AR5000 OPERATING MANUAL PAGE 73
at VHF/UHF frequencies than for short wave. 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 centre
pin of ANT 1 (N-plug) or ANT 2 (SO239). 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 (one metre) of insulating
material 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 the wall.
Keeping the aerial away from supports and building will
reduce the loss of signal from the wire aerial and prevent
unwanted noise from entering the aerial system.
Magnetic balun long wire aerials are becoming very
popular as they allow coaxial cable to be used as the
down-lead from the wire aerial to the receiver. The balun
transforms the impedance to a low level suitable for 50
OHM coaxial cable. In this instance the path of feeder is
unimportant and chances of noise entering the aerial
system reduced. The 50 OHM aerial input of the AR5000
is ideally suited for connection to a magnetic balun.
Dipoles
For the very best results you should consider a dedicated
aerial such as a single or multi-band dipole or similar aerial.
The problem with a wide coverage receiver like the
AR5000 is that for the ultimate results, many dedicated
aerials are required to cover the whole spectrum. This
may involve complex aerial switching and reduces the
ability to quickly monitor many bands unless the automatic
aerial switching system is carefully planned using the
optional AS5000 aerial switch.
As a compromise it may be worth making up a dipole
aerial for one band of particular interest and have a
VHF-UHF discone plus second random wire for general
listening.
It is quite easy to make a dipole for short wave, for that
matter one can be easily made up for VHF or UHF too. If
being made for VHF-UHF, the centre connection of the
coaxial cable feeds the upper element set vertically. Short
wave dipoles on the other hand 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 90 degrees to the direction the aerial is
laying, however if used at two or three times it’s
fundamental design frequency, reception is best closer to
the direction the aerial is lying.
A dipole has two legs running in opposite directions and
can be mounted vertically or horizontally (most VHF-UHF
activity is vertical). 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 coaxial feeder
cable. If mounted vertically, the centre of the coaxial feeder
should be connected to the leg facing upward.
A simple formula can be used to calculate the required
length of each leg for a half wave dipole:
75
-------- = Length of each leg in metres
Frequency in MHz
i.e. For 14.2 MHz
75
------- = 5.28 metres (i.e. the total length
14.2 of the aerial is twice 5.28m.)
For increased performance and directivity, additional
elements may be added to the front and rear of the dipole.
Directors - shorter than the dipole element and placed to
the front and a reflector - longer element to the rear. Many
designs have been published for such aerials and
mathematical formulae may be used to calculate the
required length and spacing between elements. This type
of aerial is usually refereed to as a yagi or beam.
Coaxial cables
When constructing dipole aerials or connecting VHF-UHF
aerials 50 OHM coaxial cable should be used. For short
wave or short runs of VHF URM43, URM76 or RG58U
are ideal, for longer runs of VHF-UHF feeder it may be
worth considering a heavier URM67 or RG213 (or better!).
