Tag Archives: transmitter

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.

Changing the fluid… on your transmitter… P1

On most high power UHF, some VHF and some FM transmitters, the transmitter’s “residual transmission heat” is dissipated by liquid cooling systems.

Transmitter Liquid Cooling Plumbing for a Comark high power UHF transmitter.
Transmitter Liquid Cooling Plumbing for a Comark high power UHF transmitter.

Here in the US, it is very common to use a mixture of distilled water and Propylene Glycol (“Glycol”). The mixture varies based on the environment of the transmitter and the location of the external cooling systems (heat exchangers).

Transmitter heat exchangers mounted outside.
Transmitter heat exchangers mounted outside.

Glycol is used because it will prevent the outdoor systems from freezing. It also prevents the water from boiling when transferring heat. Glycol is a nasty substance if you have to handle it, with an invasive pink dye, which is very difficult to wash out when dried, and a perfume agent that is pungent at best. However Propylene Glycol is considered non toxic. (Unlike Ethylene Glycol, which is used in radiator fluid and plastics manufacturing.).  Glycol is a chemical that can leave behind residue, which over time, especially when mixing with water, can cause the pH balance and other properties of the glycol to change. This breakdown of the elements of the glycol, causing deposit buildups in both transmission components and the cooling system itself. It is important when using a piece of transmission gear, that this chemical be tested every other year, to determine if it needs to be changed out before that damage occurs.

We take the sample using the drain port on the IOT tube bottom.
This is done by running the fluid into a small bucket for about 15 seconds to flush out the crusted contaminants. Then using the sample container, we fill it slowly with the fluid.

Glycol sample collected underneath the IOT
Glycol sample collected underneath the IOT

The company we use, Dow Chemicals, provides the sample containers to put the samples in. They give you documentation and a little box to ship the samples out.  Because we have two discreet cooling systems, two samples of the chemical are sent.

Dowtherm SR-1 contained and ready to be sent for analysis
Dowtherm SR-1 contained from both systems and ready to be sent for analysis.

In a few weeks, we will get the results, and based on the results, will determine if we need to flush the entire cooling system and replace the Glycol in it.

What lurks behind the walls…

We had an ongoing issue in the “satellite office” (bathroom) where the light would cycle, like we took a quick power hit. I always just attributed it to the ballast or the bulbs being funky, but today I decided to inspect the wall-switch, just to check. Sure enough, this is what I found. Those who know electrical theory know why this is extremely dangerous. The “electrician” who may have installed this switch, never pre-twisted the wires before the wirenut, and never twisted the wirenut tight. So the neutrals both became loose and over time arced to the point of creating oxidation on the wires.

Needless to say, the switch has been replaced and the wiring is all cleaned up now.  You never know what lurks behind the walls sometimes…

burnedwires
Well cooked neutrals.

 

After waiting for the floor buffers to finish drying, I was hanging outside , watching these ominous clouds go by outside.

Storm Clouds at the transmitter in Needham.
Storm Clouds at the transmitter in Needham.