[RFI] 97 Chevy Tahoe noise problem

Scott Neader KA9FOX ka9fox at qth.com
Sat Jul 31 23:56:27 EDT 2004

Hey guys!  I have an ICOM IC706-MKiiG installed in my 1997 Chevy Tahoe, 
with a Hi-Q-Antennas 3/80 mounted to the towing hitch.  System works very 
well, except some nasty engine noise.  I have put up with the noise thus 
far by using the excellent Noise Blanker on the IC-706.  However, there is 
a penalty for using the Noise Blanker... loud signals that are within 
10-25Khz of the frequency cause a lot racket.  So, I'd like to fix the 
problem, if possible.

To help determine the source of the noise, I hooked up a separate battery 
to my rig, so that it had no connections to the car power, and the noise 
did not change at all.  Also, I would not describe it as "clean audio 
whine".  More of a low rumbly sound that increases in pitch with engine 
speed.  In other words, I don't think it's the alternator.

I have created a 60 second WAV file of the noise... it is about 1.2mb.  The 
file is here:  http://www.qth.com/ka9fox/noise.wav.  This file was created 
with the radio being powered by the external battery, with the radio tuned 
to about 14.200.  As I remember it, the noise seems to be heard equally on 
all bands, but if it is important to analyze the noise as it relates to 
frequency, let me know.

What you are hearing on this sound file is:

-- 00 seconds to 05 seconds:  engine off and the key turned off.

-- 05 seconds to 10 seconds:  engine off, but with the key turned on.  I'm 
told that fuel pump noise would appear here, if that were the problem.

-- 12 seconds to 13 seconds:  car is started.

-- 13 seconds to 23 seconds:  engine idle speed.  Noise is about S7 (NB 

-- 23 seconds to 52 seconds:  engine being revved up, then back to idle, a 
few times.

-- 52 seconds to end of sound clip:  car is turned off.

I have not made any attempts to cure the noise yet.  Some have suggested 
installing a Radio Shack 12v filter near the battery.  Others have 
suggested RFI surpressing spark plug cables, such as those from 
http://www.magnecor.com.  And others have suggested bonding all the body 
parts together with copper straps.  I'm pretty handy with a computer, but 
when it comes to cars... well, it's not my bag.  So, my question is... 
before I put in the big effort to implement these suggestions.. .are any of 
them likely to help this particular noise problem?  Anyone out there have a 
Chevy truck in the 1997 era that have had similar noise problems, that have 
been solved?  If so, what seemed to do the trick?

Thanks in advance!

73 - Scott KA9FOX

Scott Neader, KA9FOX    ka9fox at qth.com
KA9FOX Contesting & DXing Site: http://www.qth.com/ka9fox
Free Ham Radio Classified Ads:  http://swap.qth.com

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