The utimate PIM test site discovered in Brussels Part two of my quest by Pedro M.J. Wyns ON7WP AA9HX
First a summary of the "offending" station: Two DAB digital radio COFDM transmitters adjacent channel 224 MHz each 1 kW multiplexed using BPF combiners into 8 twin dipole panel antennas by kathrein Two DVB-T digital video COFDM transmitters 482 and 503 MHz each 10 kW multiplexed using BPF combiners into 8 1- dipole panel antennas by kathrein Five FM analog transmitters 10 kW each and a 1 kW multiplexed using BPF combiners into 16 4-crossed dipole panel antennas by kathrein. One analog TV transmitter 189 MHz power unknown probably multiplexed into the DAB antenna. receive systems on this high powered site: A UHF NBFM receive station on a gain collinear (works fine) A VHF NBFM repeater station into a half wave collinear (Big disaster) A TNG (terrestrial news gathering system) using 4 receive gain panel antennas on 2300 MHz. (works fine) The problem: All works fine but the VHF repeater. We start by disabling the transmitter, as the problems are purely receive related. Tracking down the problem: A high end R&S spectrum analysers FSEA is used, connected straight to the antenna, not using the duplex filters but just using some protection against the FM and DAB power, so HPF on 136 MHz and LPF on 175 MHz. When connecting the antenna the analyser noise floor goes up from -117 to -97 dB within this prefiltered band. No overdrive is seen on the analyser, as there are no high level transmitters in the band 136-175 MHz. In the noise floor variations can be seen looking like the shape of the 2 MHz wide DAB signal. Then for test purposes one of the two DAB transmitters is switched off temporarily. Half the rubbish disappears So we suspect the DAB transmitter. We move the analyser to the DAB antenna feeder and using the necessary filters we connect the analyser input sequentially to the forward -60 dB and the return -40 dB port. Analyser sensitivity maximum, NO INCREASE of the noise is observed. So the noise is NOT coming out of the transmitter and is not generated in the DAB antenna. SHIT . So apparently the noise and mixing products are generated OUTSIDE the DAB transmitter system. Then we went to the roof with a Yaesu FT817 on the VHF RX frequency using the same filter setup. A three element antenna is used for pinpointing the problem. Noise is coming from multiple sources A very large airconditioner air outlet (6x8 meter) on a lower roof 50 m from the tower, maximum (S9) INSIDE the very large cover ???? Metal shieldings around the building. The lightning protection connection (S6) on the major coax cables, small airco outdoor units (S8) straight under the tower, the mount of the VHF antenna (S7), the DAB antenna (S8) Levels are off course not absolute but it is very clear that multiple objects are radiating in this very strong field. I have no indication of the actual radiation value but the lowest FM antenna is only 10 meters above the walking area. DAB is at 20m, DVB-T at 25m. The next step ??? I would like to try other VHF RX antennas as I have the idea that maybe the biggest IMD is made inside my own repeater antenna (Brand new Procom ½ wave radialless). The UHF antenna is not bothered by this noise, nothing is observed and it is located at the same height and distance from the FM panel antennas ???? Please your input if you have any idea how to handle this case, and hopefully solve it Pedro ON7WP-AA9HX
