Topband: Mother of all ferrite common-mode coaxial chokes

[donovanf at starpower.net]

If ferrite isolators or beads have been determined to improve the shield performance of a coaxial cable in your hamshack, something is fundamentally wrong either with the cable (but most likely the connectors at either end) or the shielded enclosure(s) its connected to.

I've seen literally hundreds of improperly installed PL-259, type N and BNC connectors.  Years ago I stopped allowing anyone to bring coaxial cables into my shack because so many are improperly installed.

Outdoors the situation is fundamentally different, especially at the feed point of a balanced antenna.  Ferrites are a good choice for well designed broadband baluns (but many popular baluns are poorly designed).  For monoband antennas I prefer 1/4 wavelength and 3/4 wavelength current forcing coaxial cable baluns.  See the latest ARRL Antenna Handbook, 22nd edition, page 24-50, or 

http://www.qsl.net/i0jx/balun.pdf

73
Frank
W3LPL


---- Original message ----
>Date: Fri, 13 Jul 2012 10:31:24 -0400
>From: "Tom W8JI" <w8ji at w8ji.com>  
>Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes  
>To: <topband at contesting.com>
>
>Hi Carl,
>
>> Youre welcome to your opinion based upon your experience Tom.
>
>It is much more than experience. Good science can be proven or illustrated 
>through experiments and measurements. Opinions are just opinions, and have 
>the same value as the effort that went into confirming them.
>
>For example, a bead does not "keep a signal inside a cable". It simply 
>changes the outside shield impedance. The outside of the shield, at a few 
>hundred kilohertz or higher, is isolated by skin depth from what the filter 
>affects. If we have significant or problematic outer shield currents on desk 
>gear, it would be much better to fix the bad cable, bad connector, or figure 
>out why the cabinet is behaving so poorly that unwanted currents spill over 
>to the outside of the cable.
>
>There are valid applications for coaxial isolators, but they are all outside 
>the shack or away from the desk equipment.
>
>> While 160 never presented any problems your rebuttal to the YCCC paper was
>> generic and on 20-10 and also 6M I beg to differ based on my and others
>> experiences.
>
>Personal opinions, without reasonable technical backbone, don't mean much. 
>Data, or at least good logical explanation of how something works, goes a 
>whole lot further than opinions, guesses, or personal declarations.
>
>You can see how a wired connector disturbs common mode sensitivity here:
>http://www.w8ji.com/coaxial_cable_leakage.htm
>
>Scroll down to Connector Mounting and you will see just an inch of open 
>connection to a non-chassis mounted connector on 8 feet of cable ( actually 
>8' 3" with that pigtail installed) increased 40-meter common mode response 
>by 40 dB over an 8-foot long cable routed normally.
>
>If I had common mode ingress or egress issues on higher power RF cables in 
>my shack required illogical band-aids and hole plugs, I'd figure out what 
>was really wrong with my equipment, cables, or wiring.  :-)    If stuff is 
>built and wired correctly, there isn't even a reason to use RF grounds on a 
>desk. They'll make no difference at all. Neither will isolators or beads on 
>RF cables.
>
>73 Tom 
>
>_______________________________________________
>UR RST IS ... ... ..9 QSB QSB - hw? BK