[OZAPRS] HF APRS upsets the car's climate control

Ray Wells vk2tv at exemail.com.au
Fri Dec 19 11:02:05 EST 2008


I'll say it again. You have an antenna radiating lots of RF and you're 
worried about radiation from the coax?

You have vehicle wiring in the presence of a strong RF field from the 
antenna, not some piddling, insignificant leakage from the coax which, 
by the way, won't be affected by VSWR. Coax doesn't mysteriously become 
"leakier" just because the VSWR is high.

Think about the nature of the situation, and when you have, dump any 
notion that radiation from the coax is likely to cause the problem.

You need to prevent the RF on the wiring from getting into the 
controller by applying RF suppression techniques to the controller 
wiring. As somebody else commented, make sure the bonnet is bonded. Use 
DECENT bonding straps (the braid from RG213 coax should be considered an 
absolute minimum. Don't waste your time with RG58 braid) around the 
hinges (yes, both of them). Don't expect that to be a cure-all, because 
the front of the bonnet is a long way (in RF terms) from the grounding 
points, when the hinges are at the windscreen end. Is the problem any 
worse/better with the bonnet up/down. That will give an indication of 
the effectiveness of bonnet bonding. Fit ferrite "filters" around the 
wiring into the controller. RF bypass caps will likely have no effect 
because of the difficulty of providing a SHORT, DECENT ground point, 
which could also be alive with RF.

Make yourself a field strength meter (a 1mA FSD moving coil meter, 
diode, bypass cap and a sensitivity control) in a metal enclosure with a 
coax socket for the input. Use a length of coax with just 25mm of the 
inner conductor protruding at the far end (away from the meter). Use 
that as a probe to assess the level of RF at various point on the car 
wiring. If you need more sensitivity, use a DC blocking cap (say 0.01uF) 
attached to the inner conductor of the coax and touch various points of 
the wiring with the capacitor. It's extremely simple and effective.

Suppressing interference both to and from transceivers in a vehicle 
situation can be very time consuming and, sometimes, very ineffective. 
What works in one case won't work in another, and what you might think 
is "earthed" isn't necessarily so. A major trap with heavier 4WD 
vehicles is that the body is not necessarily bonded to the chassis, it 
sits on rubber mounts. Bonding around those mounts can return a huge 
improvement.

Some years ago a neighbour's diesel Landcruiser produced S9+ noise on 
27MHz. The source of the problem was a combination of the oil sender 
unit and the wiring to the instrument panel. I used a probe like the one 
described above, attached it  to the affected radio, and probed the 
engine compartment. The entire engine was alive with crud. I reduced 
that noise to absolutely nil by bonding around the two front rubber body 
mounts.

Suppressing interference problems can take a lot of time, but that time 
can be reduced if you don't go chasing improbable causes.

I had 25 years of installing two-way radios, and some of what I learned 
hasn't escaped via that sieve between my ears :-)

Ray vk2tv

Tim Ahpee wrote:
> Hi Folks,
>
> I spent sometime last night re-routing the coax from the left side of the car
> to the right side of the car. Now the coax is away from the fan. I also
> checked the antenna match using a network analyser and it reports an SWR of
> 1.1:1. I also added a clamp on ferrite torroid onto the coax near the antenna
> end. 
>
> So far, no change. At least it's only the fan that it seems to be upsetting
> and nothing crucial like the cruise control :-)
>
> I'm going to have a play with grounding over the weekend and also get a hold
> of a dummy load. 
>
> If worst comes to worst I can mount the antenna up the back of the car away
> from the over-sensitive electronics.
>
> Cheers,
> Tim
>
> On Thu, Dec 18, 2008 at 11:40:13AM +1100, Damien Gardner Jnr wrote:
>   
>> Yeah ditto on my the VR Calais I had - ended up using all remote head  
>> gear in the boot, with antennas mounted off the boot...  Havent gotten  
>> around to looking at what this VHF radio i swapped a mate an MTR-8000  
>> for many moons ago is yet, so haven't put radios in the VT wagon and see 
>> if that has a problem... :)   The old '87 diesel luxie never had any 
>> issues at all. It's just a shame it was such a dog to drive ;)
>>
>> --DG
>>
>>
>>
>> On 17/12/2008, at 8:52 PM, Norm VK3XCI wrote:
>>
>>     
>>> I've been here with my old VN Calais. Make sure the bonnet is bonded  
>>> to the rest of the body!!! also the bullbar, tho I think you said the 
>>> spotlights are on it. Maybe bond the base of the antenna directly to 
>>> the rest of the body. Use double shielded coax?
>>>
>>> Worst possible case, put the antenna at the back of the vehicle. Good 
>>> luck, these things are usually simple or impossible.
>>>
>>> Norm VK3XCI
>>>
>>>
>>>
>>> Tim Ahpee wrote:
>>>       
>>>> G'day Folks,
>>>> I've just got HF APRS running in preparation for my trip at the end  
>>>> of the
>>>> year and have come across an interesting one. Each time the rig  
>>>> transmits the
>>>> fan in the climate control goes to full power. Nothing else seems to 
>>>> be
>>>> affected just the fan. It was a bit of a shock the first time it  
>>>> happened. I
>>>> was driving along and heard the TX relays go click then I was  
>>>> blasted in the
>>>> face with air. Details:
>>>> Rig: Icom IC-7000
>>>> Car: Toyota Prado 2008 Diesel
>>>> Antenna: Diamond HF30FX mounted on the bull bar
>>>> Any thoughts? I'm wondering if the RF is getting straight into the  
>>>> motor
>>>> controller for the fan because if the fan is off nothing happens.
>>>> Cheers,
>>>> Tim
>>>> VK3TZA
>>>> _______________________________________________
>>>> Ozaprs mailing list
>>>> Ozaprs at aprs.net.au
>>>> http://aprs.net.au/mailman/listinfo/ozaprs
>>>>         
>>> -- 
>>> Regards de Norm, VK3XCI
>>> Mildura on the Murray
>>> QF15bt
>>> _______________________________________________
>>> Ozaprs mailing list
>>> Ozaprs at aprs.net.au
>>> http://aprs.net.au/mailman/listinfo/ozaprs
>>>       
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