> topic. Additionally, you seem to stress that using cheap RG-6 foil for
> Beverage feedlines (apart from not being very durable over time) may be
> not so wise if I am serious about common mode rejection.
RG-6 is a type of copper shield cable rarely seen, but same-size CATV and
MATV cables are commonly called RG-6. Any radiation through shields of
typical "F-6" CATV or MATV drop cables, and actually most other cables, is
for all practical purposes immeasurable and unnoticeable without very
special measurement systems. It is excellent cable for receiving, except
perhaps for immunity to lightning damage on foil shields.
I occasionally (once a year) lose one somewhere from lightning currents when
the cable connects to big ground systems, like on my vertical arrays. I
rarely have a problem with feeds to Beverages, because the grounding
resistance is generally fairly high resistance for lightning paths, making
ground loop currents low in Beverages.
We can use the biggest shield in the world and still have common mode, which
is the dominant issue by far.
> acquired a 1000 foot roll of Belden of double shielded RG-59 with BNC
> connectors affixed to each end. This will give me some better feedlines
> when I do a general replacement in the fall. I also have a nice piece
> of 75 ohm triax (two independant shields) and at one time I believe by
> grounding both shields at one end and leaving the outer one float at the
> other I could create a Faraday common mode noise shield.
Won't do a thing. It's electrically impossible to shield an inner conductor
for common mode by adding more shields, and neither the electric field or
magnetic field makes it though a shield anyway, so long as the shield is
more than several skin depths thick.
Also, beads can be a waste of time and effort, unless augmented by ground
rods or other things that force common mode impedance low.
I might use beads inside my room where I have less than perfect shield
ground resistances, like where I use phono plugs on cables, but they
generally are useless outside in cables using good fittings.
It's all about the ratio of series common mode impedance compared to
existing system common mode impedance at the bead insertion point. This is
why adding beads to a cable near a small loop, which has a very high common
mode impedance, is generally useless. As a matter of fact, if the common
mode impedance is capacitive, adding inductive beads or an air-core choke
can make common mode worse.
73 Tom
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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