RFI (Radio Frequency Interference) can plague the modern Amateur Radio station, especially 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 radio disconnect, shut down, SWR Warning, RF on the audio, or erratic operation during transmission are all symptoms of the same basic issue.
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.
Use this procedure to help diagnose the issue if you suspect Common Mode interference:
How to Diagnose Common Mode RF Interference. – FlexRadio
Resolving Common Mode Current Issues:
Build your antenna system properly
Always engineer your antenna system design to mitigate common mode currents. In addition to reducing RF in the shack, it will improve reception by lowering the noise floor too.
For example: Using a 4:1 or 9:1 balun on an Offset Fed Dipole, or a 49:1 Balun on an End Fed Half Wave antenna is important for impedance matching in the antenna system. But it does little to affect or prevent Common Mode current from being imposed on the outside of the coax shield.
Install a 1:1 Common Mode Choke after the impedance matching balun to prevent the Common Mode currents from flowing on the shield of the coax. You can use a commercial Common Mode Choke or several turns of Coax cable in an "Ugly Balun" configuration. Some amateurs use 8-12 turns in an "air coil" about 12 inches in diameter.
Use a Common Mode choke at the base of vertical antennas before the main coax feed is attached.
Even so-called "Balanced" antennas (Dipoles, Loops, etc.) can benefit from Common Mode chokes.
Shunt Common Mode currents to an earth ground before they enter the shack.
From an electrical circuit perspective, a common mode choke prevents currents from passing and the blocked currents need a low potential path to ground to dissipate them properly.
You do this by using a common mode current 1:1 balun (also known as an "Un-Un") installed on your coax in conjunction with grounding the shield of your coax before it enters your home.
Visualize the common mode current's direction of travel: from the antenna to the radio, the coax shield ground comes first, then install the common mode choke between the Grounding Block and the coax entry to your shack. The ground point must be an RF (low impedance) earth ground, not a ground connection to the AC safety ground or a water pipe. This ground point should be outside right before your coax enters your shack. A lightning arrestor is an excellent coax shield ground point. See the signal path diagram below:
Antenna => [Coax Feedline] => [Grounding Block/Lightning Arrester] => [Common Mode Choke] => [Coax leading into the QTH] => [Amplifier, if used] => FLEX-6000.
This important step is only one of many recommendations regarding proper station grounding.
See also this great article about grounding your Amateur Radio Station:
Grounding Systems in the Ham Shack - Paradigms, Facts and Fallacies – FlexRadio
ISOLATE YOUR NETWORK EQUIPMENT, TOO
You may also need to isolate your network equipment by wrapping several turns of your device power input cables through MIX-31 or MIX 43 "Snap-on" Ferrite cores. Install this choke closest to the device's power input, not at the wall plug.
Also, take a few turns of your radio's power cable, and other power cables, through MIX-31 or MIX-43 "Snap-on" Ferrite cores.
Use MIX-31 if the issue is primarily on 20 Meters and Below.
Use Mix-43 if the issue is primarily on 20 Meters and higher frequencies.
(Note there is an overlap at 20 Meters when using booth mixes of ferrite cores.)
Each turn you take the cable through a core multiplies the effectiveness of the ferrite. i.e. two turns through the same snap-on is more effective than simply snapping two cores on the same line.
(NOTE: Putting Ferrite Beads on the power leads of your various equipment is also a good preventative for RF "hash" leaking OUT of those devices, increasing your receiver noise floor.)
For more information: see this article: How to Build a Quiet Station for Better Reception – FlexRadio
If you still have RFI issues in your network:
Take a few loose turns of your ethernet cables through a large FT240-31 or FT240-43 toroid core.
In severe cases or long or resonant lengths of CAT5/CAT6 runs, you may need to do this on each end of the cable. Or you can employ special ethernet cable isolators available from suppliers such as DX Engineering or Palomar Engineering.
(NOTE: Ethernet cables are balanced twisted pair transmission lines using differential signaling and generally do not benefit from using ferrite cores or beads. Wrapping too many turns on the toroid, or wrapping them too tightly, can result in degrading the signals in the ethernet cable, reducing the effectiveness of your network.)
P.S. Always obtain good quality ferrites, don't trust the cheap "hamfest bargain table" stuff.
Many of these are designed for VHF/UHF frequencies to reduce computer interference to Television and FM Radio signals, and will not be effective at HF frequencies.
A properly grounded station with proper treatment of Common Mode RF currents is not only a safer station, but it will be a much quieter station, because the measures taken to prevent Common Mode RF from causing trouble when transmitting, will also prevent RF Noise from entering the antenna system as noise when receiving!