TDR Tutorial and Riser Bond TDR Product Review
APPLICATION #1 Broadcast transmission line bullets
A waveform TDR such as the 1220 or 1205 is a good preventative maintenance tool for broadcast tower transmission lines.
A broadcast transmission line that is made of rigid elements bolted together needs periodic maintenance. The point where the sections are bolted together wears and deteriorates with time. The hollow center conductors are spliced together with a small solid copper section, commonly known as a bullet.
As the transmission line warms and cools with and without power, from day to night, and from summer to winter, the sections expand and contract. This expansion and contraction causes the hollow center conductor to rub and wear against the solid connection sleeve. This wearing can generate small filings that will drop onto the nylon spacer directly below the connection. Eventually, these filing will create an RF power path to the transmission cable sleeve and ground.
This path to ground will cause momentary power shorts to ground and momentary loss of output power. A waveform TDR can easily see the deterioration of the bullets and help the tower maintenance crews to identify deteriorated bullets . When compared to the trauma of an instantaneous short to ground that the transmitter and transmission line experience, the cost of refurbishing these bad bullets is low. The TDR is instrumental in the location and evaluation of transmission line bullets.
APPLICATION #2 SWR and dBRL
Engineers that work with transmitters, transmission lines, and antennas are very familiar with SWR (VSWR or Standing Wave Ratio). SWR is caused by an impedance mismatch. In the transmitter/ transmission line/antenna system, the mismatch usually takes place at the transmission line/antenna connection. Usually the mismatch is caused by an antenna that is not tuned to the frequency of the outgoing signal. The mismatch can also be caused by other problems such as bad cable, moisture in the cable, or bad connections.
Other transmission line technicians, such as Cable TV cable workers, are more familiar with the term Return Loss, or dBRL. There is a definite relationship between SWR and dBRL. Both SWR and dBRL should be read at the point of the mismatch. But, in reality, they both tend to be read at the most convenient point, usually the point of the transmitter. The error in the measurement is the cable loss. An additional error in the SWR reading is the actual phase of the voltage with respect to where the meter is actually placed.
SWR is a passive reading using the transmitted signal as the signal source. This is because an SWR meter can always stay in the line. Monitoring the reflected signal and the transmitted signal is the most accurate example of a signal that creates the SWR. The dBRL is normally measured using its own signal source.
SWR is usually thought of as a narrow band frequency problem and dBRL as a broad band problem. SWR is thought of in terms of the mismatch between the transmitted signal frequency and the antenna cut frequency. If the transmission line goes bad from moisture in the cable, the SWR will go up and the engineer's first reaction is that the antenna has gone bad.
The mathematical definition of each are stated below:
Where VO is the voltage of the outgoing signal and V R is the voltage of the reflected signal.
For SWR, a value of one means a perfect impedance match. SWR of infinity means a total mismatch, such as a complete open or dead short. For return loss, a value of infinite is a perfect match and a value of zero is a total mismatch, such as a compete open or dead short.
The following table shows the SWR and return loss for the span of all possible mismatches. NOTE: The top line is a perfect match, the bottom line is a complete open or dead short.
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