AOR AR5000A Operating Manual page 75

ATU & preselectors
Aerial tuning units (ATU) may improve the short wave
section of a wide
range receiver (such as the AR5000) by rejecting
unwanted signals and only allowing a specific band of
frequencies through. These ATUs are normally
constructed in small boxes with about 3 controls on the
front. The disadvantage is the need to constantly retune
the ATU when changing frequency. An ATU of this nature
is passive, this means that no power is required to operate
the ATU and no extra circuit-noise is introduced into the
receiver.
Loop Aerials
Short wave desktop loop aerials have the advantage of
small size (such as the AOR LA320). They too have tuning
controls to reject unwanted signals. As the loop is within
easy reach of the operator it can be rotated to provide
directivity. Loops can be particularly useful for DX'ing the
lower bands.
Generally speaking they offer excellent portability but
cannot compare on the higher bands with a well sited long
wire aerial.
Active aerials
Active aerials are normally quite compact (AOR WA7000)
and combine a wide coverage aerial (30 kHz - 2 GHz)
with a preamplifier mounted within the aerial its-self. They
require power to enable them to operate. Not all designs
allow you to switch the preamplifier off although some
have a gain control.
As with loop aerials they tend to provide good results on
the lower bands when compared to poorly sited short'ish
wire aerials. Overload can be a problem on the busy 7
and 9 MHz bands. If you have a small garden space, an
active aerial may be worth considering.
Discone
For wide coverage in the VHF-UHF bands a compromise
has to be met and the most popular aerial is a discone
(AOR DA3000). Their appearance is like a large spider
or umbrella without the covering material, the better
models have about 16 elements.
Typical usable coverage starts from about 25 MHz and
extends continuously to 500 MHz, 1300 MHz or even 2000
MHz. The coverage peaks and dips throughout it's range
as the elements interact to provide the widest possible
coverage. Due to their necessary construction discone
aerials are a little prone to wind noise due to vibration
and possible damage in severe gales.
PAGE 74 AR5000 OPERATING MANUAL
Stub filters
Should you encounter breakthrough when using an
external aerial (and the attenuator does not help) a simple
stub-filter placed in the coaxial cable may help. This
comprises of a 'T' connector with an open circuit 50 OHM
cable length (the stub) attached to the 'T' piece. A rough
calculation for the stub length is as follows:
(75 / Freq in MHz) x 0.67 = Stub length in metres
i.e. To reduce the strength of 88.3 MHz on VHF Band-II:
(75 / 88.3) x 0.67 = 0.57m or 57cm
Commercial filters - ABF125
A VHF civil AIRBAND FILTER is available from AOR
called the ABF125. This will help minimise the possible
effects of breakthrough when listening to VHF airband in
BAND-II VHF high signal areas or when connected to
external aerials.
Other manufacturers are providing tunable filters to notch
out unwanted signals typically in the range of 75 to 175
MHz.
Earth systems
A separate EARTH connection made to the outer (braid)
connector of the ANT 1 or ANT 2 plug 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 when your a.c. mains supply
uses a Protective Multiple Earth (PME) system, consider
the implications carefully. If in doubt consult an
experienced electrician.
Connecting an external earth wire may greatly reduce the
local noise encountered when listening on the short wave
bands. It is very important to provide a good earth should
you use an aerial tuning unit.
A short length of thick gauge earth wire may be connected
to a nearby central heating radiator or water pipe but never
use a gas pipe for earthing. Ideally a separate earth
rod should be used but the length between the receiver
and rod becomes restrictive, if too long the earth system
may well pick up noise rather than remove it.
If a long run of earth wire is necessary, it may be worth
considering a screened earth system. This simply
comprises a coaxial cable (such as URM43 or URM76
for short runs with URM67 or RG213 being used for longer
runs) shorted inner to outer at the earth rod end with only
the centre core connected to the outer of the AR5000
aerial plug, the outer braid being cut back and insulated.
This provides a screen for potential incoming interference
and passes any noise down the cable away from the
receiver and toward the earth rod.