Line Work FAQs
From equipment to technical aspects, our Frequently Asked Questions section will equip you with the knowledge you need to navigate the world of tall tower line work confidently and efficiently. Below are some commonly asked questions on the essential aspects of line work in the broadcast tower sector.
Line Work FAQs
Flexible coaxial line doesn’t need internal maintenance, but you need to make sure it is well supported by ensuring hangers (butterfly hangers and hoisting grips) are in good working order. Ensuring good RF properties with electrical sweeps is also recommended.
Ridgid coaxial line does require maintenance. In addition to having spring hangers being in good working order, internal watchband springs require replacement at approximately 15 to 20 year intervals. Again, RF sweeps are critical so you can detect when watchband springs need to be replaced. Replacement can then be scheduled as a maintenance item instead of taking a chance with a burnout situation.
Some industry experts recommend you sweep the line on your broadcast tower on an annual basis to identify when performance falls out of normal parameters. Many tower owners have moved to constant monitoring with technologies like Dielectric’s RF Hawkeye. https://www.dielectric.com/rfhawkeye/
The size of line and frequency both contribute to the power rating. Tower owners should consult with the manufacturer for their specific line rating.
Yes. Coaxial line can range from ½” to 9 3/16”. Additionally there is also a wide range of waveguide to choose from.
There are several factors that come into play when identifying the most efficient line for your applications. These factors include:
• Transmitter power
• Antenna gain
• Length of line run considering line loss at the needed frequency.
• Who manufactures broadcast transmission line?
There are several manufacturers of broadcast transmission lines. Major manufactures include Dielectric, ERI, RFS and Myat to name a few.
Waveguide is an alternative to coaxial line. Instead of having an inner conductor, waveguide simply provides a linear aperture for RF. Some frequencies respond better to waveguide based on power and frequency. Consulting with a manufacturer will help you identify the best solution for your application.
Rigid line consists of multiple sections that are installed the length of the tower. Flex line has the ability to have a single run up the tower. Rigid line is sold in specific standardized lengths (usually 20 feet or less) whereas flex line can be sold in a spool and have longer continuous lengths. Their installation methods and supporting hardware also have different requirements.
Coax is short for coaxial transmission line. Coaxial line directs RF power from the transmitter up the tower to the antenna.
If your tower has a rigid coaxial line, then you have a connector that allows small amounts of movement between the inner and outer conductor between line segments. Most of the time, that includes a watchband spring.
There are several considerations to know what pressure you should keep your line at. It is important that you do not exceed the capacity of your line and/or antenna. Consult with both line and antenna manufacturers to identify specific requirements for your system.
If you identify there is moisture in the line, the first priority is to identify where water is getting into your line. Once this is identified and addressed, two options for removing moisture are:
- Purge your system with nitrogen
- Vacuum out the moisture
If the moisture has caused additional damage, further remediation may be required.
In situations where there is significant moisture in your rigid line, the vacuum may be the best method of removal.
Depending on what type of line you have will determine the exact nature of the patch. Generally the hole is covered with tape to ensure patching material does not get in the line. The hole is then sealed with appropriate bonding material including epoxy putty. Preparation of the repair area is vital to get the putty to adhere. Putty is just a patch. Line replacement should be considered in some situations.
Determining where your line is leaking can be a challenge. A leak can be inside a radome or behind the transmission line. It is easiest to find an air leak on a calm day and is usually best found by hearing the air escaping the line. For smaller leaks, soap can be applied to the line and you can visually see bubbles appear where the leak is located. It is best to start from your pressurization hardware and work your way out. You don’t want to climb the tower if the leak is located on the ground.
Positive air pressure on a line protects the line from moisture or contaminants from entering the line. When a leak is present, that indicates a hole that can allow both moisture and contaminants to enter the line. This can cause a rise in VSWR all the way up to a burnout in your system.
Depend on the type of transmission line you have will determine the approach:
When replacing flex line vertical support of the line when lifting is critical. This is usually achieved by using wire as a cable messenger (wire supporting transmission line) in multiple locations along the new line. This is usually done in 150 to 200 ft. intervals based on manufactures recommendations. After securing the weight of the line in the tower, additional hardware such as butterfly clamps are added to the line to secure horizontally for wind protection. This is traditionally done in less than 3ft intervals.
Rigid line is addressed by starting with line support hardware in the tower. Then the transmission line sections are brought up starting from the lower levels and moving up. By contrast you hang flex line and you stack rigid line. Upon completion, the springs supporting the hard line need to be tensioned to manufacturer’s specifications. Also, appropriate rigid hangers need to be added to the top of the line run.
Performance criteria is generally systems specific and include parameters such as power handling capability, line loss factors, line run length, transmitter power and antenna input power to mention a few.
Watchband springs can have a useful life of 12 to 20 years. However, regular electrical sweeps should be performed to ensure that your springs still have remaining capacity.
There are several factors that will indicate what type of line you need for your antenna. These factors include:
- Available transmitter power
- Length of line run
- Line loss factor
- Antenna gain
- Desired ERP
Spring tensions are provided as a length and can increase or decrease based on the ambient temperature and distance below the nearest rigid hanger. You need to follow manufacturer’s recommendations to determine the exact point to point measurement on your hardware.
Proper support of the transmission line will ensure you maximize the full useful life of your line. Inadequate spring tension puts undue stress on the flanges. Over tensioned springs can impede proper thermal expansion of the inner conductor in the line. Excess in either direction can put undue stress on other components causing issues such as cracks in elbows.