Tune In – Antennas are the Eyes and Ears of Signal Leakage Systems

Despite huge improvements in computing power available to design tools and simulations, RF antenna design remains at least somewhat of a black art.  The best antenna designers are often brilliant Engineers, but most will admit that past experience and quite frankly luck often factor into the best antenna designs.  This blog will skim over the surface of antenna theory and focus on how antenna performance impacts overall cable signal leakage detection efficiency and effectiveness

Antenna Design and Applicability

The primary purpose of an antenna is to convert electrical signals into radio waves (transmitter) and vice versa (receiver).  Many factors impact the effectiveness of an antenna including the materials chosen, shape, size, geometry, and others.  Design details for a given antenna will vary widely based on their specific application:

• Omnidirectional vs unidirectional
• Broadband vs Narrowband
• Fixed vs portable/mobile
• High-gain vs low-gain
• Form-factor (size, weight)

Antenna Designs

Antenna designs run the gamut from simple whip antennas to complex mobile and microwave designs.  Some of the most common designs and their characteristics are listed below

• ½ Wave Dipole – Ideal for field-localizing VHF leaks
• Polarized antennas help pinpoint the leak type.
• Matching the polarity helps isolate the leak source by indicating vertical or horizontal plane.
• Loop – like a dipole minus the polarization properties.
• This antenna provides direction but not polarization
• Monopole or ¼ Wave Whip – Good for vehicle use, not broadband or directional
• 360-degree radiation pattern ideal for vehicle use.
• Rubber Duck – Compact/durable for general field use, not broadband or directional
• Yagi – ideal for field-localizing UHF leaks, often bulky form-factor
• Log Periodic Dipole – Ideal for field-localizing a wide range of frequencies in a reasonably sized package.
• Wide band antenna array providing direction and gain over a wide frequency range.
• Shark-fin – Wide band agility with additional gain over a standard whip antenna
• Near Field Probe – Ideal for pinpointing leaks in crowded pedestals, non-frequency specific

Antenna Size

The size of an antenna is inversely proportional to the frequency of leak it is designed to detect.  Also, the type of antenna will dictate if ¼ wave, ½ wave, or full wave antenna is the most efficient transmitter/receiver.  In many cases, a full wave antenna does not make an efficient transmitter/receiver.  Take the dipole for instance:  ½ wave dipole is a very good transmitter where a full wave dipole cancels itself out and is a very inefficient transmitter.

Below is a general formula for calculating the optimal half-wave dipole antenna length for any give frequency.  For the purists, this basic formula does not take into account end effects so is only an approximation but is close enough for back-of-napkin calculations.

• General formula: Half-wave dipole length (in) = (5905*A)/Freq (Hz) where A is factor normally close enough to 1 to make general approximations
• Ex: 5905/900MHz = 6.6in, so for 900MHz leak a 6.6in antenna length is ideal
• Ex: 5905/120MHz = 49in, nearing the upper edge of what is practical for field use

Why This Matters:

Understanding the relationships between frequencies and antenna length can yield clues about where/how signal may be leaking out of the plant.  If you are hunting a high-frequency leak, the “antenna” or shielding defect allowing the egress is likely short (think about length of a radial crack on 0.500 hardline or loose hardline connector circumference for example).  If hunting lower frequencies think longer “antennas”.

Antenna Impact on Cable Signal Leakage System Effectiveness

For cable drive test, sensitivity is far more important than directionality since precise localization is accomplished via quadrangulation of multiple samples.  Broad spectrum coverage is also important with the advent of leakage detectors with complete frequency agility.  It does little good for an operator to have detectors which can have their monitoring frequencies changed with a few keystrokes if every truck in the fleet must have antennas swapped out to capitalize.

For walkout, directionality is the most important factor in quickly pinpointing leak sources in the field.  Sensitivity must be adequate to detect distant leaks, and broadband coverage plus field tunability help minimize the number of antennas that are needed.  Dipole, Yagi, or Log Periodic antennas shine in this use case, especially when they are designed specifically for Cable usage (small, durable, fit behind seat of small trucks).

Summary

Antenna design is neither easy nor a precise science, but when done right the results can be remarkable.  The best cable leakage systems will excel by creating an antenna portfolio which:

• Provides maximum channel layout flexibility by supporting detector full-band frequency agility
• Facilitates fastest field find and fix via purpose-built walkout antennas with superior directionality
• Enables detection of leaks that lesser systems miss via high-gain antennas

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