Topband: Question Re: Coax Shield vs. Ground.

[Bill Tippett <btippett@alum.mit.edu>]

W8JI wrote:
>> We really don't want the coax shield having a direct path to the
ground the antenna uses. The ground for the antenna and the antenna
should never connect to the coax.

AA6VB replied:
>How do we isolate the coax shield from the radial system?  My coax goes
from the amp output to the input of my balun.  One of the balun outputs
goes to the radiator and the other to the radials.  Is that what you had
in mind?  And does it matter which of the two outputs from the balun go
to the radiator and the radials?

W8JI responded:
>I'm specifically talking about receiving systems. An
isolation transformer is cheap and easy, and since most
receiving antennas have poor grounds or no grounds, they
should never be tied into the coax shield...which would
allow common mode currents to pass. I just can't imagine why
anyone would build a receiving system where the coax can
contribute signal to the receiving antenna through a
shield-to-antenna path.

>In transmitting systems elevated radials should never have a
ground path. Grounding elevated radials reduces efficiency.

        Bob and Tom, I think two thoughts are getting
mixed together here...RX antennas and TX antennas.

        I agree with Tom above about RX transformers.
A simple check for this is to make sure the low side of
both the Hi-Z and Lo-Z outputs is isolated.  If shorted, as
with a tapped trifilar winding, then you should either go
to a binocular core (easiest) or to a quadrifilar winding on
a toroid where the trifilar Hi-Z winding is isolated from the
4th Lo-Z winding (but much more difficult to construct than
using a binocular core).  See a binocular core transformer
on Tom's website below (photo near bottom of webpage):

http://www.w8ji.com/core_selection.htm

        With elevated radials in a TX antenna, you also
need to isolate them from the coax shield.  Most use
a current balun at the feedpoint for this which prevents
RF from flowing outside the coax shield and thus
interfering with the performance of the elevated radials.

        BTW ferrite sleeves can also be used on the
receiving antenna coax as Tom explains below, but the
best solution is an isolated transformer at the feedpoint
which prevents RF from ever reaching the outside of
the coax (as Tom said in his first response above).

http://www.w8ji.com/common-mode_noise.htm

        Hopefully this helps clarify the two separate ideas
(RX and TX) being presented.

                                73 & HNY!

                                Bill  W4ZV



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