenna modeling via NEC or MININEC and ionospheric propagation prediction software, we find a nexus that goes under a deceptively simple title: the type-13 file. In these notes, we shall look at three questions. What is a type-13 file? Why is it important for at least some modelers to develop such files? How can we make a type-13 file within NEC that is compatible with the most common propagation programs? Our account will be very general relative to the first two questions, since our focus will be on the modeling aspects of the Type 13 file.

What is a type-13 file?

Like modeling cores, such as NEC and MININEC, propagation software has one or two primary calculation cores with embellished implementing software. The older core is IONCAP, although the most commonly used package is VOACAP. In 1985, the Voice of America (VOA) adopted the Ionospheric Communications Analysis and Prediction Program (IONCAP) as the approved engineering model to be used for broadcast relay station design and antenna specification. As the program was modified for these purposes, the name was changed to the Voice of America Coverage Analysis Program (VOACAP) to distinguish it from the official National Telecommunications and Information Administration (NTIA) IONCAP program. The Fortran code for VOACAP is readily available, allowing a number of implementations, such as ACE-HF, available from antenneX (Note: This product was dropped by antenneX).

Although developed for shortwave broadcast interests, the VOACAP program is equally useful for predictions of ionospheric propagation conditions governing long-range two-way communications in the HF range. Hence, we find the program widely used in government, military, commercial, and amateur installations designed for such communications. Within these installations, there are almost innumerable different antennas in use, too many for any program to contain as samples. Within VOACAP, we define an antenna not solely by its geometry, but as well by its height above ground (including ground mounted monopoles with various types of radial systems) and the quality of ground beneath the antenna at a specific frequency. In fact, VOACAP is not interested specifically in the antenna geometry, but in the far-field radiation pattern produced by the antenna at the selected frequency. Geometry (including electrical features that affect the far-field), height, frequency, and ground quality together determine the far-field pattern for a station interested in propagation predictions.

To make the most accurate predictions of propagation potentials for a given station, VOACAP requires a frequency-specific radiation pattern file for any subject antenna. The file must meet certain standards. It must provide a 360-degree azimuth pattern in 1-degree increments. The azimuth pattern must proceed in compass-rose order, that is clockwise from the starting point--ordinarily North or 0 degrees. For each azimuth increment, the file must list the signal gain in dBi for each elevation angle from the horizon to the zenith in 91 entries. The result will be an ASCII file that is over 250 kB long. Moreover, virtually all implementations of VOACAP require the older file-name entry of no more than 8 characters, with a file extension of no more than 3 characters. In most cases, the extension will be .13.

The file has several other requirements, illustrated by the partial file in Fig. 1. The figure shows only the first 3 azimuth headings of the 360 required by the file, but the remaining entry groups follow the same pattern as those shown. The filename is TF50280C.13, indicating a terminated folded dipole antenna that is 50 m long over average ground. The file is one of a large series of type-13 files for this antenna, one for every MHz of the anticipated operating range. Since terminated folded dipoles come in a considerable variety of lengths (and other details), the file name should use a code that allows ready identification of the antenna, the frequency, and the ground quality (where C indicates average ground with a conductivity of 0.002 S/m and a permittivity of 13 on the scale used in this particular coding system).

For any group, the initial entry is -99.99 dBi, indicating an elevation angle of 0 degrees. The final entry in each group is the same, since every 90-degree elevation or zenith angle records the same far-field direction and hence the same gain. Note the internal grouping limits and the spacing required to have a file that is readable within VOACAP.

Equally important are the initial entries. The first entry is a limited space for recording antenna details. The next 4 entries are standard except for the frequency entry, which should indicate the frequency for this particular file. Note again the spacing of the entries from the left edge to ensure that VOACAP can read the data correctly.

Although this antenna creates a series of files at 1-MHz intervals (from 2 through 30 MHz), a developer of type-13 files should use the specific operating frequencies of operation. In general, a single frequency within each amateur or similarly narrow band will suffice for accurate propagation forecasts. Consult the applicable directions within specific implementations of VOACAP for recommendations on how to correlate type-13 files with the actual use of the propagation prediction software.

Why is it important for at least some modelers to develop type-13 files?

Accurate propagation forecasting depends upon using a reasonably accurate far-field projection of the actual antenna used at the site that is interested in such forecasts. The term "reasonably accurate" is subject to all manner of external considerations. For some very generalized applications, one of the sample models usually included in the VOACAP package may be sufficient. However, the variety of antennas available and in use precludes replication of all of them at all heights and over all types of ground. Hence, a customized collection of type-13 files may be necessary.

Ideally, one should develop a model of the entire antenna installation so as to show all potential interactions among the antennas and relevant non-antenna objects. This extensive modeling is practical under two conditions. First, all antennas should be fixed (that is, not rotatable). Second, the modeling program must have a very large maximum segment count in order to include all antennas and relevant objects. In most cases, practical models will include only the subject antenna.

However, subject antennas should be modeled accurately. A generalized label, such as "3-element Yagi," may not be specific enough for critical applications. Fig. 2 shows the azimuth patterns in free space of 2 3-element Yagis at the same frequency. The difference between the two is the boom length (and its consequences for element placement and length). The result is a full dB difference in forward gain.

Under some conditions, the use of a single frequency within an amateur band may not suffice. Some antennas show relatively equal gain across and amateur band, while others may show considerable differences between the low end (CW and digital) and the high end (SSB). Compare the gain curves for the two Yagis of similar boom length in Fig. 3. The gain values are identical at mid-band. However, the 6-element version varies only slightly from one band edge to the other. In contrast, the 5-element version varies in gain by nearly a full dB from one band edge to the other. For maximum accuracy, if needed, one might wish to create separate type-13 files for the 5-element Yagi, one for the lower end of the band and one for the upper end.

One questionable presumption used by many amateur operators is to treat tri-band Yagis of similar boom lengths as having similar characteristics. Fig. 4 shows the azimuth patterns of two different designs at 100' above average ground. The figure lists the modeled maximum gain values for each design at the TO angle (10, 7, and 5 degrees for 20, 15, and 10 meters, respectively). Although both designs use 24' booms, the band-to-band performance is quite different. The differences include not only the maximum gain on each band, but also the rearward lobe performance.

The illustrations make a case for developing type-13 files for the specific antenna in use at an amateur station at the height and over the ground that applies to the site. The modeled performance may differ considerably from the values used in antenna specification sheets. These considerations also apply to non-amateur antenna installations. Commercial and governmental installations often assume that vendor specifications sheets are precise or that calculations by internal engineering staff are transferable without checking to propagation programs. In most cases, a better procedure would be to model each antenna, using the actual values for height and ground conditions, with a single modeling core. For non-amateur use, NEC-4 may be the most generally usable package, since it allows the modeling of buried ground radials for any monopoles at the site. We cannot assume that propagation predictions are "accurate enough" using program samples until or unless we compare the results with those obtained from more precisely modeled versions of the site antennas. Of course, once we have more precisely modeled antenna far fields, we need not make the comparison, since the resulting type-13 files will take precedence.

How can we make a type-13 file within NEC that is compatible with the most common propagation programs?

1. The first step in developing a VOACAP type-13 file is to orient the antenna properly. Using ACE-HF as an example of a VOACAP propagation forecasting and analysis program, we may heed the following guidelines.

1. The software assumes that all antenna patterns (or mathematical antenna models) have their main beam energy pointed at zero degrees azimuth (north).

2. For a rotatable beam, like a Yagi or log-periodic, the user simply sets an azimuth angle after choosing the directional antenna model. The angle is on a spinner that can be set from 1 to 360 degrees. This action points the antenna toward a distant target along a great circle line, just as a real operator would point the antenna at his station.

3. To simplify the setting, there is a "Point At" control, which when checked, automatically points the antenna toward the distant station along a predetermined path. There are independent controls to do this with antennas at both ends of a circuit.

4. For the case where a station uses a directional antenna but leaves it at a fixed setting, then the user sets the azimuth to his preferred direction and does not use the "Point At" control. This means that stations not on his predetermined great circle path will receive radiation off the side of the antenna's main beam, and will be so simulated.

5. For fixed directional antennas, like a horizontal rhombic or a sloping V, the user must know the physical direction in which his antenna's main beam is facing. He then merely sets the azimuth control to that fixed angle and avoids the "Point At" control.

6. For fixed high-gain directional antennas like the curtain dipole arrays used in International Broadcasting, the azimuth-angle control may be used to simulate the use of phased feeds to create slew angles. In that case, the slew angles are usually expressed with respect to the main beam's nominal angle, so they must be added (or subtracted) from that nominal angle. (It is, of course, an approximation to "slew" such models by varying the azimuth setting in this manner. For more accuracy, use separate models for each slew angle, since patterns for each slew angle may vary slightly from the broadside pattern.)

The obvious consequence of these guidelines is that initial type-13 files should point North to 0 degrees azimuth if the antenna is directional. For bi-directional arrays, such as a lazy-H or a W8JK flattop, one of the two main lobes, which are symmetrical on each side of the antenna-wire plane, should point North. There are a number of nearly symmetrical arrays, such as unterminated long wires, Vees, or rhombics having several wavelengths of wire per side. In these cases, the end with the higher gain, normally away from the feedpoint, should face north. Vertical arrays with more than one (omni-directional) element should also be set into type-13 files with the main-beam lobe facing north, with one possible exception. A number of broadcast arrays undergo development using compass-rose azimuth bearings and directions--often figured from one of the elements. Hence, they already have fixed geometric characteristics that correspond to world map standard. One might create a type-13 model directly from the developmental (and licensing) model, with the understanding that the subject antenna should make use of no azimuth-changing controls available within the VOACAP program. There are a number of vertical arrays with switchable main lobes, such as the 4-square and similar phased arrays. The modeler faces some alternatives in this type of case. One is to create a single model with the main lobe pointed North and then to use program controls to point the lobe in one of the four main directions corresponding to the switching arrangement. A second alternative is to create 4 separate immovable models, one for each of the main lobe directs referenced to a compass rose.

The final class of cases does not readily admit to any primary direction. A center-fed doublet and a terminated folded dipole represent one subclass of this group. In this kind of case, one may create a single model and set the antenna wire lengthwise along one of the compass axes. Then, one would use the azimuth-changing control to orient the wire to reflect its position on the actual site. This procedure would be necessary if one uses a pre-set collection of files, such as the set of terminated folded dipoles included with the ACE-HF package. Alternatively, one may create a fixed antenna model with the wire length having the actual compass directions used at the site. This model would require that the propagation software user make no changes to the azimuth. A second subclass emerges when we use off-center feeding. When such a wire antenna is 1/2 wavelength long, its pattern is virtually identical to the pattern of a center-fed antenna of the same length. However, as the operating frequency increases, the patterns of an off-center-fed antenna depart from the center-fed pattern, but are not identical to the patterns of an end-fed unterminated wire (the so-called end-fed Zepp). Since the patterns at many operating frequencies will be asymmetrical, the modeler and the propagation software user must be very careful that the final orientation of the antenna corresponds to the physical layout. Otherwise, the stronger lobes of the model may not reflect the stronger lobes of the real antenna.

If an antenna site has multiple antennas of different types, such as some that are rotatable, some that are fixed, and some that are switched, all propagation software users should be alerted to the rules that apply to each antenna at each frequency within the collection of type-13 files for which propagation analysis may be relevant.

2. The second step is to coordinate the compass-rose bearing for the antenna, even if simply pointed North to 0 degrees compass azimuth, to the modeling software to be used. NEC operates by using phi angles that count counterclockwise from a 0-degree point that corresponds to the X-axis of the wire layout in the model. (NEC also uses the theta convention, but the simple conversion to elevation angles is normally an automated feature in NEC implementations.) To create a type-13 file correctly--taking into account any asymmetries in the pattern--the software must be able to convert to a compass-rose or clockwise azimuth pattern.

The required conversion may occur in one of two general ways. Programs like NEC-Win Plus employ a polar plot graphic that places the X-axis in a vertical position and labels the top point as zero degrees. Hence, for a directional antenna such as a Yagi, the modeler simply lays out the elements that are broadside to the main directional lobe along the +/-Y-axis. When creating a type-13 file, the program "merely" interrogates the NEC output data for the radiation pattern in reverse order.

EZNEC allows the creation of type-13 files at its Plus and Pro levels (beginning with Version 5.0 of the program). However, EZNEC creates polar plots using a different convention, with the X-axis aligned horizontally in the plot, so 0 degrees is to the right. This convention places 90 degrees, which corresponds to the +Y-axis, at the top. The program offers within the polar plot function a compass-rose alternative with 0 degrees at the top, but the direction still corresponds to the +Y-axis. Therefore, to use this option and to create a pattern with the main lobe pointing North, the modeler must set the Yagi elements along the +/-X-axis. Fig. 5 contrasts the two conventions by showing the same antenna oriented each way and the resulting polar plot using the compass-rose pattern option. There is a shortcut relative to type-13 files and we shall discuss this mode of model creation as we proceed in step 3.

3. Creating an EZNEC VOACAP type 13 file is the final step in the process. EZNEC's latest version provides perhaps the easiest means of creating type-13 files that are compatible with virtually all versions of IONCAP and VOACAP. The process begins by setting the antenna at the desired height above a real ground that best approximates conditions at the antenna site. The sample antenna will be a 3-element Yagi at 100' above average ground.

As indicated by the starred items in Fig. 6, the next requirement is to select a 3-D pattern and to set the increment to 1-degree, as required by the VOACAP file. The resulting plot, shown as an inset on the EZNEC main screen, is not usable in determining lobe structures. A more normal step for that work would be a 5-degree increment. However, our goal is not to analyze the lobe structure, but instead to produce the type-13 file. The plot is clear enough to reveal that the model has its main lobe directed along the Y-axis for direct use with the compass-rose set-up.

After the calculation is finished, there are three places where you can initiate the file writing action. If you chose the 3-D plot option, open the File menu in the 3D Plot Window, and select Write IONCAP/VOACAP File. If you chose the far-field table option, you can click the Write IONCAP/VOACAP File button at the lower left of the formatting dialog box which opens when the calculation is complete, or you can choose a format and use the option to write the IONCAP/VOACAP file in the File menu of the tabular data display. We have chosen the Plot rather than the Table option to verify that we have everything in the model correctly oriented. Therefore, we shall open the File option within the 3-D Plot window.

The program will offer an option to "Write IONCAP/VOACAP File." Had we chosen to create a table rather than a plot, we would have received the same option. When we select this option, the program offers a default directory for storing such files, although the user may select a different directory. Use the earlier notes to give the file a distinctive name, perhaps identifying the operating frequency and the antenna type, with possibly a ground code, if relevant. EZNEC will add the extension .13. The program will write the file, virtually instantly on most modern computers. Fig. 7 shows the first 3 degrees of azimuth for our sample model with the main lobe oriented toward North as defined by the +Y-axis, corresponding to the EZNEC convention for compass-rose patterns. The antenna description line is seriously deficient for use in any serious context.

The same process can be used with the pattern that we obtained when we oriented the main lobe along the X-axis. We assign a ground, select a 3-D pattern with an increment of 1-degree and obtain the 3-D pattern shown in Fig. 8. Again, the pattern itself has only one main use: to keep us informed about where the main lobe lies.

When we select the option to write the type-13 file and choose a directory and filename, EZNEC will flash a new screen that only appears when the main lobe bearing does not coincide with the compass-rose North bearing. Fig. 9 shows this screen, which gives us the option of letting the plot value of 0 degrees be North or of setting the main lobe's maximum gain bearing to be North. Since we are dealing with a rotatable beam, we select the pattern maximum as the file's 0-degree bearing.

If we had set in place an antenna having a fixed position and had already used coordinates that correspond with the real antenna, we likely would have received the same set of options. However, we would have chosen to let the plot North be the file's 0-degree bearing. Likewise, we might have set up a fixed antenna for multi-band use with the prospect of rotating it to its fixed position within the propagation program. For such an antenna, we might have files at many frequencies, reflecting a wide range of use. The patterns for each frequency would differ. In such a case, each frequency's type-13 file would again opt to let North = 0 degrees. Using the option of allowing the pattern maximum to be north applies only to rotatable and other directional antennas whose azimuth we may set within the propagation program.

Fig. 10 shows the first three azimuth entries for the Yagi's pattern maximum = 0 degrees selection. Compare this partial file to the corresponding entries in Fig. 7. The values are identical, since the antenna has not changed other than turning 90 degrees. (In fact, I created the earlier compass rose version of the Yagi by rotating the present version by 90 degrees. The type-13 file creation function performed the same action, but at a different stage, namely, by operating on the NEC output radiation pattern data.)


The Yagi samples with which we have experimented are, of course, simplistic, since their main function was to show a procedure and process, not to produce a type-13 file for an actual antenna. That fact is clear from the incomplete file descriptions in the first line of each type-13 file. A more complete model would have used the stepped diameter structure for the actual antenna structure. As well, it might have included relevant surrounding objects, including inert antennas for other frequencies that we might have stacked above or below the subject antenna. In all cases, a serious type-13 file would have used the antenna's actual height above ground and would have included the most accurate ground specification one might be able to derive from local sources or measurements. (Ground quality precision is less important for horizontal antennas than for vertical antennas.) The degree of model complexity will always be a user judgment.

Nevertheless, the addition of VOACAP type-13 file capabilities to NEC software provides a means for both amateurs and professionals to make better use of propagation software in the pursuit of more reliable communications.

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