If you were in the Philadelphia area in early July, 2003, you may have heard a strong unmodulated carrier on 1650 kHz in the expanded part of the AM broadcast band during daylight hours. That was WP3XEN, a special field test station affiliated with WPEN Radio.
Since the construction of a new or upgraded broadcast transmitting facility can be a very expensive undertaking, stations may conduct field tests under Section 73.1515 of the FCC rules. These tests determine the suitability of the transmitter site. WPEN is a popular AM station in the Philadelphia area. With 5,000 watts on 950 kHz, WPEN was Philadelphia's only adult standards station, now broadcasting a sports/talk format. It is also the originating station for broadcasts of Philadelphia Phillies baseball games. The recent field tests were conducted to collect data for a possible upgrade of the WPEN signal. I was the transmitter operator during the WP3XEN project.
Although field test stations are experimental in nature, they are licensed under Part 73 of the FCC Rules, which governs regular broadcast stations. Experimental broadcast stations that are used for other purposes are licensed under Part 74, while experimental stations in the nonbroadcast services are licensed under Part 5. In all cases, the callsigns look like amateur radio callsigns, but the experimental callsigns always have the letter X following the numeral. These callsigns are taken from the block KA2XAA-KZ9XZZ and WA2XAA-WZ9XZZ. The numbers 1 and 0 are not used. For field test stations that are affiliated with currently licensed broadcast stations, the callsign is derived from the callsign of the parent station.
WP3XEN did not broadcast any kind of music or talk programming. As a field test station, it was limited to transmitting nothing but a steady, unmodulated carrier, with the call letters being announced once per hour. It was a complete AM broadcast transmitter site, running 10,000 watts into a nondirectional antenna. Several engineers measured the field strength of the WP3XEN signal at numerous points along radials extending through Pennsylvania, Delaware, Maryland, New Jersey, and New York. The field strength readings were then tabulated and used for calculating the ground conductivity, as this determines the coverage of a mediumwave station and provides valuable information for designing a directional antenna system. Any upgraded WPEN facility will need to use a directional antenna to maximize signal coverage into Philadelphia and its suburbs while minimizing interference to other stations on the same and adjacent frequencies. AM field test stations are limited to operation during non-critical hours, thereby avoiding skywave interference to other stations and preventing distortion of the field strength readings by skywave interference coming from other stations.
The expanded portion of the AM band is especially suitable for these tests, as those frequencies are not congested. Even in the crowded Northeast Corridor, there are vacant frequencies available for field tests. There are no regularly-licensed AM stations on the expanded band in Pennsylvania.
The WP3XEN site was located in a field near Spring City, PA, approximately 30 miles northwest of Philadelphia. It began operations on June 29, 2003 (the Sunday of Field Day weekend) and ceased operations on July 15, 2003. The station was dismantled on July 16. Not a trace of it remains.
Acknowledgments: Many thanks to Philip K. Harris, WB2IQG, who provided the pictures for this page and to Greater Media and its subsidiary, the Greater Philadelphia Radio Group, for permission to use this material on my site. Any reproduction of the text or pictures on this page without the express written permission of Philip K. Harris or Greater Media is expressly prohibited.