Re: [Elecraft] Mobile operation RFI from QRO (was KPA500 mobile)

[Jim Brown]

On Sat,4/15/2017 8:55 AM, Colin Brench via Elecraft wrote:

My expertise is in EMI control and so I can provide some background to how 
unexpected things happen.


Great post, Colin.


RF fields around the antenna will be high and most mobile antennas have a low 
input impedance.  To fully understand EMI risk you need to understand where the 
RF current will go, and simply put you need to consider the antenna as every 
conductor in the vicinity of the actual radiator.


This is FAR too simplistic a view. The metal body, chassis, and frame 
serve as the counterpoise for the antenna. These metal parts of the 
vehicle are the return for antenna current and for the field generated 
by the antenna. This makes them part of the antenna, they are carrying 
as much power as that vertical radiator we CALL the antenna, and current 
will be distributed around the vehicle's metal parts just as in any 
other antenna. The only difference between current distribution in the 
vehicle and in a wire is that the shape of the vehicle is very different 
from a wire, so that modeling it is far more complex. This current can 
easily couple into vehicle wiring running in proximity to that current. 
And, since the VEHICLE carries RF current, the vehicle itself is part of 
the radiator.


73, Jim K9YC

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Re: [Elecraft] Mobile operation RFI from QRO (was KPA500 mobile)

[Wayne Burdick]

This is why I run 10 watts mobile with the KX2 or KX3 :) Last time I tried 
this, using 10 W into a Hustler 40-m whip, I worked JA1NUT on CW and had no 
trouble checking into a statewide net on SSB. Also had quite a few SSB QSOs on 
17 m. 

So if you find all this excellent information about mobile/QRO overwhelming, 
try mobile/QRP. 

Wayne
N6KR


http://www.elecraft.com

> On Apr 15, 2017, at 8:55 AM, Colin Brench via Elecraft 
>  wrote:
> 
> Greetings to all,
> I thought I would start a separate thread to expand this topic is a slightly 
> different direction as this topic touches on a number of areas.
> The exposure discussion is very valid especially in today's vehicles that 
> often contain composites rather than metal panels, but I will leave this to 
> others.  My expertise is in EMI control and so I can provide some background 
> to how unexpected things happen.
> The DC issues are clear - you need to provide enough power cleanly to an 
> inverter.  Well over 100 Amps peak with acceptable drop.  This is all very 
> low resistance but not necessarily low impedance at the high frequencies 
> being used.  So some serious EMI filtering may be needed to keep things 
> stable.  
> RF fields around the antenna will be high and most mobile antennas have a low 
> input impedance.  To fully understand EMI risk you need to understand where 
> the RF current will go, and simply put you need to consider the antenna as 
> every conductor in the vicinity of the actual radiator. The one rule of 
> antennas is that current goes to zero/reflect back from an end.  This is how 
> NEC works - it solves for the current distribution over the entire structure 
> be it a dipole or frigate, these currents are then used to calculate the 
> antenna effect.  Now for a vehicle the antenna and car body are the most 
> obvious conductors to consider and may be OK for the antenna analysis.  
> However, considering all conductors is essential for EMI analysis, so add in 
> all the cables, door slots (well insulated breaks in the 'Faraday cage'), all 
> other antennas, engine and mounts ground straps etc.  Picking which can be 
> ignored in a given situation requires serious expertise.
> For QRO operation these directly induced currents are very critical.  
> Unwanted high current on any electronically controlled function can have 
> unexpected and potentially unpleasant results. The addition of the ham 
> antenna, DC wiring, other control wires combined with the possible positions 
> for each part of the system may drastically change the results of the system 
> analysis performed by the vehicle designers.  So caution is certainly called 
> for.
> EMI testing has a number of aspects, using a radiated field of 200V/m is one 
> typical test for remote EMI risks.  Direct current injection is used to 
> simulate near effects such as cells phones of an on-board transceiver of 
> 'moderate' power.  If you can find out how the vehicle you have was tested, 
> then it is possible to get an idea of how much risk there might be (using NEC 
> to guesstimate the induced currents on each band).  this is not easy
> Today's vehicles have so many safety critical electronic circuits that 
> manufacturers do go to extremes to keep everything bullet proof, but 1kW to a 
> (by necessity poor) antenna is most likely outside their considerations.  EMI 
> issues won't show all the time, as effects can interact, there is a stream of 
> data being passed at all times and upsetting some data patterns can be easier 
> than others.  Because of this testing takes a long time to get a high 
> confidence that vehicle performance is reliable or at least will fail to a 
> safe mode
> OK. all said what is practical here?  First I would consider how much power I 
> really want to use as the problems grow rapidly with higher power.  Seek the 
> advice of someone familiar with both QRO operation and your specific vehicle 
> if possible (as was done on this list).  Be cautious and aware of the vehicle 
> behavior until you are sure all is well.  I would happily run a 1kW in my 
> 1969 vehicle, but would be extremely cautious abut that in my 2015 minivan!  
> Though ensuring good general RF design consideration I would not worry about 
> 100W in the minivan, maybe 200W, after that I would be in caution zone.
> A call to the vehicle manufacturer might shed light on what they consider 
> reasonable or possible (though they might just say don't do that :-/  )
>   Very 73,
>   Colin.. WDJR
> 
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