Earth grounding of radio shack

Discussion in 'General Electronics Chat' started by wpri268, Jul 14, 2014.

  1. wpri268

    Thread Starter New Member

    Feb 18, 2012
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    I question if I have created a ground loop problem between the AC ground and a vertical antenna radial system that may be causing some RFI.

    My AC safety ground consists of two 8' rods one vertical, one horizontal driven into the earth. The horizontal one is several feet below grade and both are nearby the breaker panel.

    The radial system for the transmit antenna is made from thirty, 60ft long #12 radials fanned out in a semicircle.

    My receive antenna is comprised of two ground independent pennant antennas galvanicaly isolated from the coax feedlines via transformer. The coax outer braid however is common with the radial system at its center. Coax from both antenna systems runs about 90ft on top of the ground to receiver and transmitter.

    There is no direct connection between the radial system and AC ground except through the coax outer braid as they connect to the transmitter and receiver while both receiver and transmitter are AC grounded. This seems to me to create a large loop. Should I connect the radial system to AC ground directly and if I did, should I connect to the center point or just extend one radial to make connection?
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
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    This was covered in some detail in a few instalments in the UK magazine; Practical Wireless.

    Have a look for their website - there may be an option to order photocopies of the original articles.
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    Chasing down the source of RFI in an antenna system is some art and a lot of cut and try. The best I can do is make some suggestions.

    My first impression is that the problem is induced current on the braid of your transmit coax from the radials. Try a 1:1 isolation transformer if you have one. You can also strap ferrite to the cable. Also maybe a single loop through a T200-2 toroid core.

    If power plays a roll in the RFI, then look for corrosion. Oxides are non-linear.

    Mark KG7VH
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If you want protection from lightning, then the ground loop you describe is inevitable. Presumably, your station is AC-line powered, which means it is intrinsically connected to the ground rods under your AC distribution panel through the power cord. The coax shield is intrinsically connected from the radio(s) to the antenna radial system, therefore, there is an Ohmic connection between the grounds (as there should be).
     
    Last edited: Jul 14, 2014
  5. vk6zgo

    Active Member

    Jul 21, 2012
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    This has been chewed over at length on QRZ.com,so you may be able to get some more info there.
     
  6. wpri268

    Thread Starter New Member

    Feb 18, 2012
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    I am a firm believer in single common point grounding in regards to nearby lightning strikes. The ground potential rise will induce current to flow between points at different potentials which makes perfect sense. Flashing back to my original post however, what I was asking is whether or not the coax outer shield of my three antenna cables was sufficient to consider the radial system and AC safety ground as common, or should I extend a radial for direct connection to AC ground.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    also, if there is a difference in ac voltage between the house ground and antenna, it can induce current on the centerconductor of the coax. un hook the coax at the radio and check for ac voltage between the shield and your station ground.
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    No, it's not a grounding conductor for safety or lightning protection. What kind of ground rod system is at the antenna? I would assume you have a RF bulkhead grounding plate outside the house/shack that all coax/power/control lines connect to while entering. That would be the tie point for the AC safety ground, any buried bonding cables from the antenna grounding system and shunt lighting protection devices.

    For information on grounding/lighting protection systems this should have about everything but it's likely overkill for a Ham shack.
    https://www.wbdg.org/ccb/FEDMIL/hdbk419a_vol2.pdf
     
  9. wpri268

    Thread Starter New Member

    Feb 18, 2012
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    Lots of good information in the referenced military handbook....thanks.

    SHACK LAYOUT.jpg This illustration is a basic layout of my shack.

    There are no ground rods attached to the radial system which consists of 1800' of #12 tinned copper wire.

    The coax shields of the two amps are common at the center of the radials. I installed 75V GDT's inside each amp from the center conductor to GND. The amps are powered thru the coax.

    The ATU of the vertical broadcast antenna has a factory installed surge suppresser from antenna to GND. This coax runs 90' directly to the TX through an underground conduit. The TX has the same factory installed suppresser as the ATU.

    RX coax runs on the ground directly into the shack. Its center conductor is always and automatically grounded via an open frame relay except when the radio is turned on.

    ###################################################

    This is what I would like to do....

    Run the TX and RX coax underground thru the same conduit.

    Install surge suppressors on each RX coax where they enter the shack and connect to common point AC ground.

    Float the RX and TX from house wiring via iso transformers and install a separate RF ground of copper foil connected directly to the common point AC ground.

    Last but not least, extend a radial to connect directly with the common point ground.

    That should eliminate any possible ground loops and isolate the RX from shack wiring which has been a known source of RFI.
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    are the radials insulated or bare?
     
  11. wpri268

    Thread Starter New Member

    Feb 18, 2012
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    The wire is uninsulated tinned copper. Back when I bought it, it was cheaper and easier to find than bare copper.
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    then whats the difference between a ground rod and the bare wire in the ground?
     
  13. wpri268

    Thread Starter New Member

    Feb 18, 2012
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    There is no difference between the two when DC is concerned but AC follows the rules of the skin effect. At line frequencies, like DC, there is little difference but as frequency increases up into RF currents flow only on the surface of conductors. The same is true with the earth. For that matter, radials that are buried too deep will loose their effectiveness. A single ground rod is basically useless at RF.
     
  14. alfacliff

    Well-Known Member

    Dec 13, 2013
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    and wire buried in soil does not exhibit skin effect. mostly just reduces ground resistance around antenna for better radiation path and less radiation resistance. check for current on shield of coax, 60 or 50 hz on shield will induce hum and noise into center conductor. then when you know which way the noise is getting in, you can find a solution.
    the military has gone to using three ground rods dirven in the ground under a vertical antenna in a triangular pattern about 1 1/2 foot on a side. makes a very good ground for verticals.
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    AFAICR; radials can be either insulated or bare as they're usually used for the antenna ground plane - which is not necessarily the same as earthing.

    I've seen photos of an installation with a large sheet of metal covering most of one wall of a shack, all the incoming RF connectors were secured to it and it was earthed by multiple grounding rods.

    There are certain rules you have to follow to avoid current loops between the antenna grounding and the mains earthing to equipment chassis.

    Its also worth keeping in mind the ground potential gradient that spreads radially around a direct lightning strike, the voltage across the distance of the fore & aft legs of cattle frequently kills some even when the strike was some distance away!

    You don't want that coupled into your house wiring!

    If I could remember what they're called, I could google for some pictures - I've seen a photo of what looked like a glass tree branch, lightning had struck several feet of sandy soil and fused the sand into glass - one of the branches found an underground power cable, presumably the glass bit was dug up while they were repairing the cable.
     
  16. alfacliff

    Well-Known Member

    Dec 13, 2013
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    there is also a voltage gradient across the earth due to ciculating currents between grounds in power lines. thats what I was refering to when I said you should check for voltage betewen the coax shield and radio.
     
  17. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    The antenna RF ground (radial current driven reflector) is used to create a proper virtual image of the driven element for a vertical antenna. This makes the properly matched antenna into a electrical equivalent of a 1/2 wave dipole for a 1/4 wave element as the charges in the driven element wire react the same with the ground plane as the would if the opposite element existed and was feed the same signal 180 out-of-phase but the radiation patten is the upper half of the dipole so only ground and sky waves are normally generated. The radials can be bonded to earth ground in addition to the feed-line ground for lightning protection, it's not required for proper RF operation but it's a really good idea to have shunt protection devices and a good earthing system at the base to reduce the amount of energy that will be should be shunted to ground at the shacks earthing system using grounding and protection devices.

    Typical RF Surge Protection devices for commercial and military systems that I have used before. http://www.smithspower.com/brands/polyphaser/products/rf-surge-protection

    Grounding notes by Polyphaser
    http://mobileers.com/main/getting-on-160/grounding-information/grounding-notes-by-polyphaser/
     
  18. wpri268

    Thread Starter New Member

    Feb 18, 2012
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    Note: so called petrified lightning is sand that has been fused into a thin glassy tube. They are called fulgurites.

    It has been demonstrated that better results are had by not burying radials because of earth losses. Having them on top of the ground is a close second but elevated is the way to go. The primary reasons for burial are safety and security. Imagine an AM broadcast station with 120 elevated radials hundreds of feet long. The pic is an example of such an installation that was taken nearly 40 years ago. It has since been modified.

    Raised radials.jpg

    RF energy does not penetrate deeply into the earth. Thus I refer to this phenomenon as the skin effect. Therefore burying radials more than a few inches deep gains nothing.
     
  19. alfacliff

    Well-Known Member

    Dec 13, 2013
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    most of the local stations use a copper grid burried undeer the antenna field. not radials, but a grid of bonded copper. makes for very low radiation resistance, and you can change frequency too.
     
  20. wpri268

    Thread Starter New Member

    Feb 18, 2012
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    Although a grid may be used in the immediate vicinity of the base of an AM tower it is in addition to the 120 radials that are required by the FCC.
     
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