Cecil Moore wrote:
B29 wrote:
Do folded dipoles have better noise rejection than single wire dipoles?

There's some controversy about that but here's my
take. Folded dipoles receive *electromagnetic noise*
just as well (or just as bad) as ordinary dipoles.
However, IMO, there is one type of noise that is not
as bad on folded dipoles as open-ended dipoles and
that is precipitation static noise defined at:

http://www.atis.org/tg2k/_precipitation_static.html

If a charged particle dumps its charge on the floating
wire of an open-ended bare wire dipole, there is only
one discharge path to the other side of the dipole
or to ground and that is through the transceiver.

Hello Cecil,

It is not certain that there will be a "discharge" (i.e., a change from
a charged state to a neutral one) at all. One possibility is that
charges on the wire will simply redistribute themselves after the
addition of a new charge. It also depends, obviously, on whether the
charged particle is of the same polarity as the preponderance of charges
that may already be on the antenna. My pet hypothesis is that these
charge redistributions cause changing magnetic fields which appear as
noise impulses in the receiver.

Remember from electrostatics that a grounded conductor, such as a tower
or an antenna, may not be at ground potential due to induction caused by
a cloud of oppositely charged particles, alien space ships, or whatever
charge concentration is in the vicinity of the conductor. Even though
the conductor is grounded! Of course, when the influence departs, the
grounded conductor returns to is neutral status.

Make sense? ;-)

Chuck


However, if a charged particle dumps its charge on
one half of a folded dipole, there are two discharge
paths to the other half of the dipole, one through
the transceiver and one through the antenna wire.
The antenna wire discharge path is usually a lower
impedance than the discharge path through the
transceiver and the entire folded dipole is usually
referenced to ground.

In addition, we know that insulated airplane antennas
are less susceptible to precipitation static than
uninsulated airplane antennas. Folded dipoles are
often made out of insulated transmission line.

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