White Paper
Testing RF Lightning Protectors without Indicators
IS-50 / IS-B50
11/2011 | 1474-003RevB
White Paper | Testing RF Lightning Protectors without Indicators
Testing RF Lightning Protectors without indicators
RF lightning protectors are designed to protect RF equipment by equalizing the potential difference
between the center pin of the coax cable and its shield. The majority of RF protectors do not feature
indicator lights to show whether or not they are still functioning. Since power is not readily available at
some installation points, an indicator light is not practical and would add significant cost to the protector.
So how do we know if the protector is still functional?
Since the IS-50 and IS-B50 are the most widely used PolyPhaser RF protector series, finding their
application in Land Mobile Radio, SCADA systems, amateur radio and many others, this paper focuses
on their specific testing condition.
IS50/B50 DC block RF protectors
The IS-50/B50 series products are capacitive
coupled gas tube protectors with multi-strike
capability. As DC block products, they do not
provide continuity from center-pin to center-pin,
or from center-pin to shield. Low frequency surge
current induced onto the center-pin from the
antenna side is blocked by the capacitor until the
gas tube reaches its “turn-on” voltage to shunt the
surge to ground (assuming the IS-50/B50 unit is
properly grounded).
Once the gas tube ionizes, the potential on the center conductor is equalized with the protector’s shield.
The lightning protector’s gas tube is connected between the center conductor and the coax cable shield.
Its purpose is to bring the center conductor up to the same potential as the shield during a surge event.
Once the center conductor and shield are equalized, current cannot flow between the circuitry connected
to the center conductor (base station – etc.) and the shield/equipment chassis’ ground. The radio
equipment is protected.
Conversely, at the equipment input little potential difference exists, which hinders damaging current from
flowing through it.
It should be mentioned that the majority of the lightning current is generally grounded through the protector’s
shield, not having to pass the gas tube at all. The only current through the gas tube is caused by the
potential difference between the shield and the center conductor from the antenna (surge) side, because
the gas tube, in a PolyPhaser protector, is connected between the shield and the center conductor on
the surge side (before the capacitor on the equipment side). Although tests still estimate peak currents
between ~100 to ~500 Amps, the current is further divided by the number of coax cables on the tower.
Hence, the gas tubes generally last a very long time and their failure rate is more influenced by the site
installation and number of thunderstorms per year than component failures.
Damage assessment
According to gas tube manufacturers, the components can take 1,000 events @ 500 Amp, 10/100
microsecond, or 5 events @ 20,000 Amp, 8/20 microsecond surge. Expected life of the gas tube without
experiencing a surge strike is 175K hours (20 years). However, predicting a gas tube’s lifespan is
impossible since lightning is a natural event with varying current waveforms and return strokes. The weak
link in all protectors is the contacts in the connectors. The center pin that makes the connection between
the cable connector and protector can get damaged from high currents. If that is the case insertion loss
and SWR (Standing Wave Ratio) increase. The IS-50/B50 protector should be replaced if insertion loss
and SWR are greater than they would be through a straight through barrel adapter, as full failure may or
may not pass RF.
How to test for damage
Since the IS-50/B50 protectors are DC Block devices, they cannot be tested with a resistance or ohmmeter. A properly functioning product will show the same results as a damaged one, because the unit
does not have any continuity between center-pins. Furthermore, roughly 95% of the time a gas tube will
fail open, which is also indicative of a functioning state, as well as a failed state.
To test the IS-50/B50 units, without expensive test equipment, check the VSWR (Voltage Standing
Wave Ratio, or return loss) of the radio transmission line. If it has increased significantly from the time
it was first installed, then damage to the unit can be expected. To determine if the damaged component
is the protector, it should be removed from the line and replaced with a barrel adapter. If the radio’s
VSWR performance has improved far beyond the VSWR specification of the protector, the unit should
be replaced.
The failure rate is affected by where and how the protector is installed and the natural variation in the
intensity and number of return strokes per lightning event. The tower size and placement, the ground
conductivity, and number of storms per year also influence the life expectancy of the unit.
Generally gas tubes will withstand multiple hits of <20kA each. However, since that much current could
only ride on the shield, it would indicate that the coax cable shield or tower ground connection may not
be grounded properly. The N connector center pin can also degrade with numerous smaller hits. This
degradation of the center pin will happen on any connector in the transmitter line. For this reason a
scheduled yearly testing of the system’s VSWR is recommended.
Please contact us for questions or further information on this topic.
Contact:
Tel:(+1)208-772-8515
Email: sales@transtector.com
www.transtector.com
11/20211 | 1474-003RevA
© Infinite Electronics, Inc., 2017. All Rights Reserved.
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