All solid state transmitters are subject to IMD issues in a multiple RF environment like you describe. However my experience is that different PA designs have more problems than others. I'm not familiar with how the BE compares with the Harris model, but the general rule of thumb is that a bandpass filter is always a safe bet when you share a tower site with other full power RF signals. Depending on the frequency separation of all of them, the filters can be simple one or two cavity models rated at your transmitter TPO.
If you are able to do so, you can check to confirm your need of a filter by turning off or at least greatly reducing the RF from the other stations on the tower and observing the VSWR on your transmitter. If the absence of their RF causes your VSWR to go away, then you will need a filter to operate properly and to bring your IMD into FCC compliance levels.
I just recently installed a Kintronics FM combiner unit and am extremely pleased with the cost point as the performance of the unit. I know their bandpass filters are similarly constructed and priced and would encourage you to ask your rep about them if that becomes your path of action.
Jim "Turbo" Turvaville
SBE Certified Radio Engineer
Turbo Technical Services
From: Bill Hurne <firstname.lastname@example.org>
To: 'CRTech' <email@example.com>
Sent: Thursday, September 7, 2017 10:09 AM
Subject: RE: [CRTech] Combiner-Isolator?
I’m asking this question below because we replaced the BE FM 1C1 transmitter with a GatesAir FAX unit, and now having high SWR issues. Maybe the SWR is caused by lack of output bandpass filter in the crowded location? Maybe adding an output filter would help?
But this is complicated by the fact we also replaced the 3-bay antenna so we’re troubleshooting.
In a BE FM 1C1 1 KW transmitter, would the RF output combiner also act as an isolator to help reduce IMD in a crowded location? This transmitter has two RF output modules with an RF combiner with a reject load resistor on one port.