On 1/13/2015 Jerry greatly enjoyed demonstrating shielding effects and challenges to UIC students in Professor Danilo Erricolo’s class ECE 423 on EMC . http://www.ece.uic.edu/bin/view/ECE/ECE423
27 MHz Radio controlled toy car or 460 MHz FRS handi-talkies
were the sources and receptors sending their waves through a shielding “pot” and its variations . The car receiver was modified to excite an acoustical alert sounder, while the ordinary speaker of the handi-talkie “spoke” evidence of reception action.
First the lid , a single disk shield plate (far right in photo) , was held between the transmitter joy stick controller (in Jerrys hand) and the race car (red 2nd from right in photo ) . That was shown to be totally ineffective.
Thus one could initially hypothesize that radio waves at these frequencies do not behave like rays of light traveling in straight lines , to be blocked or reflected by “opaque” objects. Rather they behave more like acoustic or fluidic analogues flowing around ( diffracting ? ) or bouncing off other nearby objects.
Second , the solid metal open “cup-shape” or pot (in photo behind “shielding demo” white paper sheet) completely covered the race car with the open side facing away from the transmitter. It was also ineffective (for the same reasons) .
For maximum performance , one needs a fully enclosed “Gaussian shell” . It was demonstrated that huge shielding degredations occur from even Slight corruptions of the shell enclosure by modifying the fit / contact quality of the soup-pot lid against the pot opening. It was more remarkable at 460 MHz vs. 27 MHz.
A strainer version of the pot made of perforated metal (near Jerry’s hand) with ~5 mm diameter holes and using the same tight fitting lid , was effective at 27 MHz but largely ineffective at 460 MHz (why ? HINT Wave-guide beyond cutoff attenuation. Also handi-talkies have much more transmit power and receive much lower signal levels than 27 MHz toys , thus requiring more shielding attenuation to block transmission)
When an insulated ~12″ long wire penetrated a hole in the sealed strainer pot (as might be used to power the car within) , attenuation at 27 MHz was severely degraded. It acted like a receiving antenna to the transmitter outside the shield then the current it carried caused a re-radiated emission within the enclosure. However, when the wire was filtered with a sufficiently low impedance shunt capacitor, as a simple filter ( 2 nF not enough 10 nF enough) , shielding effectiveness was restored , as the current it was carrying was attenuated by diversion (bypassing) in to the shield structure.
Enjoy doing your own extensions and variations. Lets hear from you !
Jerry at JDM LABS 1/14/2015
Note : the inspiration for this demo was Dr. Amy Pinchuk of http://infieldscientific.com/eng/about.html
who demonstrated it at the IEEE EMC Society 2004 Symposium in Santa Clara .
Hey see you all in Santa Clara for the EMC Symposium this
March 16-20 http://www.emc2015usa.emcss.org/