Radio Interference

[Kyle Dixon]

We have been experiencing radio interference while transmitting from a Kenwood VHF TK-790 mobile radio which is installed in a 2010 Freightliner M2 ambulance.  This problem has been on-going for the past three months and we have nearly exhausted all options and resources in determining the source of this interference.

The ambulance is equipped with a Kenwood VHF TK-790 radio with dual heads. The first radio head is installed in the cab of the ambulance with the second installed in the patient care compartment of the ambulance. The RF deck is installed in the cab of the ambulance behind the drivers seat.  The cable for the radio head in the patient care compartment ran through the wire chase with many other electronic cables and wires from the cab to the box. A "wind noise" can be heard in the radio transmission when transmitting and seems to become worse when the engine speed is increased.This wind noise at times causes the crews radio traffic to be very difficult to understand. We have swapped RF decks, microphones, radio heads, and cables with unsuccessful results in eliminating the wind noise. There is an in-line filter installed on the power cables for the RF deck and that has also been replaced.  The alternator has been tested by our Freightliner service center and is reported to be okay. 

One radio service center believes the interference is being caused by engine, while a second believes it is from other electronic devices or equipment in the ambulance. Any ideas or suggestions?

[Kevin Delmonico]

Anytime radio interference frequency seems to increase with engine speed it is related to a part of the engine. Although the alternator tests fine, I would have to say that is the culprit. Rarely will any other component act this way because the alternator acts as an electric generator. Along with electricity comes residual RF. if this RF Is not shielded properly you get your interference. The first step I would take is to buy a couple feet of 1" braided grounding strap. I would tin one end with soldier and drill a hole in it. Then I would attach this strap to one of the mounting bolts on the alternator. Prep the opposite side of the strap the same way and bolt that to the closest solid chassis piece. If this doesn't kill the noise try taking another piece of strap and attach it from the firewall to the hood mount bracket on the hood. Traditionally the hoods aren't grounded and get a very poor ground through their hinge. If the hood is fiberglass you may still get benefit by grounding the metal frame beneath it. It is important to use the ground strap since wire doesn't conduct the RF in the  same manner.

If you have an oscilloscope you can scope the input voltage to see if there is any RF. If it fluctuates  with RPM then it is a voltage problem (not likely). In this case placing a ferrite trap over the voltage line and a 47 microfarad capacitor across the supply line usually absorbs the noise. Lastly if all else fails take the ground strap and run a piece or two between the frame and the rear patient compartment. The pt compartments are mounted via rubber grommets and don't get good ground conductance through the mounting bolts. I hope some of this helped. If not let me know and we will go back to the think tank. 

Sent from my iPhone

--
You received this message because you are subscribed to the Google Groups "NEMSMA-EMSfleet" group.
To view this discussion on the web visit https://groups.google.com/d/msg/nemsma-emsfleet/-/IHC3KI1d0F4J.
To post to this group, send email to nemsma-...@googlegroups.com.
To unsubscribe from this group, send email to nemsma-emsfle...@googlegroups.com.
For more options, visit this group at http://groups.google.com/group/nemsma-emsfleet?hl=en.

[Nora Helfrich]

I forwarded your email to our radio repair service to see if they had any ideas or suggestions for you

[Allen Lovett]

We use Kenwood KLF-2 line filters (there are other brands as well) on our units with success.