[RFI] Mobile RFI Help Please

Mack McCormick w4ax.mack at gmail.com
Wed Sep 7 04:31:36 PDT 2011


I did some more grounding of the frame to the body yesterday but it's making
no difference at this point. I believe I'm at the point of dimishing returns
and plan to cease work. Here are some spectrum screen shots of the noise. It
is all generally less and -115 dBm and removed with the radio's internal
noise blankers. *http://tinyurl.com/3eeaj2d *On 80M/40M it is below the band
noise floor and not noticeable.

I also tried running the radio from a separate gel cell battery supply with
no improvement. I did an A/B comparison of another hams mobile installation
and his ignition noise was similar to mine.

Other answers are in your original email. Thank you again for working with
me on this but incremental improvements do not seem possible at this point.

I think I'll just key the rig and work the world!

Vy 73,


On Mon, Sep 5, 2011 at 5:36 PM, Jim Brown <jim at audiosystemsgroup.com> wrote:

> 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? <Mack> 5 dB would probably remove the noise.
> 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. <Mack> I powered the battery from a
> separate gel cell pack with zero improvement. I don't think the power line
> is the source. The source seems to be the ignition coil packs at the engine.
> I don't know a way to improve these.
> 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.<Mack>
> I've tried both way with zero difference. For now, the chassis is bonded to
> the body.
> 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. <Mack> I'll test this theory by placing a
> dummy load at the end of the coax that connects to the antenna to see if
> there is still RFI.
> 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.  :) <Mack> I've added about eight straps at
> this point with zero improvement from the beginning. I'm done being under
> the car.

<Mack> Thank you very much again!

> 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.


Mack de W4AX

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