HELP! RFI causing analogue instrument deflection

Thread Starter


Joined Jun 26, 2019
Help! I have been pulling my hair out trying to find the root cause of this inference. any Avionics techs in this forum?

This system is 24vdc Aircraft radio

The Situation

I currently have two resistive based instruments that deflect when radio is keyed.

I have determined RFI is coming from the Antenna as when a load or test set is direct connect to the antenna feeder at the antenna (RG-142) the deflection stops.

Radio is out putting approx. 10-12W at 117-137MHZ

T/Shoot carried out so far;
1; All grounds have been removed and cleaned, some light corrosion was found but nothing extreme(adding a grounding wire direct from the ground terminal of the instrument to the supply battery made the symptoms worse)

2; Power wires and Signal wires have been replaced with shielded wires

3; Relocation of antenna to achieve spacial separation

4; the use of a longer coax to add attenuation

5; Antenna SWR checked peak 1.4:1

6; Antennas removed grounding between antenna and Airframe checked, treated with conductivity promotor

7; New antennas


1; when transmiting a terminal voltage increase of approx. 1-1.5vdc is observed at the battery with no AC component (24vdc System) even with the use of an external battery cart the terminal voltage of this is also seen to increase the same amount

2; movement of the loom and separation of individual wires helps but not practical for longterm solution

I feel like the increase in terminal voltage is an indication of a unit possible resonating with the RFI but I am stumped if I can find a source, Have isolated most systems but can never get a definitive source. You start to see a small improvement but randomly the symptoms start to return.

My next step was to try a choke/ferrite bead on the loom to the instruments or possibly a capacitor across the positive and negative terminals of the the instruments? IF it was believed that this may achieve a constant voltage supply to the instrument, how would you determine the size of the capacitor for use?


Joined Nov 6, 2012
Increasing Voltage at the Battery is an impossibility,
and points more towards a Meter, or Testing-Method problem.

The Coax, and the Antenna, are the "A-number-One" target.
The number 2 target is the Grounding scheme used.

I'm assuming that the instruments that You are referring to are Temperature or Oil-Pressure Senders.


A bad/loose Ground at the Mounting-Base of the Antenna can also make crazy things happen.
This can turn the Coax-Cable into an Antenna running inside the Cabin.

It is highly unlikely that the RF being transmitted from the Antenna mounted on
the outside of the Craft,
is getting back inside the Craft to any significant degree, unless there are Coax problems.

The Transceiver must be mounted in the exact location recommended by the manufacturer of the Craft.
The Coax-Cable must be in perfect operating condition in every respect.
The Coax-Cable must be routed exactly as installed by the manufacturer of the Craft.
The Antenna must be mounted exactly as recommended by the Antenna manufacturer,
to a large, conductive, metallic surface, unless otherwise specified by the manufacturer of the Craft.
The Antenna must be in perfect, "like-new" condition,
and specifically designed for the Frequency-Range of the Transceiver.
The Grounding-Wire of the Transceiver must go directly to the Negative-Terminal of the Battery.
No Ground-Wire for any accessory should be attached to the metal body of the Craft in any way,
with the exception of a single Ground-Wire from the Battery to the Body of the Craft.


No Ground should be connected to the Engine-Block for these Sensors.
They should be 2, or 3-Wire Sensors, no single-Wire-Sensors allowed.
The Sensors should have a dedicated Ground-Wire all the way back to their respective Gauge.
The Grounds for each Gauge should go to a "Star" Grounding-Point,
located behind the Dash, and then from the "Star-Point" directly to the Battery.

The Engine should not be used for any Grounding of anything excepting
the Ignition-System(s), and the large Ground-Cable for the Starter-Motor.

Any Shielded-Cabling "Shield-Wire", or "Drain-Wire" should ONLY terminate
at the "Star-Grounding-Point" behind the Dash.
They should not be attached to anything else.



Joined Aug 21, 2017
Probably the coax braiding is radiating much. Try the ferrite cores on it. Often some 5 to 10 pieces are enought to stop any leakage. Low SWR is clear sign that leak is happening. May walk around with field meter and watch where it is going out.