This section describes several models of a signal-level measuring instrument used in the cable television industry. In its typical configuration, it measures the peak voltage of a modulated carrier, and displays the results in voltage units (millivolts or microvolts), power units (dBm or dBmV), or some combination of these units.
This meter goes by two names: signal level meter (SLM) and field strength meter (FSM). These terms are not synonymous, although they are often used interchangeably by cable TV personnel. The difference lies in the way in which the two meters are used:
- A signal level meter (SLM) is a variable-frequency voltmeter designed to measure the peak voltage of an RF signal inserted at its input connector:
- A field strength meter (FSM) is a signal level meter that, when connected to an antenna of known characteristics, is used to measure the signal strength of an electromagnetic field:
Adding to the confusion, Wavetek identifies its line of meters with the term "Signal Analysis Meter (SAM)."
FSMs were originally developed for the broadcast industry. Station engineers needed equipment to measure the signal strength — i.e., field strength — of a station's signal at various locations within the station's coverage area. Manufacturers responded to this need by offering an entire package — SLM, antenna, and interconnecting cables — as an integrated system, tested and guaranteed to produce accurate results when used as instructed. Manufacturers called these meters field strength meters to indicate the intended use.
When the cable television industry came along, it needed SLMs to measure signal levels at various points in the cable distribution network. The obvious choice: the same FSMs that the broadcasters were using. They were readily available, relatively inexpensive, and sufficiently accurate for the purpose.
Furthermore, the cable industry actually did need FSMs for some applications. Since the earliest days of the industry, cable engineers have used FSMs to measure the field strength of television broadcast signals received at their headends.
Nevertheless, FSMs designed for broadcast use were not ideal for cable TV work. Depending on the manufacturer and model, these meters exhibited some or all of the following disadvantages:
- Poor sensitive, rendering them unable to measure the low voltages typical of cable TV networks.
- Broadcast-specific input-frequency selectors highlighting VHF and UHF broadcast channels.
- Output displays calibrated in dBm.
- 50-ohm inputs, necessitating the use of special adapters for use with the cable industry's 75-ohm circuits.
As the cable television industry developed, equipment manufacturers began to offer specialized SLMs specifically for the industry. In many cases, they simply repurposed the FSMs they'd been selling to broadcasters, but they included modifications to address the disadvantages described above. Cable-specific meters featured:
- Improved sensitivity.
- Cable-specific input-frequency selectors highlighting channel bands used by the industry.
- Output displays calibrated in dBmV.
- 72- or 75-ohm inputs.
In most cases, an F-connector was provided for input, although some meters were fitted with BNC connectors. Some really early meters (including a couple shown in this section) were fitted with C-connectors, and a few were even fitted with 300-ohm inputs to match 300-ohm dipole antennas connected with twinlead.
These meters were true signal level meters: they were designed specifically to make signal level measurements in cable TV networks. Yet, for historic and cultural reasons, many of them were (and still are) called field strength meters by industry personnel. Equipment manufacturers have contributed to this confusion: manufacturers that had repurposed FSMs as SLMs often used the same housings without modifying identification labels or nameplates.
Even today, the term field strength meter is so universally recognized that the distinction between FSMs and SLMs has all but disappeared.
This section contains three pages:
- Signal Level (Field Strength) Meters, Group 1: Amphenol to Jerrold
This page illustrates meters manufactured by Amphenol, Avantek, Benco, Blonder-Tongue, Industrial Television, and Jerrold. Each meter is identified as SLM or FSM according to the manufacturer's designation.
- Signal Level (Field Strength) Meters, Group 2: Jerrold Model 704
This page is devoted to a single meter — the Jerrold Model 704 — because of the special place this particular meter holds in the history of the cable television industry.
- Signal Level (Field Strength) Meters, Group 3: Radion to Wavetek
This page illustrates meters manufactured by Radion, Sadelco, Sencore, Simpson, Texscan, Transvision, and Wavetek. Each meter is identified as SLM, FSM, or SAM according to the manufacturer's designation.