Leakage Calibration in a CATV Environment
Calibration of leakage equipment used in a CATV system can be performed by two different methods. One is a direct coupling into the detector of an RF signal with predetermined characteristics. The other is transmitting a field of energy also with known characteristics from a fixed antenna to a receiving antenna coupled into the detector. The explanation of these two methods is as follows.
Direct coupling-
Utilizing the formula dBmV = 20logE – 20log(20.7 X FMHz) we can derive an RF level equivalent to any desired microvolt-per-meter level.
In the formula E is microvolts-per-meter and F is the frequency in Megahertz.
Sample: We will use 20 microvolts-per-meter and a frequency of 133.2625 MHz
dBmV = 20log20 – 20log (20.7 X 133.2625)
dBmV = 26.02 – 68.81
dBmV = -42.79
In other words this dBmV level is equivalent to a 20 microvolt-per-meter at 133.2625 Megahertz.
If the RF detector is tuned to the frequency of 133.2625 MHz and this level is applied to the input, the device should register 20 µV/m. This number may vary slightly due to the specification of accuracy.
Antenna method –
In this method there are various factors that must be addressed before beginning the calibration procedure. They are antenna types and their usage in CATV applications. The FCC requires that to measure leakage for recording purposes, a horizontally positioned half-wave tuned dipole antenna must be used.
A quarter wave monopole whip antenna, which is installed on many service vehicles, has a different polarity. It is mounted in the vertical position thus giving it the ability to be most efficient in the presence of a vertical wave front.
If calibration is to be done using the monopole antenna as the receiving antenna and the dipole antenna the transmitter, the dipole must be reoriented to the vertical position. Thus keeping apples to apples.
To make accurate calibration measurements, a certified antenna range is the ideal location. However this becomes impractical for most CATV systems. Variables such as antenna placement for both transmitter and receiver are less than suitable. Ideally a transmitting antenna would be placed on a wooden pole in an area free from reflective objects such as chain -linked fences, dumpsters, automobile traffic, etc. The receiving antenna would be at least 5 wavelengths away again keeping clear of reflecting objects. The distance between the two antennas should also be consistent. This becomes difficult to do for each vehicle has a different configuration of ladders, radio antennas, booms and safety lights fixed on its roof. Both transmitting and receiving antennas should be well off of the ground to keep ground reflections at a minimum. As you can see calibration in a CATV parking lot is not the best location. However, the industry seems to do the best with what it has.
If you need to have some type of calibration procedure set up at your system, just remember the basics. Keep the polarity of both antennas the same. Keep as far away from any reflective object as possible. Do the math to determine the signal level to the transmitter, keeping in mind free space loss and give it your best shot.
