[Elecraft] RF feedback

[Jim Wiley]

Gentlemen -


No amount of adding low-pass filters, inserting antenna tuners, 
grounding your set, etc.  will clear up these problems.  Don't let 
anyone tell you different.  Facts is facts, and that's that. 


While all these steps are good things to do, and follow what is commonly 
known as "good amateur practice", the problem is outside the 
radio-amplifier-antenna "package".   This sort of problem is caused by 
the strong signal from your transmitter overloading the audio amplifier 
circuitry in the powered devices.  There is nothing you can do at your 
own station, other than running much lower power, to reduce the problem.


Consider this:  The speakers and headphones, wired telephones,,  
intercom systems, etc. are by definition, not designed to receive radio 
signals directly.  Ergo, they are malfunctioning in the presence of 
strong RF fields.  The problem is created inside  the device by the 
strong RF field that your antenna (hopefully) is radiating. 


Manufacturers of consumer devices, in their attempt to be competitive 
(and to maximize profits) tend to leave out the parts that would prevent 
this type of problem.  They take the chance that their product will  not 
end up in  near proximity to a ham transmitter (or other strong RF 
source) . This gamble works often enough that they get away with 
sub-standard construction,  plus they count on the scenario that the 
general public won't understand the real problem, and blame it on the 
poor ham.  In other words, they prefer to skate on their 
responsibilities by shifting the blame to someone else, anyone else, 
rather than doing their job. 


The fix, both for your own equipment and the neighbor's stereo, 
telephones, etc. is to install filtering or suppression devices on each 
device that is experiencing problems.  Installing  ferrite RFI 
suppressors in series with speaker or headphone leads,  wrapping the 
cables around ferrite rods or cores, capacitors across speaker leads 
(also from each side of the speaker connection to ground), and similar 
techniques are what is required. 


A PR problem exists here.  It is difficult to get a consumer to 
understand why his brand new (whatever) is malfunctioning.  The problem 
happens only when you transmit, so it must be a defect in your station.  
The PR problem lies in convincing the poor neighbor that while you are  
the cause of the problem, it is not your fault.  A fine definition to be 
sure, and one where you r local club's RFI committee can help no end.  
No club or RFI committee?  Start one, and become the local expert.  Once 
your have solved a couple of cases, you become the "guru", and your word 
will be taken as gospel. 


As an aside, also remember that SWR or other mismatch issues will not in 
itself increase interference problems, other than in extreme cases high 
SWR might have the effect of reducing interference because it causes the 
applicable transmitter to back off it's power output so much that little 
signal is reaching the antenna - but then nothing is left to make 
contacts either! 


- Jim, KL7CC


Jack Regan wrote:
> Dick,
>
> I have an 811-H connected to my K3 running 500 to 600 watts cw. I have
> "problems" that might be similar to yours.  I get loud, fuzzy, cw tones on
> both my noise canceling head phones my external speakers.