[RFI] Auto quieting

Michael Germino ad6aa at sbcglobal.net
Mon Oct 5 18:13:52 EDT 2015

I did everything that Jim recommended and no problems.  I also avoided running all cable and coax along the vehicle wiring and crossed all wiring at right angles.

Don't be afraid to oversize your power cables to the battery.

Some new vehicles have a current sensor in the battery ground lead and your negative cable will need to be connected at the engine block (where your battery connects)  for your computer to know all current draw from the battery. 


73, AD6AA

From:"Jim Brown" <jim at audiosystemsgroup.com>
Date:Mon, Oct 5, 2015 at 12:19 AM
Subject:Re: [RFI] Auto quieting

On Sat,10/3/2015 6:28 PM, Michael Coslo wrote:
> Anyone have any ideas?

From an RFI/EMC point of view, I strongly recommend floating the radio 
from the vehicle frame, powering it with a twisted pair of big copper 
straight from the battery to the radio, with the only connection to the 
chassis being the coax shield at the antenna. Even better, add a high 
quality RF capacitor between the shield and the chassis. What this does 
is cause everything above low audio frequencies to return on the return 
side of the DC pair rather than on the chassis/frame.

In addition, I would also put SERIOUS multi-turn ferrite chokes on that 
DC cable near the battery, and also on the coax where it feed the antenna.

Fundamental principle -- you want the chassis/frame of the vehicle to be 
a counterpoise for the antenna, but you don't want it to carry DC or AC 
current, and you don't want any of the wiring you have added to be a 
receiving antenna for the RF you are transmitting, nor to radiate the 
noise on the chassis or other wiring.

73, Jim K9YC

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