Measurements and detection of cable TV leakage sources have become absolutely necessary in order to avoid harmful interference with terrestrial radio communication systems and reduce ingress,especially in the return path. Many CATV operators have introduced leakage monitoring programs in their networks. However, a substantial number of CATV networks is not monitored at all or on an unsatisfactory level. Today, we have a very serious threat – LTE. Many CATV operators may have problems with ingress (service quality) and egress (problems with national regulators and LTE carriers). The problem of leakage measurement and control is again a “hot” topic which requires new methods and tools.One of the very important things is recognition of the measured signal. One must be sure that the received signal comes from the network under test and not from competitive networks or other EM energy sources (transmitters or radio noise sources). The signal should be tagged (marked) to allow a leakage meter to detect leakage correctly and reliably. There are three major methods of detecting CATV leakage:
- spectral analysis,
- correlation of QAM signals,
- insertion of narrow band carrier(s).
Each method has its advantages and disadvantages. Let us shortly analyze them. The spectral analysis method assumes the reception and visual analysis of the whole spectrum occupied by CATV networks. The advantages of the spectral analysis method:
- usage of general purpose receivers/spectrum analyzers,
- suitable for analog and digital TV networks,
- wide frequency spectrum analyzed (determined by the measurement antenna),
- directional antennas used (easier source direction-finding),
- no degradation of transmitted signals,
- no equipment in the headend.
Major disadvantages of the spectral analysis method are:
- the need for a well-trained technician/engineer to analyze the received spectrum on a screen and verify the origin of the leakage (network under test or other CATV network),
- not fully automated (signal recognition by a human – no tagging signals),
- not recognized source network of leakage (in case where there are two or more CATV networks in the area transmitting QAM signals),
- higher cost of wideband receivers/spectrum analyzers,
- mobile patrols of the CATV area usually not possible (directional antennas not suitable for mobile measurements),
- very long measurement time of large-area CATV networks.
The correlation method is based on the correlation of the QAM signals sent from the headend and the signals received in the field. A strong correlation is a sign that the received signal is radiated from the network under test. The advantages of the method are:
- no need for separate test channel or signal,
- no degradation of the signals transmitted in CATV networks,
- mobile patrols usually possible (fast coarse localization of leak sources, fine localization with directional antennas).
However, although one can say that the method is able to isolate the leakage signals (as if they were tagged), it has substantial drawbacks:
- very sophisticated equipment in the headend and in the field,
- higher hardware and software costs,
- the need for separate transmission channel (usually mobile network) between the headend and the field meter to perform signal correlation.
The methods based on the insertion of the tagging signal assume that a special narrow band signal is inserted in the headend. The signal can be placed in the guard bands of analog or digital TV channels. The field meter receives the tagging signal, measures its amplitude and detects tagging information. If the tagging signal has not been detected, it is not taken into account. The advantages of these methods are:
- very simple, fully automated and inexpensive,
- similar to well-known methods used in analog CATV networks in the past,
- suitable for analog and/or digital CATV networks,
- flexibility in placement of the tagging signal frequency,
- no need for separate measurement channel,
- mobile patrols possible (fast coarse localization of leak sources, fine localization with directional antennas),
- no degradation of the signals transmitted in CATV networks.
The major disadvantages of these methods are:
- the need for a tagger in the headend,
- the inserted carrier could be a source of leakage in the spectrum part which should be protected (e.g., aeronautical VHF or LTE band).
The last drawback is rather questionable – usually, there are other signals at similar frequencies transmitted over the network which can be also strongly radiated. The CATV operator should detect and remove leak sources as fast as possible, so for properly monitored network the risk of long-lasting leaks is small.
KABELKOM has been involved in leakage measurement and control for 15 years. Based on rich experience, KABELKOM has developed the AMS2010 leakage meter with a new tagging method (patent pending). The method belongs to the third group of methods (insertion of narrow band carriers) due to the numerous advantages, mainly simplicity and low price. The KABELKOM’s method has improved tagger detection reliability and can be used to differentiate leaks coming from various parts of the network. One can find some reports claiming that narrow band signals can degrade QAM signals. The KABELKOM’s method does not decrease MER more than 0.1 dB, so in practice the degradation is negligible.
The second important thing is the measurement frequency of the inserted tagging signals. Most of tagging signals are placed in the VHF aeronautical band. Nowadays, there is a tendency to place a tagging signal in the protected band (e.g., LTE). The problem of the measurement frequency choice is quite complicated. The results of many experiments show that it is quite difficult to find correlation between leakage levels at various frequencies for the same physical plant. The received power level depends on the physical damage type, spatial orientation of a broken cable or damaged connector, and the electrical size of the “hole” in the cable (increases with frequency). It is impossible to predict at which frequency the leak magnitude will be the highest. However, if the technical condition of the network is poor (quite common situation in many European countries), the leakage level is high both on low and high frequencies.
Another important factor is the free space loss which is a function of frequency. For example, let us have two sources at 150 MHz and 800 MHz radiating the same power. The free space loss 15 m away from the sources is 39,5 dB and 54 dB at 150 MHz and 800 MHz, respectively (see the graph below). It means that the leakage power received by the meter will be 28 times lower for the higher frequency. If we assume the same sensitivity at both frequencies, in some cases the weak and medium leaks at higher frequencies may be undetected. One can use the Yagi-Uda directional antenna to increase the input signal power level. The Yagi-Uda is not handy inside buildings and not suitable for mobile patrols which require an omnidirectional antenna. For this purpose, we can use an active monopole antenna with an amplifier. The amplifier gain should be as low as possible. Otherwise, we can expect high levels of intermodulation products which can mask leakage signals with small or medium amplitude.
Free space loss (FSL) in terms of frequency (blue line) and difference between FSL at a given frequency and FSL at 150 MHz.
Intermodulation is a very important phenomenon for leakage detection. If we want to measure leakage directly in the LTE band, we must be aware that there will be many high-power sources – portable terminals and base stations (in some countries also jammers). Those high-level signals increase the likelihood of intermodulation. It seems that it is better to measure leakage at frequencies which:
a. are not too high (free space loss)
b. are in the spectrum band where terrestrial radio transmitters or other strong radio noise sources are as far away from the measured area as possible (intermodulation, receiver/meter desensitization).
Sometimes it may be necessary to use narrow band passband filters or frequency traps to reduce the level of the interfering signals, especially if the measurement frequency is close to the LTE/GSM band. The choice of the measurement frequency belongs to the CATV operator.
KABELKOM has developed the basic AMS2010 leakage meter which can measure signals up to 200 MHz. Having in mind that some customers may want to measure leakage close to the LTE band, we have developed a new version – AMS1G14 – working up to 1 GHz . The upgrade to the new meter is free for these customers who will buy the leakage start pack (AMS2010 meter + AM signal tagger) after September 1, 2013. We have also developed an active vehicle omnidirectional antenna with a 12 V amplifier equalizing the increased free space and cable loss at higher frequencies. The meter is calibrated together with the antenna in order to give actual and correct values of the electric fi eld intensity. We offer professional trainings on leakage control for CATV technicians and engineers.