[TZ] Third order intermod in FM Station

[Rolf Sandmeier]

Dear Braintrust,

A client of mine owes an FM station, 10 kW TX power, 4-bays top mount  elliptical polarized antenna, full wave bay spacing.

He is planning to install a second station , side mounted 2-bays antenna, 
V- polarized, about 30 ft vertical spacing below the 10 kW antenna. This new station will operate on 1 kW TX power.

I suggested him to make a preliminar test , as I sniffed some intermodulation in the air, given the close spacing (physical and carrier f0 ) between both stations.

10 kW current station:    92.1 MHz

1 kW future station:         92.9 MHz

Intermod prevision products in FM Broadcast band:  93.7 and 91.3 MHz.

We hastily mounted the new system, and the test results were positive for IM above my expectations, worsened by the fact that the station stays in downtown commercial center. We took a drive around and the 93.7 came loud and clear up to 1.5 mi in line of sight of the tower , whereas the 91.3 faded away within few hundred yards.

Plan A : install Notch filters to block carriers in both TXs and possibly also bandpass filters to to knock out the residual IM  91.3, respectively 93.7(at the output of both TXs).

Plan B: Desperately finding a new aerea for the 1 kW station, with high costs involved.

Taking into account the 10 kW power @ 92.1 MHz transmitter,  the reduced spacing between carriers (800 kHz) and the MUX baseband modulation which will not exceed 70 kHz, (Stations are and will stay analog for the predictable future), that is Standard Stereo MUX 53 kHz band + 2 SCA- RDS , Is that technically feasible and what would be your recommendation for the choice of manufacturer, that would be in the Diplexer  market?

Thanks a million for your precious advice!

Best! 

Rolf Sandmeier
Consultor autonomo em radiodifusão, sistemas irradiantes AM/FM
Broadcast CE, Brazil
13 / 99776 0150
sistemasradio.hd at gmail.com