Topband: QST Jun 06 RX Loop
k3bu at optonline.net
k3bu at optonline.net
Wed May 17 19:30:20 EDT 2006
> Could it be that my co-ax loop is rejecting something in its very near
> field,(the TV of my neighbour that I suspect is about 10 meters
> away from my
> RX antennas!)or is it a case of unbalance in the Flag
> antennas,(that would
> make sense)I do not have to rotate the loop to get rid of the
> offendingnoise, it simply is not there!I also tried K9AY (various
> ones) with the same
> problem as the flags.
> Raoul Coetzee
the shielded or coax loop is providing electrostatic field shielding from the near by interference by its virtue of capacitance of the shield to the ground. Shield is a shield and NOT an antenna as W8JI claims, otherwise you would not see the effect (signal is a signal he claims).
Shield provides symmetry and contributes to better, deeper nulls. It also serves as a capacitor plate that is grounded and shunts the near field interference to ground and minimizes its pickup by the antenna wires inside the shield.
Here is the excellent description by KB5WZI from the rec.radio.amateur.antenna news group:
>>>Tom, W8JI wrote:
"There is absolutely nothing that causes (sic) noise to electric field
dominant and the shield absolutely does not "filter" the time-varying
electric field from the time varying magnetic field."
A "Faraday shield" is designed to allow magnetic field coupling while
disallowing electric coupling. See page 38 of Terman`s 1955 edition:
"It is possible to shield slectrostatic flux without simultaneously
affecting the magnetic field by surrounding the free space to be
shielded with a conducting cage that is made in such a way as to provide
no low-resistance path for the flow of eddy currents, while at the same
time offering a metallic terminal upon which electrostatic flux lines
I`ve previously described the Faraday picket fences or Faraday screens
used in the medium wave broadcast stations where I worked that were used
to avoid capacitive coupling to the antennas while permitting magnetic
coupling. Capacitive coupling would favor harmonics of the operating
frequency. These are undesirable. The Faraday screen effectively rejects
the capacitive coupling. It shorts the lightning strikes to ground too.
In a Faraday screen one end of pickets or wires is grounded. Their other
ends are open-circuited. So, circulating current can`t flow through the
wires. Thus, no counter-field can be generated to oppose magnetic
coupling but capacitive flux lines land on the wires and are shorted to
ground. It all works very well.
Look at Terman`s shielded loop on page 1048 of his 1955 editiomn.
There`s a gap in the shield opposite the feedpoint. The gap prevents
current circulation in the loop shield thereby making it permeable to
magnetic coupling while shorting the electric field to ground.
Therefore, this loop cover is a Faraday screen.
Why should we care if noise comes from near or far? The near field has 3
components. See "TV and Other Receiving Antennas" by Arnold B. Bailey.
The first near field component is produced by the electric vector and
decays by the cube of the distance. The second is the induction field
and decays as the square of the distance. The third is the radiation
field electric vector which becomes the volts per meter at a great
distance. This decays inversely with distance and its power decays as
the square of the distance. 6 dB every time the distance doubles.
Point is we don`t have to get very far from a noise source to make a big
improvement in noise received, especially if we avoid electric field
coupling which decays especially fast in the near field.
Best regards, Richard Harrison, KB5WZI <<<
The point is that those properties of shielding the ANTENNA inside the electrostatic shield are only noticeable when the antenna is close to the interference source.
Thank you for providing unsolicited proof of performance and function of the shield!
73 Yuri Blanarovich, K3BU
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