To the editor: A plastic trash bag full of balls of crumpled aluminum foil has sometimes been suggested as a useful emergency radar reflector, should you lose your primary reflector in a dismasting or other accident. I recently (March 2007) had the opportunity to test this suggestion in a radar anechoic chamber.
Two rolls of Reynolds kitchen aluminum foil, each 50 feet by one foot, were used. The foil was cut into one-foot square pieces, and each piece was crumpled into a ball. The result was 110 balls of crumpled aluminum foil about two inches in diameter. These were put into a plastic kitchen trash bag forming a ball approximately 12 inches in diameter.
The “size” of an object as a radar target is described numerically by the radar cross section (RCS). The RCS specifies the amount of power in the incoming radar pulse that is reflected back toward the radar antenna. It is measured in units of square meters. The radar cross-section changes with changes in target orientation so a complete description of a radar target involves specifying the RCS at all orientations, azimuth and tilt. Commonly, the description is limited to a single set of RCS vs. azimuth data for zero angle of tilt. This is how I ran the test. The bag of balls was placed on the rotating turntable in a radar anechoic chamber, and the radar cross section measured continuously as the target was rotated 360°. Measurements were conducted at 9.4 GHz with horizontal polarization (marine X-band). The following figure shows the measurement trace scanned from the strip chart covering 180° of rotation. The horizontal axis is degrees azimuth and the vertical scale is dB (decibels). The horizontal black line shows the radar cross section of a conducting sphere 12 inches in diameter that is approximately the same size as the bag of aluminum balls.
The RCS of a bag of aluminum balls fluctuates wildly as aspect azimuth changes; the average RCS is similar to the radar cross section of a conducting sphere of the same diameter. While a bag of aluminum foil balls does provide some radar reflectivity, it isn’t a practical radar reflector for a recreational vessel for two reasons.
First, even though the bag of balls acts, on average, somewhat like a perfect sphere of the same size, there are numerous gaps in coverage, meaning that roughly half the time the bag of balls provides no useful reflected signal.
Second, the theoretical radar cross section of a sphere is quite small compared to its physical size. In our experiment 100 feet of aluminum foil produced a 12-inch diameter bag; a sphere of the same size has only a 0.073-square-meter cross section. Using 0.625 square meters as the lower limit for a useful radar reflector, one would need a sphere (or bag of balls) three feet in diameter, which in turn would require a huge amount of aluminum foil. Clearly, the bag of crumpled aluminum balls approach is not a practical radar reflector.
