----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Shoppa, Tim" <tshoppa@wmata.com>; <topband@contesting.com>
Sent: Thursday, July 12, 2012 12:37 PM
Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes
>> Figure 5 of W1HIS's writeup. 15 pounds of ferrite beads.
>>
>> http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf
>
>
>
>
> Unfortunately that can lead to a waste of time and material, and even has
> some bad advice.
** Considering the YCCC's long time reputation for top level engineering
ability plus the others mentioned such as K3LR Id consider that article a
must read.
>
> Common mode chokes NEVER belong between transmitting device like a radio
> and
> an amplifier, or between things on the desk in a system running chassis
> mounted connectors. The reason we do not need beads or common mode chokes
> in
> this application, and should never use beads or common mode chokes, is
> simple and obvious.
** Not really
>
> If we have a potential difference between chassis of gear causing common
> mode, we need to fix the source of that problem. The very last thing we
> ever
> want to do, unless our goal is to make money selling choke, is increase
> impedance between things on the desk.
** Ive been using beads over short runs of coax between the rigs and low
pass filters and again at the output of the amps into other LPF's for at
least 25 years. They do an excellent job of making sure RF goes thru the
filters and not around. With almost no RF ground possible its proven
effective for TVI/RFI.
>
> The above rule applies to connectors that are properly mounted to
> enclosures
> or shield walls.
** As with any standard equipment and filters.
>
> The reason we throw beads at audio lines and other lines inside the shack
> and see improvements is actually rooted in improper connector mounting or
> connector grounding to the enclosure. This virtually never applies to
> transmitting systems, where enclosures are metal and connectors are bolted
> to the enclosure. Beads or CM chokes on lines only applies to systems with
> inherent flaws in cable entrance or exit shield integrity.
** Again, not true; those are just one example.
For example, a
> power cord is not shielded for any of the length, let alone the entire
> length exiting the area of the station. There can be no shield integrity,
> so
> beads and CM filters are useful. They are also useful near where the
> system
> is nether perfectly balanced nor perfectly unbalanced, like at many
> antennas, or where a plastic box is used.
>
> It is absolutely certain though, common mode isolation does not belong on
> RF
> cables between enclosures with higher power RF on someone's desk.
** As I said it works well here and I suggest others try it rather than
unilaterally discount it on one persons say so.
With a
> defect in the antenna system, they can be useful just outside the room at
> the cable entrance or exit to the station area, but not inside the
> station.
> Used inside, on unbalanced RF cables that properly enter or exit boxes,
> they
> will increase problems more easily than decreasing problems.
** Thats about the 5th time you have said the same thing in this post, and
just moved some words around.
What works for you doesnt mean others cant benefit.
Carl
KM1H
> 73 Tom
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
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>
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