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Lightning conductor for ametuer radio antenna mast - electrician says regular PVC sheated yellow green wire is ok from mast to earth rod

Former Community Member
Former Community Member
As per title I am getting a corner of my house setup as a radio shack and wanted to protect it appropriately.

The antenna is made of fibre glass but sits on an aluminium pole that brings the total height to 10m tall (only house in the area).


We have agreed to setup an earth rod below the antenna mast and earth it but the question is - will regular 35mm green/yellow sheated earth be good enough to provide earthing for antenna mast while being exposed to the elements? How long will it last?


Any thoughts on the electrical setup for the radio? We are planning to use the same earth rod and make a TT island for the power socket that feeds the radio unit (as main electrics is PME). The radio will be fed electric connection through an MCB by taking power from the main distribution box protected by a Type 1/2/3 SPD (I have 3 phase 100A supply from DNO) and we will use the earth rod for earth supply.


The radio unit will be powered by a seperate consumer unit which gets it's earth supply from the rod (I might also get a Type 1 SPD in the TT island consumer box).


The antenna and the TT island powering the radio will be linked by a 700A rated earth bar enclosed outside the property in an IP65 enclosure.


Appreciate any thoughts on my setup.
Am I being overkill and do not need a TT island for my radio setup? Will earthing the antenna be sufficient?
  • 0
    will regular 35mm green/yellow sheated earth be good enough to provide earthing for antenna mast while being exposed to the elements? How long will it last?

    BS 7671 lists bare 25mm² copper as being sufficient for a buried earthing conductor for protection against both mechanical damage and corrosion (table 54.1)  - so you should be more fine with 35mm² PVC covered on that score. (Copper water pipes seem to survive the elements both buried and above ground almost indefinitely.)


    If it's to be used as a lightning conductor though it'll have to survive lightning currents - others here will know more about that than me.


     
    a corner of my house setup as a radio shack

    I'd be a bit wary of converting a part of the house to TT while keeping the remainder as TN. When you have two different earthing systems it's imperative to keep them out of reach of each other - otherwise there can be a significant shock risk between items of metalwork connected to the two different systems. Metallic plumbing systems are one obvious way the boundaries between the two systems can be crossed (both initially and future channges) - but even things like Class I portable appliances and/or extension leads risks reducing the distance between the two systems in a way the electrical designer can't really guard against. I'll let the 'ham' types on here suggest the best solution to that problem though.


        - Andy.
  • 0
    you do not say how your antenna connects to the radio - many designs of ATU are in effect double wound RF transformers.

    There are 2 very distinct issues here -

    1) earthing PME/TT, and in your shoes I'd look to island as you propose if it is practiacl to keep TT earth and mains earth things apart (but beware of random items like radiators bringing  the PME earth into the shack room). But at install time you need to know the earth impedance, and it may be worth making provision to be able to monitor the voltage between the 2 earths.

    The other thing you can do to reduce risk, is simply to unplug the rig from mains, when not in use, breaking the earth as well as L and N, and especially when working on the antennas, and if you are not a heavy user, and do not operate when the mains voltage is misbehaving, that may well be all that is required really, and just use the PME earth for operation in a non-lightning state and when no-one is near the antennas - I'd like to think you'd not be transmitting then anyway.


    2) Lightning.

    Nothing you can do will protect against a direct strike, just insure and minimise the risk as best possible.

    Even a 10 ohm electrode will come up to 100kV plus during a modest 10kA strike - that will sideflash about 4 inches to any easier path, a megavolt will jump perhaps a metre or so.

    I tend to just drive the end of the mast into the ground - or a length of scaff pole beside it, but less than 10 ohms is a hard target to achieve.

    Induced voltages from strikes in the vicinity are more likely, but the voltages induced vary from nothing up to kV. Gas discharge devices accross the feeder or in a delat between feeder and ground help clip some of that. You need to select the voltage rating to suit the transmitter power.

    The only truly safe radio in the thunderstorm is one that is not connected to an  antenna, so make provision to interrupt the antenna feeds by a large air gap and short them to the external ground when not in use - a removable coax link is often the easiest.

    If you wish, carry a rod from the grounded mast onwards above the antennas, so that (hopefully) the mast tip is the attractor, rather than your prized rig. However, lightning is fickle stuff.


    Others will offer varying opinions - there are as many answers as circumstances.


    73 de G7VZY
  • Former Community Member
    0 Former Community Member
    mapj1:

    you do not say how your antenna connects to the radio - many designs of ATU are in effect double wound RF transformers.

    There are 2 very distinct issues here -

    1) earthing PME/TT, and in your shoes I'd look to island as you propose if it is practiacl to keep TT earth and mains earth things apart (but beware of random items like radiators bringing  the PME earth into the shack room). But at install time you need to know the earth impedance, and it may be worth making provision to be able to monitor the voltage between the 2 earths.


    The other thing you can do to reduce risk, is simply to unplug the rig from mains, when not in use, breaking the earth as well as L and N, and especially when working on the antennas, and if you are not a heavy user, and do not operate when the mains voltage is misbehaving, that may well be all that is required really, and just use the PME earth for operation in a non-lightning state and when no-one is near the antennas - I'd like to think you'd not be transmitting then anyway.


    2) Lightning.

    Nothing you can do will protect against a direct strike, just insure and minimise the risk as best possible.

    Even a 10 ohm electrode will come up to 100kV plus during a modest 10kA strike - that will sideflash about 4 inches to any easier path, a megavolt will jump perhaps a metre or so.

    I tend to just drive the end of the mast into the ground - or a length of scaff pole beside it, but less than 10 ohms is a hard target to achieve.

    Induced voltages from strikes in the vicinity are more likely, but the voltages induced vary from nothing up to kV. Gas discharge devices accross the feeder or in a delat between feeder and ground help clip some of that. You need to select the voltage rating to suit the transmitter power.

    The only truly safe radio in the thunderstorm is one that is not connected to an  antenna, so make provision to interrupt the antenna feeds by a large air gap and short them to the external ground when not in use - a removable coax link is often the easiest.

    If you wish, carry a rod from the grounded mast onwards above the antennas, so that (hopefully) the mast tip is the attractor, rather than your prized rig. However, lightning is fickle stuff.


    Others will offer varying opinions - there are as many answers as circumstances.


    73 de G7VZY


    There is no sperate room per say but it is a corner of the room with the closest socket to the house mains PME will be around 4-5 feet away.

    The radio will be unplugged when not in the property but like you said rightly I need to be careful of the two coming near to each other.


    There is a gas discharge surge protection device in line with the antenna coax and that device is also earthed through the common earth bar in the IP65 enclosure outdoors. So essentially the following are grounded through a common earth bar - Antenna mast, Coax gas surge protection device and the TT power supply.


    During a storm the radio will be disconnected and I am also looking at fixing a 40A isolator between the mains power supply and the TT power supply which I will be turning off when the radio is not in use.


    Any thoughts on this setup? Realistically do I need to do all of this stuff or could I just use PME for radio power supply and earth the antenna and coax?


  • 0
    During a storm the radio will be disconnected and I am also looking at fixing a 40A isolator between the mains power supply and the TT power supply which I will be turning off when the radio is not in use.

    How much protection will a 3mm or 4mm contact gap of an isolator give against kA/kV lightning? I might have though that physically unplugging things so plug and socket are many inches if not feet apart might be better?


    - Andy.
  • Former Community Member
    0 Former Community Member
    Will be doing both of them of course, diconnecting the devices from power supply to break the path to my device and main power supply 3 phase DB and using the isolator as a secondary break point.


    Any thoughts on why I might want to TT my radio island (and rely in PME earth) if I will disconnect everything including radio and antenna coax during a storm / lightning / rain?
  • 0
    Is this a VHF radio? Your post suggests a sleeved dipole or similar aerial type, enclosed in a fibreglass cover, which is common. It would be unwise to have any other metalwork near such an antenna because it will make a complete mess of it omni characteristic, which is probably why you chose it over a beam in any case. The chance of a lightning hit is really quite remote and anyway may well demolish your house, and as Mike suggested just earthing the pole by its base in the soil will al least take away some of the effects of nearby flashes. If it is a coax fed vertical, the coax outer is probably not connected to the support pole, it is worth checking. The best lightning protection you can get is probably a spike fixed to another pole some way from the antenna, with a good long earth spike connected. It does not need a large cable, it is not going to get hit, just reduce the local electric  field strength to your antenna.


    I have had many antennas on top of big hills for contests, and despite some thunder storms never had anything broken. No direct hits either, although the Landrover was almost blown over on top a Welsh summit once, which was interesting to say the least. Estimated windspeed was 140MPH at the time and the mast guys were very tight with a big beam at the top.


    73 de G8FNR
  • Former Community Member
    0 Former Community Member
    It is a 2m, 6m, 70cm antenna and the coax is plugged into the base but the antenna itself is made of fibreglass, just the aluminium pole (on which it is mounted) and base of antenna is metallic.

    Just behind the antenna and house is an electric substation with 33kw HV cables underground, there is also an earth mat for the substation maybe 1m away from where I want to put my earth rod.....


    So instead of doing all this and earthing the aluminium pole (which people say will attract lightning!) is it better to not earth the current antenna and just get another long lightning spike connected and grounded as a sacrifice?

    Should I also abandon this madness of making a TT island?


    Thanks.
  • 0
    I am beginning to think this is a wind up. 


    As for putting in your earth rod 1m from an earth mat of a 33kV sub to make a TT island!



    JP G8NUP
  • Former Community Member
    0 Former Community Member
    John Peckham:

    I am beginning to think this is a wind up. 


    As for putting in your earth rod 1m from an earth mat of a 33kV sub to make a TT island!



    JP G8NUP


    No I am being really serious about my question, I am not an electrician and just getting expert opinion from folks like you all......... is this frowned upon? What is the exact concern please?


    If it is that close to the earth mat would the TT island and the PME be at the same earth potential which means my initiative is pointless?


    The map of the entire layout is attached, the blue box is what the substation building is and the green dotted box is the earth map (as per DNO map symbols). The building next to it is the property and the earth rod I plan to use to make the TT island will go just to the right of where the (A) marker is on the map on the property boundary.....


    For some reason the Earth Mat is under my property ! I have no idea how they managed to do that or if it is even accurate. What depth would it be at?


     


  • 0
    Crikey - that is "a complication"

    Sod trying for the TT, all you will pick up is the substation bouncing with noise picked up on the HV lines  - they too pick lightning surges, and are in effect a good noise antenna.


    If the antenna is only VHF UHF, then the feed is presumably coax and does not need a ground at all for normal operation (I tend to think in terms of HF and open wire feeder )

    I'd plant the pole, and run the radio off the mains earth, and put something that can flash over between the coax outer and the pole if they diverge by more than 100V.

    And just unplug when the forecast is for lightning. If the antenna gets hit well bad luck - and maybe take it down when you are away for long periods.


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