RF Interference with CHT's

Somewhat recently I noticed (ok, maybe it's been going on for a while--I just never noticed ;-) ) that when I transmit on any number of different frequencies, my CHT's & EGT go bananas. FYI, Dynon EMS module.

Std CHT's & EGT's below:



When I transmit, I see CHT's & EGT's do this:



Friends at Aerotronics suggested I install this RF filter, which cost $25 and took like 5 min. Problem solved!




https://www.alliedelec.com/product/l-com/dgfc25mf/70364521/

Look in the last pic. You have coax cables bundled with other wires. Regardless of frequency, any time you transmit (think coms, DME, and transdponder/ADS-B) you will generate a magnetic field around the coax. This is what is bleeding over into the CHT and EGT sensor lines. Those lines are unshielded by the way. Coax cables should never be near other wires and should never be bundled together. Even an inch of separation can work wonders.

Web

Thank you for that great point Web. I will address this under my panel. Much appreciated!

If you need them Panduit makes small nylon squares. I tie them between bundles to keep the separation.

Web

An even cheaper option than the filter is to put snap-on ferrite cores around the sensor wires, just before the display.

Let me do some preachin'

Filters are just bandaids. They simply cover over a problem. If you put filters on a line (ferrite beads are filters too) they simply mask the fact that there is unwanted electrical noise present. The fix is to eliminate the source of the noise. If it's from a mag or alternator, have it repaired so it does not inject noise into the surrounding circuits. If the source of the noise is un avoidable, such as that generated from coax cables or from high current wires such as the alternator output wire, then separation is called for. Don't bundle those wires with any audio or sensor wires. I.e., do not allow the noise to get into the surrounding circuits. If wires need to be routed closely, such as through an access hole or up a windshield post, make sure those are the ONLY places they come into close contact. Keep all coax cables separate from each other. They are designed to both transmit and receive signals so if two or more are bundled together one will pick up the signal transmitted from the other. I've seen this run the range from making audio noise all the way to flip-flopping a standby frequency.

When doing a new install, keep high current wires and coax cables separate. If troubleshooting noise in an existing install, separate out the high current wires and coax cables.

Web