Identifying Interference


There is one issue that must be borne in mind when dealing with interference problems on licence free LPR installations - both users are entitled to be there! You have absolutely no legal recourse on anyone using the same radio channel as yourself so long as they too are abiding by the law regarding ERP limits etc. i.e. massive Yagis are not used to ensure his signal gets through by overriding yours! The best political approach to this situation is one where it is in both your interests to resolve this interference, the result being more reliable systems on both sides. If you are using a licensed frequency then once you have identified the interference do not be shy to employ the services of your local licensing authority to help locate and rid you of this thorn in your side.

The only way to start tackling an interference problem is to listen to the interference. This is easily achieved by using a scanner but is best done by using the receiver of the telemetry unit being interfered with. Scanners are not preferred as these receivers are usually designed as cheaply as possible and one of the areas skimped on is the "front end". The problem is these cut down circuits (which is the 'economical' approach to creating a large frequency range input system) are not designed to cope with large signal strengths on frequencies other than what you are actually listening to, especially if the signal strength of what you are interested in is rather weak. What happens is the input circuit becomes overloaded with all the signals reaching the antenna which in turn could create its own interference leading you up the garden path.

Very few telemetry system designers offer a pin allowing a person to monitor the audio from the receiver (and hopefully those who read these pages accept this as a constructive criticism and make a plan!). Most times, however, there is a test point available on the circuit board where the received audio is available. Note that it must be ahead of any audio limiter as hearing the interference without distortion is important, especially if it is a speech channel. It is suggested that you contact the relevant supplier of the modules who, I am sure, will be willing to help you locate this test point. Monitoring the audio is best done using an audio amplifier but a 600 ohm telephone earpiece in series with a 2uF capacitor is usually a suitable alternative. The capacitor is needed to ensure the earpiece does not place an undue DC load on the receiver circuitry although it is possible the telemetry module may stop processing commands while the earpiece is connected.

There are three types of interference:

  • On Frequency is where there is some other system (voice, data, or telemetry) using the same frequency as yourself and being in range of your receiver. In licence free LPR systems this is highly possible especially if two installers are supplied by the same distributor as most will have their 'own' working frequency.

  • Off Frequency is where the signal is so strong it overloads the antenna input system such that it blots out the signal you are trying to receive.

  • Intermod is the mixing of at least one very strong signal within the front end of the receiver or surrounding antenna or structures with the result appearing on your frequency causing you interference.

As can be seen, interpreting what you hear is extremely important in identifying the source of the interference.

Planning the tracking of the interference (oh yes!, this stage requires some planning too!) is extremely important if time spent on this unproductive process is to be minimised. Ask questions! The most important piece of information you could want is if it is related to a time of day, day of the week, etc. You want to be set up and ready to trace the interference before it starts, but not that far ahead that time spent on site is wasted. By the way, there are many types of interference where it is time or weather related and many where it is not. Accept the answers you get at face value, even if the answer is "it only happens at midnight"! Once you have established when you are likely to have the interference, you arrive on site, connect your earpiece, and then ....

.... listen!

Unless you do this sort of tracking often you will need to learn to identify the signals that are meant to be received i.e. what transmissions from telemetry units within your network sound like (so that you do not confuse these with the interference). Done when you start your listening session will help to have the sound fresh in your mind. Use a spare unit near you and then transmit with it while listening to the 'noise' it makes. Once the type of noise is known the next step is to sit and wait for the interference. The "rush" sound, much like a constant sound of the sea, is normal and any received signal quietens this noise.

Identifying that interference has occurred is easy. There are two tell-tale signs, there is either a reduction or disappearance of the received signal or the interference is heard above or with the intended received signal.

If the signal disappears momentarily, being replaced by the "rush" sound, the chances are there is a strong off-frequency signal overloading the receiver's front end stopping the normal RF amplification. If the received signal is strong the overloading signal may not blot out but only reduce the signal or impede the quality. Investigate the area for other radio systems of all natures, including two-way voice systems and specifically other telemetry systems. Try ascertaining the frequencies of them and should any be near your frequency the chances are they are the source of your headache (antennas will be similar in length/wavelength - no special knowledge is needed, just read the label).

If none of the above are evident then finding this interference without the help of a spectrum analyzer is almost impossible. Scanners can help but the time spent scanning for the culprit will soon outweigh the hiring costs of a suitable analyses or the services of a two-way radio service centre. This is the most difficult of interferences to deal with as in most cases specialist equipment is required to locate the source - to reiterate don't be shy to employ the services of a good two-way radio service company.

A slightly easier interference category is when the interference can be heard. There are two distinct subcategories being Off Frequency and On Frequency. Off frequency is usually attributed to a signal that is near but not quite on your frequency, such as another licence free telemetry system. Two distinct problems exist with radios and, although there are methods available to reduce these, one cannot eliminate the spurious emissions of a transmitter. The closer the offending transmitter is to your frequency the higher the noise level received by your receiver. The interference usually appears very distorted on the output of a receiver.

On Frequency interference is where the offending transmitter is using the same frequency as yourself. A prime example is two telemetry systems supplied by the same company working in the same area and thus interfering with each other.

A word of warning to those using 433MHz systems: Please do not attempt to operate at higher than the accepted 10mW for this band. This frequency sub-band was stolen from the radio amateurs world wide and they are justifiably annoyed, if not angered. No consideration was given to the fact that this borders on the sub-band of the down-link from satellites put into space at huge expense. Along comes a number of badly designed transmitters splattering all over the extremely weak satellite down-links - not conducive to good neighbourly relations. These fellows are not unknown to deliberately transmit on this frequency to blot out and thus make unreliable telemetry systems operating on 433MHz, and will continue to do so until the offending system is shut down. If you are suffering from such blocking try to locate the person doing it and ask him to help you sort out your interfering with him - in the long run it's worth it.

The other common form of interference is portable two-way radios that are also using the licence free frequencies, they do, however, tend to move to an unused channel but that could now become an off frequency problem! If they do not move then carry on listening, the biggest give-away is names and telephone numbers. We can personally clock up many times surprising a person asking them about a telephone call they have just received and who it was from. After informing them that it was to assist tracking an interference complaint the callers details are usually handed over.

There is one special case where the interfering signal is exactly twice the intermediate frequency away from your working frequency but gives the impression of being on frequency. This sort of interference is known as "image reception". This is more likely in UHF systems than VHF as the bandwidth of the 'front end', being related to the working frequency, is wider on UHF. Typical intermediate frequencies are 10.7MHz and 21.4MHz, the latter being more suited to UHF but not all designers use this as the crystal filters are more expensive than 10.7MHz.

If you are experiencing such interference that appears to be on your frequency but you do not appear to be interfering with them then try to obtain some receiver capable of listening to 21.4 and 42.8MHz away from your frequency (that is both below and above your operating frequency). It would be advisable to call the supplier to ascertain what the likely image frequency would be.

Being close to the border of a country could also pose a problem as the low power licence free band your side of the border may well be a high power licensed frequency on the other!

Intermod, the complex name for the simple term 'mixing'. The word is derived from the term 'inter-modulation' and stems from the older days of AM and has stuck with radio buffs to this day. As the term describes the primary frequency is modulated by a second. While all signals levels are within the limits of the equipment nothing really happens except everything is amplified by an equal amount. It's when things get stretched to the limit and beyond that weird effects start to take place.

The primary problem when overloading occurs is the device being overloaded becomes non-linear. The effect is that the primary frequency is 'modulated' by the one causing the overload. In this instance the result is usually a distorted audio output. A further effect, and the one we are more interested in here, is the non-linear operation creates a superb platform for mixing frequencies together. This means that two apparently unrelated frequencies can now be mixed with the result, if on your operating frequency, being highly undesirably.

Another sad fact of this lousy sort of interference is that not only are fundamental frequencies to be taken into account but up to as high as the 5th order harmonic i.e. 5 times the interfering frequency. And to make matters worse up to as many as 5 frequencies could mix to cause interference but there is one bit of relief, the chances of this happening are small and the mixing is usually related to frequencies within the same band. This is purely owing to the fact that most of the active RF components are surrounded by tuned circuits limiting the range of frequencies open to this sort of mixing to the bandwidth of the tuned circuits.

There are two types of circuits to be careful of as there frequency range is usually as "wide as a barn door". The first is transmitters, the second is wide band receivers. Dealing with the last first. Wide band receivers are generally only used in two-way voice radio applications thus making it highly unlikely to be found in radio telemetry equipment. Being wide band makes them highly susceptible to intermod which is why we advise against using scanners in interference tracking.

Transmitters have a slightly different problem in that they operate in a non-linear mode as it is and therefore are in themselves available to be mixers. The advantage here is the amount of signal strength required to generate a level of signal large enough to cause a problem happens to be fairly high. This sort of intermod usually occurs where two transmitters are close to each other and not properly installed using combiners (devices for coupling two or more transmitters into the same antenna system).

Intermod is usually quickly recognised, as the above descriptions has shown, as the output is a mixture of the two audios being transmitted. To ascertain the intermod is happening in your receiver insert a 3dB attenuator in the antenna cable and notice the signal strength before and after the insertion of the attenuator. If the mixing is occurring within your receiver the interference 'signal strength' will drop by 9dB (owing to the logarithmic characteristics of the mixing). If the interference is being mixed outside of the receiver (e.g. in a nearby transmitter) then the level will only drop by 3dB (same as the attenuator).

This section on interference is going to grow as more and more examples come to mind. It is a highly specialised field but hopefully these introductions will be able to steer you in the right direction. I am open to receiving (no pun intended) small .WAV files of the interference to assist with trying to determine the cause (please zip the files up before sending!). 99% of tracking interference is hearing what the it sounds like.

In the following section we deal with solving the interference.

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