AOR AR800 Operating Manual

							AR8000 operating manual
111
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.
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 2:      (75 / 88.3) x 0.67 =
0.57m or 57cm
Commercial filters - ABF125
A VHF civil AIRBAND FILTER is now available from AOR called the ABF125.
This will help minimise the possible effects of breakthrough when listening to
VHF airband in BAND-2 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 BNC
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,  consider
the implications carefully if your mains supply uses Protective Multiple Earth
(PME) system.  If in doubt consult an experienced electrician.
Connecting an external earth wire may greatly reduce the local noise
encountered when listening on the shortwave 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, URM76 for short runs or URM67 or RG213 for longer runs) shorted
inner to outer at the earth rod end with only the centre core connected to the
outer of the AR8000 BNC plug,  the outer braid being cut back and insulated. 
						

							AR8000 operating manual
112
This provides a screen for potential incoming interference and passes any
noise down the cable away from the receiver and toward the earth rod.
(23)  Propagation - shortwave bands
VHF and UHF transmissions generally only propagate relatively short
distances when compared to short wave signals.  For all intensive purposes
they may be considered as line-of-sight plus a bit.
Where as point to point communication between mobile users or when in built
up areas may only be a couple of miles, aircraft at heights of 30,000 feet may
be heard for many miles (50 to 200 with the right conditions).
Occasionally “tropospheric” weather conditions or “sporadic E” layer ionisation
enable VHF-UHF signals to travel many hundreds of kilometres.
Unlike VHF and UHF transmissions which generally propagate only on a
localised basis (to the horizon plus a small amount),  shortwave transmissions
may travel for many thousands of kilometres.  Depending upon the frequency
in use, time of day, season of the year and sun spot activity,  transmissions
may propagate completely around the World.
Radio signals are electromagnetic waves very similar to light beams.  As such
they do not readily follow the curvature of the Earth but attempt to travel out
into space.
The ionosphere
Luckily the frequency spectrum of shortwave is often reflected back down to
Earth by the upper layer of the Earth’s atmosphere called the ionosphere.
When the reflected signals reach the Earth again they may either be
received or reflected back up into space.  If lucky, they will be reflected by the
ionosphere yet again down toward the Earth providing reception into another
and possibly more distant location.
The ionosphere is constructed of many layers of ionised gas.  Of particular
interest to shortwave listeners’ are the lower “E” and upper “F1” & “F2” layers
although a lower “D” layer exists during day time.
“D” layer
During day time the lower “D” layer 
forms around 60 to 80 kilometres above
the Earth’s surface.  This “D” layer tends to absorb low frequencies reducing
the distance covered by medium wave transmissions.  In the night time when
the “D” layer dissipates,  medium and low frequency transmissions may
propagate over much greater distances. 
						

							AR8000 operating manual
113
If the transmitted frequency is too high for to be reflected by the ionosphere,
or the angle too steep,  transmissions will simply pass straight though the
ionosphere without being reflected and will travel upward to the next
ionosphere layer.
“E” layer
Above the “D” layer is the “E” layer located at a height of about 100 kilometres.
The “E” layer tends not to absorb signals as much as the “D” layer but refracts
some signal back to Earth where it may be received some distance from the
original point of transmission.
Usually in Autumn and Spring “SPORADIC E” propagation consisting of dense
pockets of “E” layer ionosphere, reflect even the higher VHF and UHF
transmissions causing patterning on television sets.  This is to the delight of
Radio Amateurs who are then able to communicate for many hundreds and
even thousands of kilometres on frequency bands usually capable of only local
reception.
Occasionally a similar effect can be caused by temperature inversion layers
creating “tropospheric propagation” selectively “ducting” transmissions
between two points.  Tropospheric propagation is usually applicable to the
higher VHF and UHF bands.
“F1” & “F2” layers
During the day time there are two upper layers of the ionosphere,  these
being the “F1” layer at about 200 kilometres and the “F2” layer at about 400
kilometres.  As evening falls,  these layers combine to form a single “F” layer.
It is “F” layer propagation that is largely responsible for shortwave propagation
over great distances.
The density of the ionosphere layers varies depending upon season, time of
day and sunspot activity which is believed to follow an eleven year cycle of
good and bad propagation conditions. 
						

							AR8000 operating manual
114
You will note that large areas of the Earth’s surface lays between the point of
transmission and reflection,  in this area there will be little or no reception.  For
this reason “F” layer propagation is often referred to a “SKIP” and the reflected
signal as “SKY WAVE”.
Generally speaking only frequencies below 30MHz are reflected by the
ionosphere.  Higher frequencies pass straight through even the “F” layers and
will continue outward into space for ever.
Choice of frequency
Depending upon the time of day and desired skip distance,  different
frequencies will be selected by Radio Amateurs and commercial users such
as Oceanic Air Traffic.
For instance the “MUF” (Maximum Usable Frequency) is often stated for a
path between two locations.  Choosing a frequency above the “MUF” will not
produce results as transmissions will pass straight into space.
Many propagation predictions and statistics are published and usually
available from most country’s National Amateur Radio and Shortwave
Listeners representatives.
Various publications are produced giving transmission and contact details for
World wide reception.  These titles include:
World Radio TV Handbook (WRTH), BPI Communications, 1515 Broadway,
New York 10036, NY USA.
Passport To World Band Radio, IBS North America, Box 300, Penn’s Park PA
18943, USA.
Listings for utility services are also widely published and available. 
						

							AR8000 operating manual
115
* Specifications subject to change without notice due to continuous
development of the receiver. E&OE.
(24)  Specification
Frequency Range: 500 kHz to 1900 MHz
Receive Mode: AM, NFM, WFM, USB, LSB, CW
Frequency Step Size: 50, 100, 200, 500 Hz, 1, 2, 5, 6.25, 9,
10, 12.5, 20, 25, 30, 50, 100, 200,
250, 500 kHz or any multiple of 50Hz
up to 999.995 kHz
Receive Sensitivity:500 kHz to 2.0 MHz
SSB by field signal strength
AM   by field signal strength
2.0 MHz to 30 MHz
SSB  1.0uV
AM   3.0uV
NFM  1.5uV
30 MHz to 1.0 GHz
SSB  0.25uV
AM   1.0uV
NFM  0.35uV
WFM  1.0uV
1.0 GHz to 1.3 GHz
NFM  1.0uV
1.3 GHz to 1.9 GHz
NFM  3.0uV
AM/SSB  S/N 10dB,
NFM/WFM  SINAD 12dB
Selectivity: SSB 4.0 kHz (-6dB),  7.5kHz (-50dB)
AM/NFM 12 kHz (-6dB),  25 kHz (-60dB)
WFM 180 kHz (-6dB),  800 kHz (-50dB)
Antenna Impedance: 50 ohm BNC
AF Output (at 4.8V): 120mW (8ohm) THD 10%
Power Requirements: 4.8V  Nicad
6.0V  Manganese Battery
EXT   9.0 to 16V dc
Power Consumption: 160mA  (nominal)
110mA (stand by)
20mA (power save)
Memory     Memory channel: 50 channel x 20 bank  - total 1000
           Pass channel: 50 channel x 20 bank  - total 1000
          Priority channel: One
Scan/Search Rate: Approx. 30 channel per second (max)