RFI (Radio Frequency Interference) can plague the modern Amateur Radio station, especially with high-powered stations.
RFI can cause a FlexRadio to "shut down" during transmission. It can also infiltrate other equipment and cause loss of ethernet connections, corrupted data transfer between devices, improper readings on remote meters, poor transmit audio, and erratic operation, among other things.
RFI can be either Radiated, (Direct signal transfer from the antenna to the affected equipment) or "Common Mode" interference. The most prevalent is "Common Mode" interference.
Common mode currents on the coax shield can take a path to a lower potential ground through one of the cables connected to the radio. A systematic approach is required to find what is causing the problem.
(In this procedure, a disconnect, shut-down, HIGH SWR Warning, RF on the audio, or erratic operation during transmission can all be symptoms of Common Mode interference.)
Common mode currents are prevalent when the antenna system is unbalanced, like when using a Vertical, End Fed wire, OCF dipole, or indoor "attic" antenna. There is never such a thing as a perfectly balanced antenna so there are always common mode currents on the shield of the coax.
Tracking Down Common Mode Interference:
Employing a "divide and conquer" approach, you need to simplify your station configuration, remove as many variables from the equation as possible, test the simple configuration, and then add devices back into the configuration, one at a time.
Before starting, you can perform a Factory Reset on your radio to ensure you are not experiencing any issues related to the internal database. Please follow the procedure outlined in the HelpDesk article How to Factory Reset your FLEX-6000 Radio. Do not restore any previously exported profiles until you have completed testing for RFI.
Disconnect all cables connected to the radio except the ones necessary to operate the radio, including any ground connections. For a non "M" model radio, it is an Ethernet and DC power connection. An "M" model radio would only need a DC power connection.
Connect a dummy load directly to the radio, using a short coax jumper. If you use an amplifier, completely remove it from the RF signal path. Do not simply put it into "standby mode."
If you use any third-party programs with the radio, do not use them while RFI testing.
If using an "M" model radio, use the front panel. For non "M" model radios, start SmartSDR for Windows. Use the SmartSDR TUNE feature to transmit with 50 Watts on the most problematic band.
If you do not get a disconnect or other symptoms of RFI, increase the TUNE power incrementally until 100W or experience the symptoms of RFI. If you get a disconnect at 100W with the minimal physical connections made to the radio, then the path to ground is through the power supply or the Ethernet cable, if one is connected.
If you can transmit at 100W without a shutdown or network interference, and you use an amp:
Connect your amp to the radio and a high-power dummy load directly to the RF output on the amp. If used, connect the TX Relay cable between the radio and the amp. If you use a USB cable for band data, do not connect it to the amp yet. Transmit with an output power level of 200 W and slowly increase the exciter drive to full power.
If you are using an amp and get a shutdown with the dummy load connected to the amp, there may be a standing wave issue with the coax jumper between the radio. Changing the length of the coax jumper is one way to test for and eliminate the problem.
If you still do not get a disconnect or network interference, keep moving the dummy load down your coax one connection point at a time and retest. Do this until it is not easy to move the dummy load any farther down the coax. At that point connect your antenna and test.
If you get a failure at full power with the antenna connected and the SWR is high or fluctuating, this indicates a problem with the coax, coax connectors, baluns, or antenna traps.
If you get a failure at full power with the antenna connected and the SWR is reasonable, this indicates you have common mode currents returning on the coax shield and they are finding a path to ground through the radio. See the article: How to Mitigate Common Mode Interference – FlexRadio
If you do not have a fault when testing at full power, start adding back connections to your radio, one at a time, and test for a disconnect or network interference at high power. Reconnect the mic, key, and any other physical connections to the radio. If you use a ground connection to the radio, connect it last.
Hopefully, at some point in your testing, you will make a connection and the problem will reappear. This will zero in on the connection that is causing the problem. Or, you will get everything hooked back up and discover that disconnecting and reconnecting everything solves the problem. (Perhaps this is due to rearranging the cable layout, or tightening loose connections.)
If this testing procedure reveals that you may have a Common Mode RFI issue, use this document to help resolve the issue:
How to Mitigate Common Mode Interference – FlexRadio