The Art of Launching Rays: A 12-Step Program to Kick the Azimuth Habit
By Russ Carpenter, AA7QU
The ARS Sojourner
It's a curious thing. When we HF types think about antennas, our reasoning is flat, not vertical. We get our thrills from looking at gain patterns in azimuth. You want proof? Just take a look at the ads for HF antennas in the ham radio rags. Buy a monstrous, expensive 4 element 20 meter beam, and you too can brag about your dBs. Apparently, nobody gives a hoot about performance in the vertical plane.

For QRPers, it's especially hard to get excited about vertical radiation patterns. After all, 99 percent of us are not big-time DXers. We're perfectly happy if we can hit a reasonable part of the U.S. And our intuition tells us that we don't need low angles of radiation for such modest goals. If we can get our 20 meter antennas 30 feet in the air, it should be no problem.

The trouble is that our intuition is often wrong. Low angles of radiation frequently make a big difference, even for single hop propagation within the United States.

Take this example. Let's suppose that you're on the West Coast, and that your goal is to cover the Midwest with your QRP signal. Your inner voice tells you that radiation between 20 and 30 degrees should work just fine. If you were trying to reach Ethiopia, you'd probably need launch angles less than 5 degrees. But you're not--Missouri will be enough.

Look at this VOA-AREA plot, which is cleverly based on Field Day, 1999. It shows coverage for an isotropic 20 meter antenna, 7:00 in the evening, running 5 watts. Looks pretty good. You've got the Midwest nailed.

The foregoing chart (and all the subsequent VOA charts) shows signal strength in terms of signal to noise ratio. (According to standard HF thinking, SNR needs to be greater than 25 for a signal to be copiable.) Here is the relevant legend:

Now let's look at which radiation angles generated that nice looking map. To make this possible, your author custom designed some weird antenna elevation tables using "HFant," a part of the VOA software package. Three hypothetical antennas were created. Antenna 1 radiates only from 0 to 10 degrees, Antenna 2 from 11 to 20 degrees, and Antenna 3 from 21 to 30 degrees. As an example, here's the elevation plot for Antenna 2.

Let's see how each of these antennas performs, using VOA-AREA. Antenna 1 generates a great VOA pattern. All by itself, it's taking care of Mid-America. Antenna 2 is pathetic, and Antenna 3 is DOA. Oops--looks like low angles of radiation matter a lot, even when you're just aiming at the backyard.

Here comes the confession. The pesky thing about elevation analysis is that Mother Nature keeps changing the rules. The fate of rays is utterly dependent on the frequency, season, time of day, sunspot cycle and current solar activity. Wait a few hours, and the rays that were doing such a nice job at 7:00 PM from Antenna 1 will be starting a long journey toward outer space.

Nevertheless, as messy as they may be, launch angles are still primary. If your antenna doesn't perform in the appropriate vertical realm, you might as well read a good book. Azimuth patterns are secondary--they are important, but only insofar as you've dealt with the vertical dimension first.

I have discovered that mastering the art of elevation angles is, in its own right, a rewarding pastime. Regular citizens, like you and me, have access to amazing software that predicts where our rays will land under defined conditions. You will soon discover that you can achieve 9 or even 12 dBi gains just by being in the right place at the right time. For about a dollar's worth of gas, you can drive to a trailhead, take a nice walk to a hilltop, put up a five pound inverted V, and smoke 'em.

And now some wonderful news. There aren't really 12 steps. There are only four.

1. Download the VOA software. It's zillion dollar software and you can get for free. Just go to ftp://ftp.voa.gov/pub/software/voacap/.

2. Get or make some custom elevation tables for VOA. Starting with the February issue of the ARS Sojourner, we will be publishing elevation tables in the Archives section. Some will be hand made, and some will be generated by a zippy little utility written by Brian Beezley, K6STI.

3. To achieve personal growth, get Eznec, by Roy Lewellen, W7EL.

4. And to have more fun than a barrel of monkeys, get TA by Brian Beezley, K6STI.

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Russ Carpenter, AA7QU, lives among the trolls on the McKenzie River, Oregon. He is the contest manager for ARS and attempts to serve as webmaster for The ARS Sojourner.