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Equipment  -->  Subject: Static / lightning protection

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ZL4NVW updated in Equipment on 2020-12-06 16:40

I've just fitted an inductive 133uH discharge coil from feedline core to ground at the base of the mast for my 80m dipole - 100 turns of antenna wire on a length of 40mm plastic pipe. Which will hopefully work to discharge static from the non-grounded half of the dipole without passing RF (comments on the value welcome, just went off a comment that '100 turns on a beer can should be enough (then remove the can)') .  Unfortunately my antenna <1.5:1 SWR band seems to have dropped from 140kHz to 100kHz either side of centre - though at the low point is still 1:1 at the usual tuned centre freq of 3.650. Not sure if that's the result of the coil, or due to something else, but is annoying.  Solves the static, but not the lightning risk.  Will build another for the (40m/15m)-20m-17m fan dipole.

Separate conversations here (https://forums.qrz.com/index.php?threads/lightning-static-buildup-protection-for-efhw-on-house-gable.737891/) have convinced me that the EFHW coax run needs re-routing down to ground outside the house rather than through the loft-space so that it can get effectively grounded at the choke.  As I should have realised the UNUN (matching transformer) on the EFHW provides a low-resistance DC path between core and shield so the discharge device connected to the feedline core is not required for an EFHW so long as the shield is grounded at the choke.

All of which is about static buildup, and none of which deals with lightning strike. The spark plug sounds like a good cheap option.   I was getting a good 15mm spark gap between centre-pin and casing yesterday. The gas-discharge lightning protection devices cost more each than my Kenwood cost me by the time they're converted to NS$ and landed in NZ.
 

ZL1LC posted in Equipment on 2020-12-06 09:31

You don't have to be in a thunderstorm to have a shocking experience with an antenna.  


When I was an apprentice in a desolate place called Waiouru, we had a dipole strung up outside the workshop.  It had the braid of the coax grounded and tapped onto the center wire of the coax was a spark plug with a 0.030" inch gap.  (No metrics for me, so I'll let someone figure it out.)  In the winter, when the snow clouds were down low, that spark plug was zapping away all day as it discharged the static electricity that had built up on the antenna.


We don't normally get the huge lightning strikes that they get in the USA, because our humidity is higher, so I suggest as a good starting point, you need to start reading some of the ARRL handbooks.  Having a high wattage 100k ohm bleed resistor across the antenna to ground is a good start, but you need a fast DC (low resistance ) path to ground if there's any close lightning.  


The best solution is not to have your radio connected to the antenna during a thunderstorm.  That keeps the expensive item safe.  Fixing an antenna or coax is a lot cheaper and simpler.


73

Jim  ZL1LC

ZL4NVW updated in Equipment on 2020-12-06 09:31

Not specifically QRP related but ... 

Last night in a particularly heave rainstorm I was sitting at the computer listening to what sounded like an arc-ing electric fence inside of my antenna switch. 

Disconnecting the antenna cable proved to me quite emphatically (shockingly, even) that this was electric discharge happening from the core or the coax / connector coming from the antenna to the case & thus earth.

Dipoles:

The shield of the feedline is grounded at both mast and shack - but the core obviously not.  I had been disconnecting at the mast when away and when lightning was forecast - but clearly that's not good enough. 

  • Putting static discharge coils between mast-ground and the cores of the 2 dipole feedlines is a no-brainer.  
  • These presumably really should have some arc-gap or similar lightening protection too ...

End-fed half wave:

  • Feedline shield is grounded at the shack only (there is no mast) 
  • Feedline core is not currently protected at all
  • The unun is on the house gable with no ready access to ground and whilst I can run a static discharge ground cable down the outside of the house, I suspect that running something sufficient to ground a lightning strike is less simple and aesthetically acceptable

So 2 questions:
1) What would you you do to safely discharge & protect an EFHW?  
2) What arc-gap or similar lightening protection devices do you use and is any of it available in NZ?


Matt
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