The FCC EAS (Emergency Broadcast System) consists of an originating location in Washington DC which is connected via land-lines to PEP (Primary Entry Points) in each state which are broadcast stations responsible for disseminating alerts in that state. All stations monitoring these PEP stations are Secondary EAS stations. All FCC licensed broadcast stations are required to monitor a minimum of three Secondary stations as dictated in each state’s Emergency Communications Plan. In most cases this dictates monitoring either two Secondary FM (88-108 MHz) stations or an AM (530-1700 KHz) and an FM plus a NWS (162.40-162.55 MHz) (National Weather Service) station.
The required apparatus installed in each station is one or more antennas to maintain adequate signal level, and an EAS receiver/decoder to decode the incoming EAS tones and automatically switch audio feed to the transmitter in the case of an alert coded for that geographical area.
AM broadcasts are vertical antenna polarization, FM broadcasts are generally circularly polarized and the NWS stations utilize vertical transmit antenna polarization. In many cases urban or suburban stations can receive the requisite three stations with an omni-directional vertical antenna. This is usually not difficult as many secondary FM broadcasters utilize 10-100 KW of ERP. In half or more of suburban and rural areas, a directional antenna is required to ensure adequate signal level, especially from the NWS broadcasts which are limited to 1000 Watts.
Due to the wide frequency range the FM band covers, there are two dual-band FM & NWS Yagi antenna configurations possible to address all needs. All will have to receive both the NWS band and either FM low (88-98 MHz) or FM high (98-108 MHz), vertically polarized. The current design consists of a two-element FM high Yagi interlaced with a 4-element EAS Yagi. There is a single feedline to the FM driven element, and the EAS Yagi is sleeve-driven. The array delivers ~8.75 dBi gain with 9 dB front-back ratio on FM and 11.36 dBi gain with 7.18 dB front-back ratio in the EAS band. Both measurements made at a 1 degree elevation angle.
This dual-band Yagi design has not been optimized after combining them on the same boom, this would be a further enhancement.
The files used in design and evaluation are downloadable here, the EZNEC file has been zipped due to file security issues:
Dual EAS Yagi layout.pdf
Dual Vert Pol EAS Yagi w-mast.EZ
Please let us know if you are interested in executing this design.
Best regards,
Howard Hoyt
info@proaudioeng.com
(919)260-3675