grounding antenna

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fireman3214

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I just put up a radio shack discone antenna outside in my back yard, its mounted on a 15 foot mast, the mast is about a foot in the ground and i have it cemented in, the coax i have has a ground wire with it, would it be ok to ground the antenna to a chain link fence post which goes into the ground?
 

SCPD

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ground it to the pole. If you ground to the fence it would be the same thing as to the pole.
 

n5ims

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Both the mast and fence post won't make good grounds and won't do the necessary job correctly. Grounding in that matter would be somewhat like using a fish tank air hose to replace the gas line in your car. It may look like it's working, but once the gas ruins the plastic, the heat from the engine causes it to melt or pull away from the connections, you'll have a real mess on your hands.

You'll need at the minimum an 8' ground rod (Home Depot has them in their electrical section) driven into the ground at least 7 1/2' worth and a good solid wire (#6 minimum) attached to the mast (in several places would be best). There are other things that are necessary, but I won't go into them there since they've been discussed in detail (some with very good specifics on what to do and what not to do) already on these forums numerous times.
 

ampulman

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Regarding grounding, this has been troubling me so I have to ask.

Isn't the antenna electrically isolated from the mast? In my way of thinking, if not isolated, any received signal would be 'grounded' out and not reach the receiver (assume antenna at top of mast, i.e., highest point).

In the case of coax, it is recommended that the shield be grounded at the point of entry. But, what about the center conductor? Doesn't the center conductor 'conduct' the lightning strike inside?

Amp
 

mrdinks

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Yes, cut the line and install a polyphaser. I do this where it enters the house, make sure it is well grounded. It will take care of the center wire in case of lightening.

Sent from my mobile world using Tapatalk HD
 

mmckenna

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Regarding grounding, this has been troubling me so I have to ask.

Isn't the antenna electrically isolated from the mast? In my way of thinking, if not isolated, any received signal would be 'grounded' out and not reach the receiver (assume antenna at top of mast, i.e., highest point).

In the case of coax, it is recommended that the shield be grounded at the point of entry. But, what about the center conductor? Doesn't the center conductor 'conduct' the lightning strike inside?

Amp
Depends on the type of antenna, but in this case, a discone is not "DC grounded". So, yes, the radiating element is isolated from ground at the antenna. Using a polyphaser or similar product is a good idea. It grounds the outer shield of the coax, but it doesn't technically ground the center conductor. There is a device inside that will ground the center conductor when it sees the sudden voltage surge.

None the less, the lightning will try to find a path to ground, and it will easily "hop" the insulator in the antenna, coax, etc. Think about it: the lightning strike has traveled thousands of feet through the air, an 1/8th of an inch of plastic isn't going to stop it. Grounding the mast is a really good idea, and actually required by the National Electric Code.

So, at least one 8 foot copper clad ground rod pounded into the earth close to the base of the mast. That should have a heavy gauge copper wire (#6 or larger) connected to the mast. The polyphaser needs a ground to, and that should be connected to the same ground rod with heavy gauge cable. NEC also has some wording about bonding the antenna ground rod and the electric system ground rod together.

Direct lightning strikes are not your only concern. Even nearby ones can induce high voltages onto the antenna and coax. Disconnecting your radio from the coax is also a really good idea during a lightning storm.

Of course lots of hobbyists never go this far and still walk the earth. It's a risk not to properly ground your antenna/mast system, but it's a risk that some people choose to take.
 

Ed_Seedhouse

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A discone antenna does not require an R.F. ground, since it is a kind of dipole although not a balanced dipole. Where lightning is a hazard (which is most everywhere) then it does need a proper electrical ground, of course. But that will make no difference to it's performance strictly as an antenna (other than that getting hit by lightning will likely destroy it and destroyed antennas don't seem to work very well).
 
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