Fixing a microstrip line in a professional Microwave ODU

A while back, I was asked if I could repair a ODU for a BMS Truck-Coder II. Someone had managed to rip the N connector straight off the casing, including a bit of the microstrip line from the distributed element output filter. See what approach I took to fix the broken stripline.

The BMS Truck-Coder II [1] is a professional transmitter for Electronic News-Gathering (ENG) applications. The transmitter operates in the 2 GHz or 7 GHz Broadcast Auxiliary Service (BAS) frequency range. A complete setup consist out of a control unit, an outdoor RF unit (ODU) and an antenna.

Even though these devices are pretty rugged, an ENG truck operator at a local TV station managed to break off the whole flanged N type connector. Together with the N connector, a piece of the output low-pass filter’s microstrip transmission line got ripped straight off the PCB.

BMS ODU with broken stripline (top right corner)

BMS ODU with broken stripline (top right corner)

BMS ODU with broken stripline, close-up

BMS ODU with broken stripline, close-up

Being under time and budgetary pressure, I came up with a pretty simple yet effective fix. I found a piece of copper wire with the same diameter as the microstrip line. To straighten the copper wire up and make it more tense, I stretched it lightly between two pliers. A short piece of this copper wire was cut and bent so that it would be long enough to connect the microstrip and the new flanged N connector.

BMS ODU, simple fix for the broken stripline

BMS ODU, simple fix for the broken stripline

I didn’t think this quick and dirty fix would degrade the output signal in any way. And sure enough, a quick check with a RF power detector and a spectrum analyzer revealed that this device was now actually performing better (output power and spectral purity) than a different ODU I compared it against.

Links and Sources:

[1] Truck-Coder II, BMS: http://www.bms-inc.com/

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