AOR AR8000 Operating Manual page 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.