Lightning & static buildup protection for EFHW on house gable

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by ZL4NVW, Dec 5, 2020.

ad: L-HROutlet
ad: l-rl
ad: abrind-2
ad: Left-2
ad: L-MFJ
ad: Left-3
ad: Subscribe
  1. ZL4NVW

    ZL4NVW Ham Member QRZ Page

    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 EFHW antenna cable proved to me quite emphatically (shockingly, even) that this was static discharge happening from the core of 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
    • Static discharge coils between mast-ground and the cores of the 2 dipole feedlines
    • I had been disconnecting at the mast when away and when lightning was forecast - but clearly that's not good enough.
    • These really should have some arc-gap or similar lightening protection too ...

    End-fed half wave:
    • Feedline shield is grounded at the shack only
    • 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 to the boss
    • Far-end of EFHW has ready access to ground - but I'm assuming that resistance along the antenna would be such that placing the discharge coil & arc-gap device at that end would still result in dangerously high voltages & currents at the shack-end in the event of a strike

    So:
    1) What would you you do to safely discharge & protect this EFHW?
    2) Any general advice on different lightning discharge device types also welcome - though this is well covered elsewhere
    3) A pointer to guidelines for cable / strap size for a lightning ground connection would be useful too - can't seem to find that anywhere


    Matt - ZL4NVW
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    The solution is two-fold: first, the coax shield from any antenna should be "bonded" to a ground rod outside of the house before the coax comes into the house. Most hams drive a ground rod a few inches out from the wall of the house where the coax enters the house. This is required by electrical/safety building codes. There is more to this requirement in that it is also necessary to run a wire from the new ground rod to the grounding system that is part of the electric utilities company grounding system.

    Second, use a coax tee to connect a 100 kOhm, 2W (non-inductive) resistor between the coax center conductor and the coax shield in such a way that the resistor remains connected even if the coax is unscrewed from the rig or tuner. Some antennas are intrinsically DC "shorted" just by the way they are built, so do not need the added resistor. My EFHWs are like that...

    With the coax disconnected, use an Ohmmeter to measure between the coax center conductor and the coax outside (shield). If you see a low DC resistance, you do not need the added resistor. If you see an "open", then the resistor will prevent the "snap", "snap"...

    The EFHWs I have studied and built do have an intrinsic "dead short" between the coax center and shield through the primary winding of the 1:7 transformer. However, if you hang one outside in the air, and then unscrew the coax from a switch/tuner/rig with no installed ground rod, the antenna and its coax could pick up enough static charge from rain droplets, snow grains, or blowing dust to periodically arc to some nearby grounded thing... The outside ground rod stops that.
     
    Last edited: Dec 5, 2020
    N0TZU and WA4SIX like this.
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    What he said...

    Ed
     
  4. ZL4NVW

    ZL4NVW Ham Member QRZ Page

    Thanks for that ... caught me thinking in block diagrams rather than circuits. Of course - the UNUN _is_ an inductive discharge coil between core and shield. So that's the static build-up issue dealt with (once I've checked my existing ground-path to see why it was not discharging correctly last night)

    But what to do about lightning protection on the EFHW
    1. Could I implement lightning-protection for the EFHW at the far-end of the antenna (I'm dubious of this one ... but putting it out there)
    2. If not I will need to run a ground cable down from the eaves of the house to ground at the UNUN rather than grounding _in the shack_ as at present
    3. If I do install lightning protection at the UNUN, what spec of cable or grounding strap would I have to run the 8m down the outside of the house to ground?
    4. Worst-case assuming 250w and 3000ohms antenna impedance (UNUN is wound 3:23 -> squared -> 58 -> times 50ohms feed impedence = 2938 ohms antenna impedance) gives me 866 volts at the antenna feedpoint when transmitting.
    • Will lightening arrestors allow 866V to be present at the feedpoint (and thus at the protection device) when transmitting, or will they discharge at this voltage?
     
    Last edited: Dec 5, 2020
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Look at how this home-brew 160m EFHW is DC grounded. The series DC resistance of the choke is a few tens of mOhms, The series RF impedance of the choke is greater than 2000 + j2000 Ohms over the useful range of the antenna.

    The presence of the choke makes the antenna "work" like a dipole; without the choke the RF performance of the antenna is horrible.

    A lightning strike to the antenna wire is going to vaporize the antenna, but you must have a code-compliant grounding system bonded to the coax shield before it enters your house...
     
    Last edited: Dec 5, 2020
  6. ZL4NVW

    ZL4NVW Ham Member QRZ Page

    Yes. All good for static - thanks for the details. But for lightning:
    1) My UNUN will vapourise in the case of a lightning strike, so cannot be relied on as a lightning ground-path from antenna to grounded feedline shield
    2) I have no tower (just a timber-framed, timber clad building) so still need to know what spec of cable or strap I'd need to run the 8m from the UNUN to ground to safely take a lightning strike to ground
    3) Arc-gap lightening protection devices provide path-to-ground when feedpoint voltage exceeds a minimum voltage. Typically these are designed for 50 ohm dipoles with low voltage at the feedpoint so do not ground during normal transmit. Will they cope with the much higher feedpoint voltage of an End-Fed without discharging during transmit?
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    ... and about a usec later, so will the piece of coax between the transformer and the required ground rod, (or not)...


    Doesn't matter. I would not try to run a separate grounding conductor parallel to the coax shield. If you did, it would also have to pass through the choke so as not to screw up the RF performance of the antenna. You do need the ground rod just before the coax enters the house, regardless. That provides a shunt path into the earth, and should be cross-tied to the utility ground.

    With your transformer at gable height, I would run the coax straight down to the choke, which sits a few inches above the top of the ground rod... That vertical coax completes the "antenna". The coax shield below the choke is bonded to the ground rod, and then goes into the house.

    I have never bothered with these, even at several mountain-top repeaters I maintained which had intrinsically DC grounded antennas, and where the antennas were not the highest thing on an otherwise grounded tower. At home, my 65 steel tower is well grounded. If I was to add a gas-gap, I would put it in the coax of the highest antenna. In this case, it would cost me $100 to protect a $100 radio....
     
  8. ZL4NVW

    ZL4NVW Ham Member QRZ Page

    Great - thanks for that.

    2) Yes - this is where the problem lies. There is currently no coax external to the house for the end-fed. The UNUN is on the wall at the gable-end, the coax enters the house via a hole in the rear of the UNUN and is routed through the loft-space to the radio. This was specifically a requirement from the 'boss' who didn't want ugly coax-runs down the exterior walls of her new house. The problem with bevel-back weatherboard houses is that cable runs are _really_ visible as they cross the undulating exterior surface.

    The choke is currently in the loft-space 6m from the UNUN and does not currently have a separate ground (because that ground would only run parallel to the coax anyway - so is not going to offer a lower impedance ground-path than the coax shield already does). However ... I feel that this might have to change and the cable needs re-routing down the exterior wall and the choke positioning at ground level as you suggest - where the underground cable runs from the 'mast' have their shack-end ground connection. Would probably have to run it over to the corner of the house and down the smooth corner-boxing rather than direct to ground over the corrugations of the weatherboard. Sounds like a compromise but an acceptable plan.

    3) Yes - by the time I've put in 3 gas-gap protectors it has cost me considerably more than the 2nd-hand radio did. The other consideration though is fire risk - so I suppose it's not just the value of the radio I should be considering.
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    Gas gaps do nothing to reduce the potential damage to your house or its contents if lightning strikes your antenna. The protection comes from diverting as much of the strike energy into the earth before the coax enters the house.
     
  10. ZL4NVW

    ZL4NVW Ham Member QRZ Page

    I thought that is what a gas-gap is supposed to do. The gas ionises and provides a short, low impedance path which connected straight to your ground-rod, capable of carrying a huge current (unlike your now-fried coax). Am I wrong?
     

Share This Page