ARLA/CLUSTER: Como instalar uma longwire c/ Balun.

[João Gonçalves Costa]

 LOW NOISE ANTENNA USING THE UNIVERSAL MAGNETIC BALUN* AND THE ANTENNA FEEDER ISOLATOR


Longwire Antenna: The Longwire and the UMB should be positioned far away from sources of interference such as fluorescent lights, TVs, computers and electrical wiring.

In most cases satisfactory results can be obtained by mounting the antenna 6-10m above ground level and at least 6m from any buildings. The antenna's length should be a minimum of 12m. The UMB should be placed near the ground at the rear of the garden. Using a sloping or "Inverted L" antenna will ensure that the connection to the earth rod will be as short as possible.




THE WRONG WAY TO INSTALL A LONGWIRE ANTENNA/BALUN:

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THE CORRECT WAY TO INSTALL A LONGWIRE ANTENNA/BALUN:

[http://www.wellbrook.uk.com/images/antright.gif]



The Isolator would normally be placed near to the receiver with a short connection to the earth rod.

The above installation will ensure that the antenna is outside the local noise zone which extends up to 5m around the house. Also the noise pickup is lower with the feeder close to the ground.

The earth return path for the antenna is a separate earth rod and not the feeder/noisy mains earth as with most other "Baluns". This can reduce noise by up to 20dB compared to other "Baluns".

Using the Isolator will ensure that mains borne noise cannot enter the feeder and couple noise to the antenna either directly or by the feeder radiation. The Isolator can reduce noise at low frequencies by up to 40dB. Without an Isolator the feeder is connected to the receiver/mains earth and is effectively an extension of the house mains wiring as far as noise is concerned. Connecting the receiver directly to earth does not generally lower the noise because it is very difficult to achieve a low impedance earth.


Fonte: Wellbrook Communications.


BALUN* - tecnicamente nas antenas de fio comprido aleatório (vulgarmente conhecidas por "random Longwire" ou somente "Longwire") a denominação correcta é UNUM ( antena "unbalanced" / cabo coaxial "unbalanced") e não Balun ( antena "balanced" / cabo coaxial "unbalanced") como no caso dos dipolos. Especificamente e neste caso pode-se prescindir do "Line Isolator AFI 5030" que é projectado para recepção, ou utilizar um para transmissão, por exemplo os conhecidos T4 e T5 Line Isolators da Radio Work ( http://www.radioworks.com/nliinfo.html ).



João Costa, CT1FBF
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