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Re: Topband: Chokes, et. al.

To: "Mike Waters" <mikewate@gmail.com>, "topband" <topband@contesting.com>
Subject: Re: Topband: Chokes, et. al.
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Sat, 14 Jul 2012 17:56:07 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
Hi Mike,

> I'm in full agreement with this, unless we consider our shack computer 
> part
> of our "gear". :-)

The context of this was entirely "transmitter" RF cables, which also are 
used at times for receiving, and devices connected in the transmitting 
feedlines on the desk. The difference with coaxial transmitter and amplifier 
feedlines, and RF ports on other well-designed RF accessories, is the ports 
and cables contain signals inside. If transmitter system connectors or 
cables "leak" or carry common mode, it is very abnormal. It strongly 
indicastes a problem that should be located and corrected.

Other accessories can be different. While **cases** generally should be 
common to the rest of stuff, not necessarily so for leads exiting through 
poorly implemented ports. This would include many things along with 
computers, like switching supplies, which might need isolation on some leads 
due to poor port system design.

> I also have a split core around my keyboard cable, to keep it from
> radiating RFI.

That's a little odd. On low bands, keyboards have a very high common mode 
impedance. They can't drive the cable very hard with common mode, so few 
beads don't generally do much on lower frequencies. They add a few hundred 
ohms in series with a CM path already in the many k-ohms. They can and do 
have large differential mode currents, but not common mode, on low bands. On 
upper HF through VHF, especially with hand capacitance, they can drive the 
cable pretty hard.

Are you sure you don't have a receiver system "leak" exaggerating problems? 
Maybe a bad box connector mounting method (like a plastic box with wires 
from RF connectors) or other RX system issues?

73 Tom 

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