Wellbrook CommunicationsWellbrook Communications

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

Universal Magnetic Balun is specifically designed to reduce noise, especially mains borne, by isolating the feeder winding of the Balun matching transformer from the antenna.

 

The UMB allows for the antenna to be isolated from the feeder with only magnetic signal coupling from the antenna to the feeder. In most receiver installations the feeder is connected to mains earth at the receiver. This can cause mains borne noise to enter into the receiver because either the feeder/mains earth is the return path for the antenna or simply by noise being coupled to the antenna by the feeder.

Most other types of Antenna Matching Baluns are simply wideband transformers with the earth side of the antenna winding connected to the feeder screen. This a serious design limitation that will result in noise pickup due to the antenna's return path being the feeder/mains earth.

The figure below shows a typical Longwire antenna installation data for other types of 'Baluns' This is probably the worst way to use a Balun, because the antenna and the Balun are within the local interference zone that surrounds most buildings up to distance of 5m. Also the feeder will pickup noise as it forms part of the antenna because there is no isolation between the feeder/antenna. Grounding the feeder with a separate earth rod is not very effective at medium and low frequencies.

THE WRONG WAY TO INSTALL A LONGWIRE ANTENNA/BALUN:

 

THE CORRECT WAY TO INSTALL A LONGWIRE ANTENNA/BALUN:

 

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.


Wellbrook Communications
The Farthings
Beulah
Llanwrtyd Wells
Powys
Wales LD5 4YD
UK
Phone 01591 620316

 

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