The Tale of the “Watchband”

When you are dealing with any medium or high level RF transmission system like we have, it is good practice to have a “line sweep done”. A trained technician connects up equipment to the transmission line to measure the line impedance and a TDR (Time-Domain Reflectometer) to determine if there are any “fail points” in the transmission systems. The TDR can determine if any point in the transmission line is starting to degrade. In just about all transmission systems using rigid line, there are many connection points in the system. Where the pieces connect to each other, and when they bend, in “elbows”. The lines are fabricated with an internal conductor (a “center conductor”) that is usually made out of copper pipe. This is the pipe that is the conduit for the RF energy, with the outer being the shield, like in any coax. This internal copper pipe is connected with a device known as a bullet.

A sample picture of a bullet. Image courtesy ERI.
A sample picture of a bullet. Image courtesy ERI.

This bullet is designed to connect the two pieces of pipe together. It makes the connection to the pipe using a small wirewround spring contact, known as a “Watchband”. This watchband allows the bullet to move within the pipe (remember these pipes are outdoors, so there is expansion and contraction with weather).
Anyways, when we had the recent inspection, our inspector found an anomaly 637′ up the tower. So we had a crew come in and “break open” the transmission line. At the piece where the line was broken open, we found one of the watchbands had become dislodged and deformed.

A deformed and broken watchband spring from 4" coax.
A deformed and broken watchband spring from 4″ coax.

This deformation causes poor contact and can even cause arcing. The watch band was replaced and the line was reassembled. Because this line is our backup, I had to wait until Saturday to “test drive” it.

Main (left) and AUX (labelled "V4"). V4 is on the air, main is into the dummy load to keep the tubes warm.
Main (left) and AUX (labelled “V4”). V4 is on the air, main is into the dummy load to keep the tubes warm.

One of the luxeries of working where I do, is we have two separate transmission systems. The primary puts out around 40kW., the backup puts out around 10kW. The two systems are 100% separate. Separate transmission lines, RF sections and antennas.

Our primary, top left, standby white hanging down, bottom left.
Our primary, top left, standby white hanging down, bottom left.