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Distance Listening to Aircraft (and more on antennas)

A while back, I documented my long distance listening of a commercial aircraft using SCARP. I recorded what I heard and I then referenced to determine where that flight was. The callsign was Frontier 2438. I was thrilled to calculate that he was 218 miles away from me.

My antenna was mounted at rooftop level, and there are lots of trees which are higher. They could block some of the signal, so I would benefit by moving to an even higher antenna elevation.

I replayed the clip. The pilot announces that he is getting light chop at flight level 330 (33,000) feet. What is the maximum distance I could hear him? There is a simple equation which will answer that question. It determines when the curvature of the earth blocks off line of sight from an aircraft, and that formula is this:

Distance (in nautical miles) = 1.25 * sqrt (height [in feet]). That's simple.

Running the math, this means that 1.25 * sqrt (33000) = 227 nautical miles = 261 statute miles. The maximum distance the airplane could be from me, at 33,000 feet, was 261 miles. Beyond that distance, he would have been below the horizon.

I heard him at 218 miles, so I achieved 218 / 261 = 83% of the maximum range possible. Given that I was partially blocked by trees, I did pretty good.

To increase my effective range, any of the following would help:

  • Ensure that the antenna is properly tuned. I checked my antenna using a VSWR meter and it is.

  • Raise the antenna height, so that it is not blocked by trees. (If a signal is blocked by objects or curvature of the earth, nothing can solve that problem.) I could do this.

  • Increase the antenna gain, so that I receive a stronger signal. By changing from a J-Pole to a more exotic antenna, I could do this as well.

  • Improve the receiver design, so that it is more sensitive. I am close to the limits for the SCARP design, especially relative to the cost of the product.

  • Decrease the length of cable, from the antenna to the receiver. This reduces losses in the cable transmission path. I could do this by moving SCARP up the cable, closer to the antenna.

  • Make the airplane fly higher, so that it rises above the effective curvature of the earth. This one's kinda tough.

  • Give the airplane a more powerful transmitter. The calculations show that the transmitter power (even at a few watts) isn't the problem.

Some of these can be done, others are unreasonable or impossible.

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