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[aprssig] Why is my AvMap G6 flaky?

Dave B dave at g8kbv.demon.co.uk
Fri Jan 4 10:23:51 UTC 2013


On 2 Jan 2013 at 20:00, Ron Stordahl, AE5E wrote:

> 
> Wouldn't this electrical noise make the AM broadcast band 
> nearly useless? And if so how would this get by the 
> designers. 
> 
> Ron, AE5E
> 
> 

< big snip >

The EMC emission tests, are only designed to protect intended (local 
area) broadcast signals, who's levels are several 10's of dB greater than 
anything we might be interested in.

It's relatively easy too, to selectivly filter something, so it scrapes 
through a test.

Then, there is the "Technical Construction File" route to passing a test.

Vehicle system susceptability/immunity (call it whatever, but EMC the 
other way round) is rigerously designed for and tested, to *Very* high 
levels (100's of V/m across the spectrum) so it is unlikely even a modern 
Hybrid would falter or ever twitch at the sort of "normal" level RF we 
might genereate in a vehicle with regular Off the Shelf commercial Ham 
kit.

Much of this I've seen first hand, visiting Automotive EMC facilities, 
commisioning and supporting their EMC test equipment, especialy the multi 
kW amps they use.   Unless you have a free 3-phase feed (63A/phase at 
least) in the shack, you won't want one, as they tend to be under 20% 
efficent, on a good day!.

The emissions test receivers/analyzers also are not as sensitive as a 
"communications" grade RX, but they are stable accurate and calibrated.  
That, and knowing the feder cable losses vs frequency, plus the measuring 
antenna performance (from indipendant calibration) also how the chamber 
or OATS behaves (Open Area Test Site) gives them a good confidence level 
when measuring emission levels.

However, the nature of some emissions, beeing very wide band and very 
short duration (narrow pulse trains) can "fool" the RX's into recording a 
lower level, than is actualy being radiated.  There are test procedures 
to detect such phenominon, but they take time, and time is $ etc.

However, all that pales into insignificance, once a local mechanic has 
had their hands on it, as there are as yet, no requirements to re-test a 
vehicle after any service (or street modification) works are done.  Yet?

It's dificult enough for Trafic Cops to measure tail pipe noise levels if 
they suspect something is "too" loud, imagine them carting arround RF 
analyzers/receivers and test antenna's?   (plus exhaust gas analyzers and 
so on...)

Happy New Year All.

Dave G0WBX.




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