Grounding Do's Don'ts & Why Part 3

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI5DH, Mar 19, 2012.

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  1. AI5DH

    AI5DH Ham Member

    I am not going to slap anyone silly, especially someone asking good questions.

    OK so here is my answer. It is better than nothing, but lacks one crucial element; A Surge Protection Device (SPD) for the center conductor. It will certainly bleed off a charge on the shield, but does nothing for the center conductor.

    I know exactly what you are talking about with the Telco ground clamp that slides under the sheath and clamps the metal shield. They are made by 3M part number 4460D and called a Scotchlok Grounding Clamp. CLICK HERE and you will see them, or at least somthing very similar. Real expensive stuff all at $2.00 :D If you want one you should be able to find them locally at electrical supply houses. Look and see if you have a Graybar or Grainger retailer in town.

    However I do not think they would work very well for your application as it would be almost impossible to slide one up inside the insulation without tearing up the braided shield. I have never tied it on a coax.

    Now what you can do is buy a device made to do exactly what you want to do that has a SPD device. You will basically cut the coax, then terminate each cut end with a connector like a PL-259, and screw each end into the protector block. They are called Ground Blocks and available from many manufactures like MFJ, Polyphaser, and DX Engineering to name a few.

    Good Luck and 73 to you.


    Dereck
     
    Last edited: Mar 20, 2012
  2. WA2LLN

    WA2LLN Premium Subscriber QRZ Page

    I really appreciate these threads, great information and discussion that has helped to clarify the concepts and methods I'll use setting up my antennas here in north central Florida (lots of lightning!).

    What about protecting a remote ATU or other remotely controlled matching device? I'm considering two applications, a 43-foot vertical and a 135 foot doublet fed with open wire ladder line. In the case of the vertical, the matching device will go at the base of the antenna and coax will run from the matching device to the entrance panel. For the doublet, the open wire feeder will come down to ground level and connect to the matching device there, and I'll run coax from the matching device to the entrance panel.

    I am considering several matching device possibilities:

    1. An automatic ATU such as from MFJ or SGC. Some of them are autonomous, just send power up the coax feedline and the device works with no other control circuitry. Others require a separate multiconductor power and control cable.

    2. A relay operated matching unit. Basically an L-C network with relays to switch coil taps and/or capacitors to set operating frequency. A multiconductor cable would be used for relay control, no intelligence or relay drivers in the box.

    3. A hybrid of (1) and (2). An L-C network, and install a frequency counter and control logic to control the relays. This would only require power sent through the coax.

    In all cases, I see a need to protect the matching device. I also need to prevent energy spikes from entering the shack via the control cable if applicable.

    Any suggestions?

    --
    Art
     
  3. AI5DH

    AI5DH Ham Member

    Art need more info to answer. Is the tower a radiating tower mounted on isolators? Or is it grounded and shunt fed? Or am I completely off the mark?

    I am also unclear on what ATU is. Is it an Automatic Tuner Unit? Or a Tower mounted amplifier?

    As for the control cable entering the shack is pretty straight foward. You are going to want to add a SPD (surge protection device) on the Surge Reference Equalizer panel. The manufactures should be able to make recommendations based on the operating voltages involved, or you could make one for a few bucks and a utility box to contain some MOV's and/or SAD's.

    If it were up to me,, I would use what commercial operators use. We send DC Current up the coax to power Tower Mounted Equipment, and it is injected through the Antenna Surge Protector (aka ADU) called a Bias Injector. On the side of the ADU is a port to inject DC current to supply power and control signals.
     
    Last edited: Mar 21, 2012
  4. WA2LLN

    WA2LLN Premium Subscriber QRZ Page

    Dereck,

    The vertical is a DX Engineering 43-footer. Just a bunch of tubing, isolated at the base and fed at the bottom. Since its a non-resonant antenna, a matching device of some sort is needed. An ATU (Automatic Tuner Unit) can be used, as can an L-C network. I'm thinking I'd use the vertical on 80/75/40 only, so an ATU is overkill and I'm leaning toward starting with an L-C network with manual band change to start, and experiment with relay controlled band change later. And if that works, I'd like to homebrew a changeover device using a frequency counter.

    The initial relay control would require more than a single conductor (three or four band settings), so a bias injector isn't appropriate. For the ATU that just needs some DC (12V) to operate, I would use a bias injector. I'm not aware of SPD's for control cable applications, but I have not researched them at all yet. Homebrewing one is within my capabilities, once I know what components are needed.

    With the vertical, I guess that to protect an ATU or L-C network, it might be sufficient to short the feed point to ground when the antenna isn't in use. Perhaps even better would be to disconnect the matching device from the antenna, and short it and the antenna both to ground. I don't know if there is a relay made (and is affordable) that would work in that role.

    --
    Art
     
  5. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    I have several ground rods around my tower and just outside the shack entrance, copper buss with surge protectors both at the tower base and shack entrance. But during the summer months when lightning is almost a daily occurrence, I routinely just keep my crank-up tower cranked about a third of the way down so the top of the tower is below tree level. If I have ample warning, I may even crank it all the way down.

    I just don't want to test my ground! I know . . . . it's going to get hit one day!! I also routinely disconnect my coax from the rigs as well. Lightning down here is AWFUL!!

    ............Bob
     
  6. AI5DH

    AI5DH Ham Member

    OK Art you can do this. Surge protection device are really simple passive devices, and easy to fabricate.

    All SPD's are is either MOV's, Zener Diodes (called Silicon Avalanche Diodes) Gas Discharge Tubes, and Carbon Blocks. Each has its own advantages and disadvantages. Regardless of which technology they are all treated the same for placement. Only thing you need to know is the operating voltage so you can make the proper selection of the CLAMPING VOLTAGE or aka as Let Through Voltage

    For example let's say this control cable is a 4-conductor cable where one conductor if ground, and the other three supply some voltage like 12 volts DC. You would need , 3, 6, 9, 12... SPD's installed from each signal line to the ground line inside a utility box. So for a minimum of 3 MOV's would be just 1 MOV connected L-G on each of the 3 control lines.

    All a SPD does is when the CLAMPING VOLTAGE is reached, they turn on and become a short circuit that diverts the current to ground or whatever point you direct them too.

    Now with that said I would bet there are commercial units available and could be used. For example if the control is say a 12, 24, or 48 volt signal there are dozens of manufactures that make Surge Protection Devices. You might even be able to use one of the devices made for Telephone, CATV, DATA, and Ethernet. Some of the best are made by Polyphaser and they have devices for all I just listed. Go Here, then click SURGE PROTECTION tab for a list and see if any fit the bill.

    If you want to home brew consider using Din Rail's
     
  7. W4JFA

    W4JFA Ham Member QRZ Page

    LJW, First, thank you for the great write ups. I did look at the web site you had a link for but I cannot tell which would be the best for twin lea/ladder line etc? Can you tell me which one? Also, I noticed when you click on "pricing" you have to have a company name so if you can give me a ballpark price I would appreciate it.
    Thank you very much, Bob WE6C
     
  8. KK4HYS

    KK4HYS Ham Member QRZ Page

    LJW,

    Great threads! I recently became a HAM and I'm in the process of setting up my first antenna. I live in Florida so lightning is a huge concern for me. Schematicforsetup.jpg My set up is going to be extremely simple. In a nutshell, I plan to setup a J-Pole over the roof of my house, although it will be mounted on the side of the house with a Y-wall mount. The mast will be high enough to get some good height. I plan to set up a Single Point ground panel and install a lightning arrester on it. My feedline from the J-pole will run down to the arrestor and back up into the house through a vent. The feedline will run through the attic into my room. It will be connected to a handheld in my room. I would have a ground cable run from the SPG to a ground rod (which the Meter box is grounded to).

    Would this set-up be sufficient to protect from lightning?

    Should I also run a ground wire directly from the mast to the ground rod??? I was planning to do that as well.

    I think a picture is worth a thousand words so I went ahead and have drawn up a schematic and have attached it here. Any advice would be much appreciated.
     
  9. K7BVT

    K7BVT Ham Member QRZ Page

    We operated from a hilltop recently in the 7th call area QSO party. Installing an 8' ground rod was not practical due to rocky soil and the difficulty in removing such a rod after the event. Are there any rules of thumb for using a number of 2' or 4' rods for lightning protection?
     
  10. AI5DH

    AI5DH Ham Member

    All please accept my apology for being so late too respond as I have been on long over due vacation in the Caribbean for a few weeks.

    OK I suggest two modifications.

    Run a at least a minimum 10 AWG copper or 8 AWG aluminum from the mast straight down to a ground electrode like a ground rod. Ensure the ground rod if added is properly bonded to any other ground electrodes used.

    Install the Antenna Discharge Unit as physically close to the ground as possible in order to keep the ground bonding jumper as short as possible.
     

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