[RFI] Mobile RFI Help Please

Jim Brown jim at audiosystemsgroup.com
Mon Sep 5 14:36:38 PDT 2011

I understand your desires to pull the noise further down -- any NB has 
some side effects that can be a problem with strong signals nearby.

Another suggestion -- if one choke made an audible difference, try two 
or more. You can also stagger-tune them to cover widely spaced bands.

Do you have a feel for how much more reduction (in dB or S-units) that 
you need to achieve?

Other thoughts.  First, I recall that you are using very large 
conductors on the DC feed from the battery.  This may be 
counter-productive, especially if they are very short.  The power leads 
may also be coupling mechanism for ignition noise, so choking them would 
be a very good idea.  If it were mine, I would downsize them to 
something like #10 so that I could wind that pair into a serious common 
mode choke.

Second, what are you doing with the chassis of the rig?  Is it screwed 
down to the frame?  Consider isolating it from the frame, realizing the 
problems with frame bonding may be putting noise on what you're bonding to.

Third, consider using very well shielded coax for the antenna line.  
There's a slight possibility that if there happens to be a lot of noise 
current on the body near where the coax runs that noise could be getting 
induced from the shield to the center conductor.  A higher quality, 
lower resistance shield could help that.

Finally, you could try to get really anal with chassis and frame 
bonding.  Depending on how bad it is, this could help both your noise 
and your transmitted signal.  Or it could be a huge amount of work for 
little reward. That's why I've never bothered.  :)

73, Jim K9YC

  On 9/5/2011 1:41 PM, Mack McCormick wrote:
> Jim,
> I've added the antenna feedline current choke per your recommendation. 
> It seems to have helped. I still have some remaining ignition noise 
> though it is about 70% better than it was when I started this and 
> everything else I've already done. I plan to add ground straps in 
> three more places and then guess I'll take a portable receiver around 
> the vehicle to try and find hot spots.
> After this, I don't know anything else to try but all suggestions are 
> still most welcome.
> The radio is completely usable if I enable the DNL on the TS-480 but 
> I'd like to try and fix the last remaining ignition/injector noise.

More information about the RFI mailing list