254 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



progresses through the atmosphere, re-radiation (reflection) from the 

 electrons in the trail occurs along its entire length to the head of the 

 meteor. A relatively wide antenna beam can cover the entire trail. 

 At an early part of the trail, say point a in figure 9, the distances from 

 the radio transmitter to the successive positions along the trail and 

 back to the receiver will vary rapidly as we move along the trail. 

 Hence the returning waves will be successively in and out of phase be- 

 cause the radio wavelength, only a few meters, is very small compared 

 to the distance, 100 or more kilometers. Little "reflected" radiation, 

 therefore, will reach the receiver when we add up the contributions 

 for an appreciable distance along the trail. 



TRANSMITTER 

 RECEIVER 



GROUND 



Figure 9. — Schematic diagram showing the geometry of radio echoes reflected from the 



ionization trail of a meteor. 



As the meteor approaches the so-called reflection point of the trail, 

 where the line from the radio to the trail meets it at perpendicular 

 incidence, we see that a considerable length of the trail will be at 

 almost the same distance from the radio transmitter. Echoes from 

 this region will return to the receiver in phase and add up to produce 

 a perceptible signal. The problem of the theoretical signal strength 

 of the received echo, as the meteoric body moves along the trail, was 

 in reality solved more than 100 years ago by Fresnel, who calculated 

 the effect of such phase phenomena for light scattered by a line. 

 The resulting signal strength as a function of distance along the 

 trail is shown in plate 5, figure 2. The echo grows in intensity as 

 the reflection point is reached, then increases beyond this value to a 

 maximum in a very short time ; then as it slowly fades out it oscillates 



