The Tortoise and the Hare

"Let's both operate 20 meters, running 5 watts and using dipoles oriented toward the east (Tortoise are Hare are both on the West Coast). I'll throw a rock over a couple of tree limbs, as I always do. You may engage in your usual foolish routine with your sling shot. We'll compare our scores at 10:00 PM, Saturday evening, and I promise you, they will be virtually identical!"

Saturday arrives. True to form, Tortoise tosses his rocks in the neighborhood trees and in no time has a dipole ready to go at 35 feet above the ground. By way of contrast, Hare selects only the tallest trees, shoots his sling shot all morning long, and gets red in the face. Finally, Hare has a dipole at 70 feet high.

Here is the elevation pattern of Tortoise' dipole at 35 feet. Not bad, thinks Tortoise. Maximum gain is at 30 degrees, which should be just about right for a domestic contest like Field Day. And I've had a pleasant morning looking at the view, while that pea-brained Hare got flustered with his sling shot.

And here is the elevation pattern of Hare's dipole at 70 feet. Ouch, thinks Hare. Look at all that radiation in the high lobe. I'll bet it will just get lost in outer space. Tortoise might be right, after all."

Field Day festivities begin at 11:00 AM. In spite of their opposite personality traits, Tortoise and Hare are both good CW operators--those little dipoles are humming! Here are the ray tracing patterns for both of them at noon (see the Tech Notes below for information on the graphs).

In both cases, the lowest angle radiation is producing single hops from the E layer. Hare is covering a larger zone with E layer propagation, because his antenna height is producing considerably stronger low angle radiation. Both stations are getting good single hops from the F layers, landing about 1800 km away. Hare's single hop signals are a little stronger. Hare is also getting a weak double hop from the F layers, about 3500 km away. Tortoise's double hop signals from the F layers are expiring before they reach the ground. In both cases, all signals radiated above 20 degrees are escaping into space.

By 2:00 PM Hare is getting a second hop from the E layer, landing about 2100 km away. Hare's second hop from the F layers is still weak, and Tortoise's double hop signals still can't reach the ground. Again, all signals above 20 degrees are escaping into space.

The situation at 4:00 PM shows the E layer refractions weakening. Hare's second hop from the F layer is still weak, but is covering a pretty good zone (3,400 km to 4,000 km). Tortoise is still out of luck on that second hop from the F layer.

At 6:00 PM the effect of declining solar radiation can be clearly seen. E level refraction has almost disappeared. Single hop refraction from the F layers is excellent for both stations, with signals landing in a zone extending from 1,600 km to 2,400 km. Tortoise's double hop signals are getting closer to the ground, but still not making it. Hare enjoys weak but distinct second hop coverage from 3,300 to the Atlantic Ocean.

Things start to get simple by 8:00 PM. Both stations still have good single hop propagation from the F layer, but Hare's signals are stronger and cover a much larger zone (from 1,800 km to 3,000 km). Hare's double hops from the F layer are now all landing in the Atlantic.

10:00 PM rolls around, the end of the challenge period. 20 meters is getting pretty quiet, especially for Tortoise, whose single hop zone is only 300 km wide.

Conclusions

What have Tortoise and Hare learned from their challenge?

1. An amazing part of their radiated energy was lost in space. In the case of Tortoise, more than half of that handsome lobe that centered at 30 degrees. And in the case of Hare, the entire upper lobe.

2. In spite of that remarkable signal loss, Hare's antenna height generated four separate advantages: (a) stronger refractions from the E layer, with a wider zone, a brief but distinct time of double hop signals from the E layer, (b) stronger single hops from the F layers, with a wider zone, and (d) a weak, but distinct, zone of double hop signals from the F layers.

3. To take advantage of the double hop signals, Hare needed to be a pretty good operator. A good QRP operator could work stations in the double hop zones, but someone inexperienced with weak signal work wouldn't even realize that his signals were landing 4,000 km away.

4. Both stations hammered the single hop F layer zone all day long. In Field Day conditions, that might be OK, because there are plenty of stations. But in a less populated contest (like all of the QRP contests), both stations would work everyone in that zone pretty soon. In that case, the E layer propagation and the double hop F layer propagation would become crucial.

Tech Notes

The radiation pattern data for both dipoles was originally prepared in Eznec and output in tabular form. The tabular data was then used to constructed custom antenna files for Proplab. Proplab was used to produce all of the ray tracing charts in this story. The graphs showing vertical radiation patterns were taken from Problab--they are a little unusual, because they are based on DBs of attenuation, rather than DBs of gain over an isotropic antenna.

Proplab assumed that transmitted power was five watts for both Tortoise and Hare. One of the fascinating things about using low power with Proplab is that it will show where signals will disappear, simply because all of their radiated energy has been absorbed. This point is marked with an "X".

Tortoise and Hare were both located on the left side of the chart, on the West Coast. The rays were traced straight across the country, with the green line marking the East Coast. In all cases, the software traced rays starting at 2 degrees elevation and ending at 30 degrees elevation. Within that range, rays were traced every 2 degrees. The ray tracing clearly showed the difference between E level refraction and refraction from the F levels.

An SSN of 85 was assumed. The date was assumed to be July 1, 1998.

I am not sure how well Proplab deals with extremely weak signals. QRP operators are geniuses at digging our faint signals, and we frequently defy the predictions of propagation software. So far, my intuitive reaction to Proplab is that it is frequently more accurate dealing with low power signals than IONCAP based software. But only time will tell. Another issue with these predictions is that they deal with only one "slice" of azimuth. Each azimuth slice will have, of course, its own propagation characteristics. And finally, any program based on smoothed sunspot numbers is only predicting averages, not the propagation on a specific day.

Russ Carpenter, AA7QU