Q: What is receiver ‘blocking’?

When a radio device is co-sited with other users’ radio equipment, perhaps on a tall building or on a hill-top antenna mast, it is possible that other transmissions can desensitise your receiver. Although the other radio equipment will be working on different channels, if the transmit power is high and your antenna is close to other antennas, then your receiver can lose its ability to pick up wanted weak signal outstations. This is called receiver blocking. It is different from on-channel interference where some other user is sharing the same radio channel as you. Blocking interference is from strong transmissions off-channel.

All transmitters generate some level of off-channel ‘phase noise’ that spreads up and down in frequency from its centre radio channel. Type approval certification ensures that the amount of allowed phase noise is strictly defined and limited. Although the signal power of this phase noise is tiny in comparison with the transmitted centre channel frequency, for nearby receiving systems perhaps a metre or two from a transmitting antenna, there can be sufficient signal power to block reception of wanted weak signals on your channel.

Whenever possible, on shared antenna sites, it is good engineering practice to space antennas as far apart as possible. Vertical separation is often better than horizontal separation. In addition, good quality coaxial cable and connectors must be used. The antenna system installation should follow good engineering practice for cable routing, antenna system earthing and overall mechanical integrity of the installation.

In some severe instances it may be necessary to install additional RF filtering on the transmitters to reduce further the amount of transmitted phase noise. Wood & Douglas has expert knowledge of such RF filtering techniques and can provide on-site troubleshooting and solution design services when needed. Contact the sales department to speak with a sales engineer.


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