Aerial (Antenna), Earths & Propagation - AOR The Superior AR820 0 Series Operating Manual

Section 21
(21) Aerial (antenna), earths & propagation
The subject of aerial choice and earth can be quite complex. There are many advantages and disadvantages to consider before
connecting an external aerial to your receiver. One interesting phenomena is that aerial theory and practice can be surprisingly
different. Keeping common sense in mind it is one of the few remaining areas for listeners to easily experiment and often achieve
fantastic results.
Whip aerial
The supplied semi-flexible whip aerial should give adequate results for general hand-held operation on the VHF and UHF bands.
A longer wire aerial or telescopic whip should be considered for listening to the short wave bands. The AR8200 is supplied with a
ferrite bar aerial for medium wave reception, for best results, rotate the receiver as the bar aerial is directional.
Mounting location
It is important to mount any external aerial as high as possible and in clear space although this is more important at VHF
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 a BNC plug then connected to the BNC aerial input on the top of the AR8200. 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.
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.
Filters
A VHF civil AIRBAND FILTER is available (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.
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
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.
Propagation
VHF and UHF transmissions generally only propagate relatively short distances when compared to short wave signals. For all
intents and 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 kilometres, aircraft
at heights of 30,000 feet may be heard for many tens of kilometres (100 to 300 with the right conditions).
Unlike VHF and UHF transmissions which generally propagate only on a localised basis (to the horizon plus a small amount), short
wave 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.
Luckily the frequency spectrum of short wave 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. 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.
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