Transcript RF Applications

RF Lightning Arrestors
&
Grounding
Mike Baker, W8CM
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The Answer:
• “it depends”
2
Now, what was the question?
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“THE QUESTION”
• What’s the right lightning arrestor to use ?
• Where should I place it ?
• Do I need to ground it ?
• OK, OK, so maybe that was 3 questions
• Or…..maybe it was a question in three parts 
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These questions are
“Guru level”
Answers: 1-800-ASK GURU
RF
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Can we “arrest” lightning?
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Or, do we really just protect from it?
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The Bottom Line:
•
•
•
•
•
Many different “expert” opinions
No guarantees
Nothing is foolproof
Direct Strikes do damage
Real Problem: induced energy from close strikes
BUT
We can improve the odds
Commercial communications sites take hits
and keep on working
Your site can too
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WHAT’S THE RIGHT ONE TO USE ?
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SPARK GAP
• Cushcraft Blitz-Bug, Hy-Gain, etc.
• Cheap, poor protection, for older tube gear
• Divides energy
• Specifications “loose”
• Passes DC
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SHORTED STUB
• Practical mostly for
higher frequencies
• Bandwidth dependent
• Divides energy
• Easy to build
• DC shorted
¼ wave
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GAS TUBE
• Wideband, shunting
• Precise specifications
• DC pass
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GAS TUBE TYPICAL SPECIFICATIONS
• Impedance “off” > 10,000 Meg Ohms
• Impedance “on” a few milliOhms
• Switch time less than 100 nanoseconds
• When “on”, voltage is less than 30 volts
• Surge current 5000 Amps (IEEE 8/20 uS pulse)
• Life > 600 times @ 500 Amp pulse
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SHUNT TYPE
• Blocks DC
• Frequency Dependent
• “Bleeds” static
• Power Ranges
C
R
F
C
R
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MODIFIED SHUNT
TYPE
• Pass DC around or insert it
• Numerous models
• Frequency Dependent
•$$$
BLOCK RF, PASS DC
C
R
F
C
R
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PolyPhaser:
the “Coca-Cola” of lightning arrestors
• Many models for various power levels,
frequency ranges, etc.
• Generally expensive
• Highly favor blocking designs
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Gas Tube Arrestors
•
•
•
•
Alpha Delta
Huber – Suhner
Etc., etc.
Favored by LMDS (etc) industry for dc
pass to equipment on roof/tower top
• Wideband (dc- 3 GHz, for example)
• Reasonable price ($30-$60)
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I.C.E. Model I348
Rotator Line Suppressor/RF
Filter
Unshielded control line conductors used to operate
antenna rotators, remote switches, weather sensors, and
so on are easily charged to thousands of volts above
ground by overhead lightning discharges which can
destroy equipment. In RF transmissions, these same
control lines often couple RF and re-radiate signals
causing RF interference to other devices. This Line
Surge Suppressor/Filter from ICE solve both problems in
one convenient unit. Easy to install, this filter shunts RF
currents and AC/DC voltages above 50 directly to
ground. Uses metal oxide varistor devices and ceramic
RF bypassing circuitry.
Use this Suppressor on 4, 5, 6, & 8
wire rotator and remote relay lines
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Grounding ?
Ask the other Guru
(ground rod)
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Well, OK:
just a few comments on grounding
• Bond the coax to the tower at the top, plus the bottom and
even in-between (if a long cable)
• Protect all rotor and switch control lines also
• Use heavy wire (2 gauge) for DC
• Use low inductance strap for RF
• Cad Weld connections vs. clamp
• Use entry ground window (heavy bus bar)
• Bond all equipment in equipment room
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.
Wrapping It All Together:
Sample Installation
.
*
.
.
lightning
arrestor
*
radio
.
ground
block
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The End
73
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