9913F7: F is for Flexible
by
Stephen H. Lampen,
Technology Development Manager and
Martin J. Van Der Burgt, Senior Product Development Engineer
Belden, Richmond, IN
When most broadcast engineers think of antennas and transmitters, they think of hard-line or helical-design flexible corrugated transmission lines. These products are the standard for most broadcasters. However, there are many areas where these designs are over-kill, besides being difficult to work with and expensive.
Any transmitter-antenna combination has to overcome the loss (attenuation) of the cable between them. Cable loss can be compensated with transmitter power or antenna gain. As long as you end up with the desired effective radiated power (ERP), your system will perform as required.
The total attenuation of any cable is determined by the loss per unit (foot or meter), multiplied by the total length of the line. A very long line usually requires lower loss, larger diameter transmission line just to overcome the length of run, regardless of the power required. Or a transmitter with greater output power may be substituted, with the excess power used up in the added loss of the smaller transmission line. In the latter case, the power company is the real winner.
However, where moderate power (under 1kW) and moderate lengths are required, there are alternatives to solid or helical-design transmission lines. One of these alternatives is Belden 9913F7. Based on the very successful Belden 9913 design, the "F7" version indicates only one of its many features, flexibility.
Belden 9913 features a helical construction, much like its bigger cousins. This means that much of the interior is hollow. In outdoor applications, poor termination or damaged cable jacket may allow water inside the hollow area of this cable rendering it useless. Belden 9913F7 uses the latest technology nitrogen-gas injected foam polyethylene dielectric, replacing the hollow tube with a completely filled interior, eliminating this concern. Originally applied in the cable television industry, gas injection is already used in many of Belden's digital video coaxes. Now this technique has made its way to RF transmission lines as well.
Gas injected foam is capable of much higher velocities compared to traditional chemical foam technology. These velocities can be comparable to the air gap dielectric designs such as 9913. Additionally, the gas injection process has a negligible residual effect as compared to the chemical foaming process, improving the overall dissipation factor of the dielectric material. This helps to make 9913 and 9913F7 almost identical in high frequency attenuation values.
Besides the dissipation factor, old-style chemically-foamed cable typically suffers from another problem: center conductor migration. Because the elasticity of the conductor material and the polyethylene is quite different, over time the conductor tends to creep or cold flow through the polyethylene. Conductor migration is accelerated when the cable is installed with a tight bend radius, continuously flexed, or exposed to elevated temperatures. This migration would radically alter the impedance at that point, causing serious VSWR, and dramatically reduce the voltage-breakdown ratings. The poor mechanical performance can be attributed to the materials used which were typically low or medium density polyethylene.
Belden 9913F7 utilizes gas-injected foam high-density polyethylene. The foam cell walls are more uniform and very stiff, resisting center conductor migration and maintaining an impedance tolerance for 9913F7 of 50 ± 2 ohms. The construction consists of a stranded center conductor for improved flexibility. Additionally, the jacket is Belden's own Belflex® compound for great low temperature flexibility and ruggedness.
Where high power handling is not an issue, such as receive antennas, Belden 9913F7 could be an ideal choice. In these applications, attenuation is the critical factor. However, comparing the cost benefit of 9913F7 to helical-design corrugated transmission lines may more than offset any increase in attenuation.
An ideal application for Belden 9913F7 would be ENG, where communications links or microwave (below 4 GHz) applications are common. 9913F7 is especially recommended where towers are raised and lowered, such as ENG trucks and vans. Other applications include RPU and two-way applications for radio - especially where antenna cables are continually coiled and uncoiled; FM translators or boosters; cellular base station equipment; low-power television; low-power FM; emergency back-up systems; 50-ohm receive antennas; and for wireless microphone receivers - especially those with extended diversity antennas.
Belden 9913 and 9913F7 utilize the same connectors and tooling. Popular versions include Amphenol 82-202-1006, Trompetor 1005-1107-1, and Kings 1205-4-9.