 |
 |
|
|
|
|
|
|
||||||||||||||||||
|
|
|||||||||||||||||||
|
|
|||||||||||||||||||
|
|||||
|
|
ARRL has significantly enhanced the elevation-angle statistical files for YT, the "Yagi Terrain analysis" program bundled with the 18th Edition of The ARRL Antenna Book. The new elevation-angle statistical files are arranged by general geographic area in self-executing files: EAST-USA.EXE , MID-USA.EXE and WEST-USA.EXE, EUROPE.EXE, AFRICA.EXE, ASIA.EXE, OCEANIA.EXE, SOAMER.EXE and the rest of North America outside the USA in NOAMER.EXE. Download the geographic area suitable for your location into the directory in which YT.EXE is located.
The new files employ a more general method of computing the elevation-angle statistics, exploiting some new capabilities just implemented in the VOACAP program by Greg Hand of the Institute of Telecommunications Sciences (US Department of Commerce). While the original Antenna Book statistical data were generated using the older IONCAP program, the enhanced capabilities of VOACAP allow a far more thorough exploration of all modes of propagation.
Users who are familiar with the old data used with YT will find that the new data generally shows a wider range of possible elevation angles, particularly for frequencies lower than 10 MHz. This is due to several reasons, the most important being that the types of antennas used in VOACAP were changed from 100-foot high dipoles at each end of the circuits to an entirely theoretical isotropic antenna with 6 dBi of gain -- that is, the gain is held flat at 6 dBi over the whole range of elevation angles. This technique allows more modes to be explored and shows all possible angles.
However, the user should be careful to keep things in perspective. For example, if you examine the W1-MA-EU data (W1 in Boston, Massachusetts, to Europe) using YT, you will find that the peak angle over the entire solar cycle is 13° , occurring 11% of the time the 80-meter band is open. There are three other lower-level peaks, at 7° , 20° and 1° in order of occurrence. The 7° peak occurs just over 8% of the time. How high would a horizontally polarized antenna on 80 meters have to be to exploit this takeoff angle?
Using YT, select FLAT ground and choose heights of 40 feet, 100 feet and 200 feet. You should be aware that these heights would be for 4-element 80-meter Yagis, one of the defaults built-into YT. Such 4-element Yagis would of course be real monsters and they would have 8.5 dBi of gain in free space, or about 6.3 dB of gain over a dipole in free space. The rather strange height of 40 feet was chosen for a specific reason -- it roughly approximates the response of a 100-foot high dipole on 3.5 MHz. This gives you a visual reference for how a pretty good antenna on that band would actually perform. Most hams would love to have a 100-foot high 80-meter dipole!
At a 7° takeoff angle, the 100-foot dipole would have a gain about 20 dB below the peak response of the 200-foot high, 4-element 80-meter Yagi. This means that the 100-foot dipole would have an absolute gain of -6 dBi at this angle. This 100-foot dipole over flat ground would thus have a gain that is 12 dB down from the theoretical antenna used in the VOACAP computations. If both ends of the transmit-receive path use 100-foot high dipoles over flat ground, the total system response would be down 24 dB compared to the VOACAP antennas, which assume +6 dBi isotropic antennas at both ends of the circuit.
For real-world low-band antennas, the actual overall statistical response would thus be considerably weighted toward the higher angles. This explains why the older statistical data for this path was clustered around a peak of 20° rather than around 13° for the newer data. You may well ask why for the new data we chose to use theoretical isotropic antennas rather than the more practical 100-foot high dipole over flat ground. There are several reasons. One is that these elevation statistics were designed to show all possible modes and the older data really showed the most likely modes mainly because of the choice of antennas. The other reason is that some particularly advantageous QTHs may well be able to emulate the response of isotropic antenna with 6 dBi of gain.
For example, a Four-Square vertical array located on a saltwater marsh will have excellent gain at low elevation angles. Calculation using NEC-4 show that such an 80-meter Four Square would have about 13 dB more gain than a 100-foot high dipole located over the same saltwater marsh! The absolute gain of the saltwater-marsh Four Square remains just under +10 dBi over a range from 3° to about 15°. Now you know why W1KM and VE1ZZ do so well with their Four Squares over saltwater on the lower bands! Just in case you wanted to know, a 4-element Yagi at 200 feet will compete directly with a Four Square over saltwater down to about 8° elevation. After that, the saltwater array takes over.
Just for reference, the same computation was done for a Four Square over ground that is considered "good" (conductivity of 5 mS/m and a dielectric constant of 13) -- although nowhere near as good as saltwater, which has a thousand-times better conductivity. At 7° the saltwater Four Square was about 8 dB better than its land-locked cousin. Obviously, verticals need saltwater to really play well. Still, the Four Square over good ground is still about 5 dB better than the 100-foot dipole at 7°.
On the higher frequencies, the new elevation-angle statistics show more "spread" around the overall range of elevation angles. This is due to the more exacting use of all modes in the computations, plus the use of the flat-gain response antennas with no nulls in their response. I want to thank Pete Smith, N4ZR, for pointing out the desirability of using isotropic rather than real-world antennas for these particular computations and Greg Hand for modifying VOACAP to do the specialized computations.
Note: the "standard" suite of antennas is still used for other propagation computations using VOACAP, such as predicting the signal levels for various transmitting sites to 40 zones throughout the world. This data is located on the disk bundled with The ARRL Antenna Book. The signal levels in that data can be scaled upwards or downwards according to the actual antennas used at each location. The standard antennas are 100-foot high dipoles over flat ground from 3.5 to 10.5 MHz; 3-element Yagis at 100 feet above flat ground for 10.5 to 20.0 MHz, and 4-element Yagis at 60 feet above flat ground for 20.0 to 30.0 MHz.
R. Dean Straw, N6BV (n6bv@arrl.org)
Senior Assistant Technical Editor, ARRL
