[RFI] RFI In My Shack

[Joe]

Thanks Jim..

 

I'll read through where you suggested I read and see where I can improve my system.

 

One thing you said, that I haven't done, and will, is to have a common ground for the home theater systems.  Gonna be tough for the home theater room, it is upstairs and 150 feet away from the grounding pedestal.  One saving grace:  I have no RF activity into the subs with the L7, only the Henry.

 

One thing I *did* do on both theater systems, was to use Cat 6 for the speaker wiring.  I forget where I heard to do that, I think it was on one of my home theater groups.

 

As for your '2 line' mode being bad, I assume [  J  ] you are speaking of twin lead fed OCFD's.  No issues there, I use coax as a feed, not twin lead.  Twin lead will not survive 100 mph winds, which are common at my location.

 

OK, off to do some reading!

 

Many thanks Jim.  I will keep you posted on anything I read, that makes sense as it pertains to my system(s), that I will have implemented.

 

Joe – W7RKN

 

-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Jim Brown
Subject: Re: [RFI] RFI In My Shack

 

On 12/2/2017 9:25 AM, Joe wrote:

> A little different question.  Perhaps some suggestions here:

> 

> I have a new amplifier in the shack, a Drake L7, that is being used in 

> place of a European version of the Henry 2K D Classic.

 

The description of your system sounds like a recipe for RFI. I've done three tutorials on the topic. They are at k9yc.com.  The most theoretical on is k9yc.com/RFI-Ham.pdf   BTW -- I tore up a lovely 18-in JBL sub-woofer about 15 ft from a dipole. It was in shreds before I realized it.

 

Several highlights.

 

AC line filters are a complete waste of money. They only work on differential mode RFI (even though they say they do common mode) and nearly all RFI induced on wiring is common mode.  SO -- Ferrite common mode chokes are the solution that works.

 

Poorly balanced antennas are a MAJOR cause of RF in the shack and the house.

 

Almost any antenna fed with 2-wire line will put common mode current on it, and off-center fed antennas are the worst. The solution is a serious common mode choke at the feedpoint, BUT off-center fed antennas put HUGE common mode on the line, so any decent choke will FRY.

 

Any antenna that WORKS will put RF current on any conductors near it. We may call them speaker cables or audio cables, but Mother Nature calls them ANTENNAS. Nearly all equipment is built with a design defect called "The Pin One Problem," which causes any current flowing on cable shields to get into the equipment. If it's AF (power line fundamental and

harmonics) we hear it as hum and buzz; if it's RF, it gets detected and amplified.

 

The solution is pretty simple -- start with proper bonding of all equipment in the shack and any home entertainment setup. Add to that multi-turn ferrite chokes on all wiring coming into those systems, and if you still have problems, chokes on interconnecting wiring. Study this set of slides for a talk I've done for several ham clubs and at Pacificon and the Visalia DX convention. It has been a large part of the basis for N0AX's new ARRL book on the topic.

 

 <http://k9yc.com/GroundingAndAudio.pdf> http://k9yc.com/GroundingAndAudio.pdf

 

Another mechanism couples RF from speaker cables to power amps. All audio amplifiers have feedback around every gain stage to reduce distortion and noise. The feedback network around the output stage of an audio power amp couples any RF on the speaker cable back to the input, where it is rectified and amplified.  Two solutions for this. First, replace all zip cord used as speaker cable, glorified or plain, with twisted pair. Twisted pair has been used for more than a century to resist crosstalk, and is about 30 dB better than zip cord in rejecting differential mode pickup.  Second, if you still have a problem, put serious ferrite chokes on the speaker cable.

 

As to why you have problems with one amp and not the other, I'd suspect issues with bonding, or some combination of monkeys and typewriters writing Shakespeare.

 

73, Jim K9YC