Aerials (Antennas) And Earth Systems - AOR AR7030 Operating Manual

11 Aerials (Antennas) and earth systems

The subject of aerial choice (and earth) can be quite
complex. There are many advantages and disadvantages
of each aerial type to consider before connecting an
external aerial to your receiver.
One interesting phenomenon is that aerial theory and
practice can be surprisingly different. Within common
sense bounds, aerial construction is one of the few
remaining areas for listeners to easily experiment and often
achieve fantastic results.
Whip aerial: Whip aerials can give fair results for casual
listening. For best results external aerials in clear space
are recommended.
Location: It is important to mount any external aerial
as high as possible and in clear space. If possible the
aerial should have a clear path to the horizon.
Results are usually disappointing when an installation
is in a loft space.
Long wire aerials: For short wave reception a
random length of long wire approximately 10 to 20
metres in length forms a good compromise. The wire
should be connected to the WIRE input of the AR7030.
If possible try to locate the receiver close to a window
so that the wire has the shortest and most direct run
from the rear of the receiver to the outside world.
Never attach the wire aerial directly to a support or wall.
Instead attach a short length (about one metre) of
insulating cord (such as nylon) to each support (house or
tree for example) and then onto the aerial wire. Allow the
wire aerial to drop diagonally into the window and receiver
rather than straight down, close to the wall. Keeping the
aerial away from supports and buildings will reduce the
loss of signal from the wire aerial and prevent unwanted
noise from entering the aerial system.
Transformer coupled long wire aerials are becoming very
popular as they allow coaxial cable to be used as the
down-lead from the wire aerial into the receiver. The
transformer converts the medium impedance at the end
of the wire to a lower value, suitable for 50ohm coaxial
cable. In this instance the path of feeder is unimportant
and chances of noise entering the aerial system reduced.
The 50ohm aerial input of the AR7030 is ideally suited for
such an aerial system.
Dipoles: For the very best results you should consider a
dedicated aerial such as a single or multi-band dipole or
similar aerial. It is quite easy to make a dipole for short
wave. Short wave are usually mounted horizontally. It is
worth noting that dipoles are also quite effective on two
and three times their design frequency so you can cover
a few bands at once. Reception using a half-wave dipole
is best at right angles to the direction of the aerial, however
if used at two or three times it's fundamental frequency,
reception is best along the direction of the aerial.
A dipole has two legs running in opposite directions. One
leg is connected to the centre conductor of the coaxial
feeder cable while the other leg is connected to the outer
screen of the coax. A simple formula can be used to
calculate the required length of each leg for a half wave
dipole:-
Length of each leg in metres = 75 / Frequency in MHz
For example, a half-wave dipole resonant at 14.2MHz
would have each leg 5.3 metres long. The total length of
the aerial would be 10.6m plus the supporting cords.
Coaxial cables: When connecting dipole aerials to
receivers 50ohm coaxial cable should be used. For short
wave URM43, URM76 or RG58U is ideal.
Aerial Tuning Units (ATUs) and Preselectors: An ATU
can improve the selectivity of any receiver when listening
to short wave and connected to a long wire aerial (other
than a very short wire of just a few metres). Preselectors
will normally work with 50ohm aerial systems although
they may include matching for long wires. This valuable
extra selectivity is created by the ATU or preselector
rejecting out of band signals enabling the receiver to single
out one band of frequencies while rejecting potentially
strong unwanted transmissions. One disadvantage,
however, is the need to constantly re-tune the ATU or
preselector when changing receiver frequency.
Passive devices, which contain no active circuitry and
generally do not need power, will not normally degrade
the performance of the AR7030. Users should be aware
that devices with pre-amplifiers or active matching units
are unlikely to have such a high dynamic range as
the AR7030 so the receivers performance will be
degraded by the device.
Earth systems: A separate connection from the
receiver to earth may improve aerial efficiency and
reduce noise. Suitable earth points include
connection to a water pipe, central heating radiator
or external earth rod. If fitting a separate external
earth rod, consider the implications carefully if your
mains supply uses Protective Multiple Earth (PME)
AR7030 OPERATING MANUAL PAGE 31