Re: Radio static/Defroster

[Pcarselle <pcarselle@worldnet.att.net>]

human wrote:
> 
> -- [ From: human * EMC.Ver #2.5.02 ] --
> 
> >At least one other lister (Huw Powell) has the same problem I do:  static
> between the rear defroster and the radio.<    (that would be me)
> 
> >The improved reception from the new antenna only made me realize that
> whenever I turned on my rear defroster I get static in the radio.
> 
> Exactly.  It is subtle enough that you need a good clean system to really be
> irritated by it.
> 
> >I only get static through the radio, not when I listen to a CD or tape.  I
> don't think it's my install, because I used to get it in the old, factory
> set-up. I've tried just about every sensible and non-sensical solution to
> rid my car of the static -- including wiring the rear defroster direct to
> the battery, bypassing all switches and wiring.  I still got the static. -
> Joe Y.
> 
> Thank you Joe for saving me the rewiring trouble!
> 
> Another data point: it is signal strength dependent.  I'll be on Route 4 in
> Durham NH listening to Maine Public Radio or WGBH in Boston, and turning on
> the defroster will trash the reception.  If I then switch to WUNH, whose
> antenna I am then right under, the static almost disappears.  It's not being
> masked by volume - even if they are playing something quiet this is apparent
> .  So, dear listers, if you have any knowledge of RF arcanery, what's up?
> Seems to me that putting the defroster into an electrical circuit is causing
> it to interfere in the antenna's ability to pick up a clean signal.  Is this
> how radio works?  can the defroster "out-antenna" the regular antenna, or
> sheild it?  Is it acting as a diffraction grating/prism?  Could there be
> some sort of filtering that would help, i.e. a small inductor in series with
> the defroster, and/or a high quality capacitor to ground right at its
> connections?
> 
> Is this quest doomed to failure?
> --
> Huw Powell
> 
> HUMAN Speakers
> 7 Kelsey Road
> Lee NH 03824
> 
> http://www.thebook.com/human-speakers
> 
> 82ish coupe
> Used parts store: 80 5k, 80 5kt, 82 coupe, 87 5k

Huw:

Re antenna assumptions....
The rear defroster is working in dc voltage and current circuit. This 
means that theoretically any magnetic field created by the heater 
current flow will build up when the circuit closes and colapse when the 
circuit opens..this activity of the electromagnetic field could cause a 
signal into your car antenna ( so fast it would probably be
undetectable ). However.... your alternator superimposes an alternating 
current component (ac) as a byproduct of the rectification into dc 
necessary to charge the battery... this evil peaks are commonly unwanted 
visitors of stereos, cd players, cassette players...etc. These 
installations do require adequate filtering of their respective power 
supply, to eliminate the whiiiiiiniiiing in the audio produced by the 
alternator. These peaks will make a magnetic field in the defroster to 
"oscillate" at "alternator frequency" and due to the proximity to 
the antenna, could attenuate the current signal of " weak " or distant  
stations. There is much higher probability that an intermittent current 
path creates the same but much more probable effect.
Recommendations:
Drive your usual route to where you clearly notice the problem.
Stop the car, with the defroster and radio on and  experiencing the 
"static"... kill the engine and swith ignition back on  to allow de 
defroster to work...( the alternator is now dead stopped ) if the static 
disappears, either the alternator or the ignition where creating the 
mentioned problem....
Cure....
Play cassettes or CD's in winter.....sorry !...
Check for good connection at defroster terminals.
You may try a "large" capacitor accross the defroster terminals, with 
some alligator clips to see if it helps..
The power handling of some filters used for high power amplifiers may be 
enough for use in the feed to the defroster.. recommend research before 
buying...
Good luck
Pablo