The PoM shield is based on a fundamental concept in the physics of electromagnetics, the Faraday cage (Wiki- pedia link). Its operation has been very extensively studied and Faraday cages have applications in many ar- eas, from microwave ovens to military security infrastructure. The shielding performance of a Faraday cage can be measured using standard laboratory equipment. On the PoM shield the Faraday cage is construct- ed using silver-based shielding fabrics and an innovative (pate nt pending) system of closure, based on gravity.
Since the first prototypes, PoM shields were being measured both in the PoM lab and in the Radio Communications and High Voltage Laboratory at the National Technical University of Athens. Both labs used certified and calibrated equipment.
The spectrum analysis measurements of shielding effectiveness are based on EM field strength measurements 1-3 me- ters from the shield. The transmitting source for the tests were a signal generator (0.1-3GHz) as well as mobile phone to measure the specific frequencies used by the different mobile networks (0.9GHz, 1.8GHz, 2.3GHz, 2.7GHz) and confirm the signal generator setup measurements. The setups and equipment used for the measurements are the following:
500MHZ to 1GHZ
NATIONAL TECHNICAL UNIVERSITY OF ATHENS - SCHOOL OF ELECTRICAL AND COMPUTER ENGINEERING Mobile Radio Co mmunication Laboratory, High Voltage Laboratory
- LINDGREN-RAYPROOF S81 SHIELDED ENCLOSURE - Radio frequency anechoic chamber SCHAFFNER SMR 4518 EMI Measuring Receiver 9kHz – 18GHz
- SCHAFFNER BHA 9118 BROADBAND HORM ANTENNA 1-18GHz
- SCHAFFNER CBL 6111D BILOG ANTENNA 30MHz-1GHz
- ROHDE & SCHWARZ SMB100 signal generator
- Pressure meter Testo / 511
- Temperature and Humidity meter Greisinger GMH 3330 N-Type cable (2m length) Huber+Suhner / Sucoflex 106 N-Type cable (4m length) Huber+Suhner / Sucoflex 106 Pressure meter Testo / 511 39106014/109 04.06.2019 Temperature and Humidity meter Greisinger / GMH 3330
Spartant RF Signal Source, 25-3000 MHz
ΚΕΝΤ Electronics Log Periodic antenna PCB-LP 850-6500MHz
RIGOL DSA832E Spectrum Analyzer
Aeroflex Spectrum Analyzer 7 KHz – 2.7GHz
Wavecontrol SMP2 Dual Broadband measurements (0 Hz - 40 GHz) Wavecontrol WPF40 probe 1 MHz – 40 GHz
RIGOL DSA-815-TG spectrum analyser
Typical measurement results:
The different setups produced highly consistent results. Measurements demonstrate an average shielding perfor- mance of the PoM shield in the area of the mobile phone bands (0.5-3GHz) of 35db or more, which is equivalent to an attenuation factor of 3000+. This level of attenuation is clearly sufficient for the intended application. For com- parison the attenuation of the Faraday cage in the typical microwave oven (which operates in similar frequencies) is around 20db.
As a reference, for reliable operation of the phone inside the PoM shield using the supplied standard antenna with 7m RG174 1.5DS cable, the mobile network field strength should be above -100dbm, and ideally above -90dbm. This typically corresponds to a signal strength of 3/4 or 3/5 lines on the mobile phone signal meters.
Testing the PoM shield at installation
The PoM shield operation can be generally verified by installing a mobile phone in it and closing the cover without connecting the external antenna. The mobile phone will normally immediately disconnect from the network. Only in cases where the PoM shield installation is very close to a mobile network base station (signal strength higher than -80dbm) the mobile phone might still operate. Of course, this does not mean that the PoM shield does not work. In this case the signal and reception are strong enough not to be affected by the 35db shielding effect of the PoM (in actuality the connector of the PoM forms a tiny antenna and reduces the resulting perceived shielding performance somewhat).