Blocking RF signals that have adverse affects on other electronic equipment

[Hillcotech]

Greetings,

I work for an Ag manufacturing company that makes products for John Deere and Case IH Combines. Almost all of these combines have CB's in them. When the mic on a CB is keyed the rf signal is causing adverse affects. The RF signal is affecting a slope sensing clinometer causing it to output a higher voltage which is then read by our controller which then causes the controller to output undesired signals. We have had success grounding the cb antenna, shielding harnesses with tin foil, redoing our power and ground to where we are direct to the battery however what works on one combine does not on the other. Is there something that we can do with our wiring harnesses that will completely eliminate this so regardless of a poorly grounded antenna or whatever our system will not be affected? We are looking into ferrite beads however there is a lot to that also. Any suggestions are greatly appreciated.

 

[zz0468]

Hire a competent RF engineer who specializes in electromagnetic compatibility (EMC). Your attorneys and accountants will thank you later.

 

[mmckenna]

Yeah, that's an issue that needs a pro.
There are two things you need to look at;
1- making sure the CB, antenna and power are all done correctly, but it's hard to control what the customer does once it leaves the factory.
2- The items that are being impacted negatively by the CB (or other radios) need to be addressed to keep the unwanted RF out of their innards. That might mean filtering their connections, shielding, etc.

Not something that's going to be solved easily by a hobbyist website. There are engineers that specialize in EMI (Electro magnetic interference) stuff. I grew up across the street form a guy that did this for some of the large electronic companies.

 

[prcguy]

Snap on ferrite products can go a long way in keeping RF out of stuff but you have to use the right ferrite mix and number of turns through the core. Just snapping a ferrite core over a wire will do nothing, you need a certain number of turns through the core to be effective at a particular frequency range.

For CB frequencies a #61 mix is probably the best but I don't know the number of turns to use for a snap on core. If you have the space I know 8 turns of wire through an FT-240-61 core provides the most attenuation around CB frequencies and that is a round doughnut shaped core that costs around $9 each.

How big around is the wire leading to or from the devices being interfered with and do you have a foot or two of slack in the wire to wrap around a ferrite core?

 

[Hillcotech]

Thanks. The wire gauge coming from the slope sensing inclinometer is small, probably 20 gauge. That is where we think the interference is coming from. At least the signal from that is what is being affected. Our harnesses are all custom so if we need to add additional length for future production that isn't an issue.

 

[phask]

Definitely needs a qualified Engineer.


About the time you think you have it fixed for CB a customer will then install a 100watt VHF/UHF radio and you have another can of worms.

 

[prcguy]

Here is my semi-professional recommendation. Buy some of these: 2X61-7281P2 - Amidon then wrap 6 turns of the same type wire used in the harness through the core with several inches sticking out each end. Every time the wire passes through the center is considered one turn and the .40 dia hole should pass many turns of 20ga wire. If you need a bigger version to pass larger power wires this model has a .514 dia hole: http://www.amidoncorp.com/2x61-4181p2/

Then cut the wires right at each affected sensor as close as possible and stick the ferrite with wires inline and you can run power and signal wires together in the core as long as they each make about 6 turns. You can use crimp butt connectors or male/female plug connectors or solder the wires and heatshrink.

This ferrite with 6 turns of wire should provide a good 25-30dB of isolation at 27MHz, meaning the level of interfering RF will be close to 1,000 times less at the sensor with the ferrite in line.

Thanks. The wire gauge coming from the slope sensing inclinometer is small, probably 20 gauge. That is where we think the interference is coming from. At least the signal from that is what is being affected. Our harnesses are all custom so if we need to add additional length for future production that isn't an issue.

 

[DJ11DLN]

It's usually the other way around with today's ag equipment -- massive RFI from all of these electronic systems. I'd think even getting a CB to work usably in that environment would be a losing proposition.

Agree with the others -- hire a pro.

 

[JayMojave]

Hello Hillcotech: Your manufacturing electrical engineer should know how to handle this fix, in a manufacturing method. The sensor manufacture should be formally notified for their input on the CB radio interference problem. Maybe there is a different part that has a RFI/EMI spec that will not have these interference problems.

PRCGUY has great solution using snap on Ferrite Chokes that have worked great for me. My GPS unit in my truck use to go "Fruit Loops" when I keyed my CB radio. And a internal audio amplifier Motorola Speaker use to pick up my CB Radio also. These snap on Ferrite Core work, I have no RFI/EMI interference any more. They are certainly worth a try. I would try one near the computer with just one turn going thru them first then as PRCGUY said using 6 turns. Good luck.

Jay in the Great Mojave Desert

 

[Hillcotech]

Thanks for all the feedback. I will let you know what we end up doing.