Interference at radio sites
Determining the source of unwanted incoming radio signals at a radio station site is largely a process of elimination. Remote sites may well prove a challenge when the signals are infrequent and intermittent in nature.
Special steps in the process are:
Determine if the interference is created on site or remote from the site
- Interrogate SMART and create an area search both wide ranging (say 80kms), and local (say 1.5 kms). This information is invaluable for comparison purposes.
- Determine if the signals are coming from the radio station site or from off site. This may require connecting the radio site aerial to a separate receiver or, if the signals are strong enough, setting up a temporary receiving system on site with direction capabilities.
- Try another receiver or other equipment as necessary to disprove equipment faults.
- A sensitive spectrum analyser is a very useful tool for analysis of the parameters of the incoming signal. Watch for possible overloading of your receiving equipment.
- Observe the nature of the interfering modulation and its amplitude/time/frequency characteristics.
- If garbled or mixed modulation is heard it may be an intermodulation product created on site. Multi transmitter sites are always suspect. If no audible noise is heard the problem may well be receiver de-sensitising – see receiver de-sensitising below.
- Note any other transmitters that may be on when the interference comes on or goes off.
- If on site interference is determined locate the transmitters causing the problem. An intermod calculation may assist. Otherwise, note which transmitters are on during the presence of the interference. Narrow down the search by observing the problem over a period.
- Check the proximity of the transmit to receive aerials. Aerial isolation is critical even between widely spaced bands e.g. UHF to VHF land mobile bands.
Locating the source of the intermodulation product
This will likely be a transmitter not fitted with a circulator, or corroded joint enabling rectification and mixing of two or more transmissions to occur. Generally the contributing transmitters will be two or more on-site transmissions though a strong off site transmission has been known to contribute.
Site operators have been known to replace entire masts with a type that does not use bolts as the most efficient way to solve on going problems.
Process and comments
- All transmitters must be fitted with filters and circulators to prevent products being formed in the final stages of transmitters and retransmitted. This is a primary source of intermodulation products.
- It is possible to direction find the source of the faulty joint in many circumstances though the use of sensitive receivers at multi transmitter sites requires the use of filters.
- Look for any bolts or screws that may have become corroded. Lattice towers where joints are secured by bolts are often a major source of rectification. Like wise, grid pack type antennas that are held together with screws are suspect.
- A connector or cable may have suffered water egress and thus corrosion.
FM noise is a major contributor to receiver de-sensing. It often occurs at sites where there are high power FM broadcast transmitter aerials close to sensitive land mobile receive aerials.
The white noise created in high power linear amplifiers raises the noise floor often over a wide range of spectrum and affects sensitive narrow band FM receivers by reducing their ability to receive signals. The affected receivers may suffer a reduced sensitivity by 10 to 30dB. It is not possible to audibly hear this noise and the receiver outwardly appears to be fine (receiver “hiss” is apparent as appears normal). It is possible to determine receiver de-sensing by testing the signal to noise ratio through the use of a noise bridge and signal generator.
Near and far fields
An understanding of radio wave propagation as relates to close and distant radiation from antennas should be understood. Signals behave differently close to radiating antennas where inductive principles predominate rather than electromagnetic electrostatic principles of radiation as observed as you move away from the antenna.
Engineering practices at multi user sites
Frequencies are engineered at sites on the basis of sound radio frequency engineering practices being employed. Where these practices are observed the chance of on site interference is minimised.
The Ministry recommends that multi user sites comply with specification AS 3516.2-1998 and that all contracts let for site construction contain this specification as part of the requirements.
Cross modulation is a form of intermodulation where the mixing occurs in the front end of a receiver. Re-radiation does not normally occur from the receiver. Commonly noticed in a mobile situation where the mobile equipment is not protected with circulators and or filters. At base stations, filters and circulators are normally fitted and this prevents this type of interference.
This is likely to be the primary problem when chasing interference sources and can lead the tracer on a wild goose chase until the effect is eliminated with filters.
Remember to use only the minimum required gain in any broadband amplifiers as overload and mixing will likely occur in the presence of a strong signal.