Electric Shock from the Coax

[hd625b]

I'm getting a small electric shock from my coax.
I have a radio shack discone antenna mounted on a collapsible mast. I have the antenna up about 20 feet above the roof. The mast is sitting on a wooden platform which in sitting on a concrete slab. The mast is grounded to a 5 foot steel shaft driven into the ground. The shaft is about 4 feet from the base of the mast. I checked the coax with a volt meter and it showed about 0.4 volts AC. I only noticed the shock when my hands were wet and I was disconnecting the coax from the radio.
Any ideas where the shock is coming from and ideas how to eliminate it would be greatly appreciated.

 

[rescue161]

Do you get shocked only when the scanner/radio is connected? Is the radio connected through AC power? Have you checked the potential between the actual ground (for the electrical panel) and your steel ground?

 

[mmckenna]

One possible reason is that while the mast is grounded, the physical radiating element of the discone (the top vertical and "spoke" horizontal elements) are not. In your area, dry warm winds can generate a static charge.
If this static charge is high enough, you could potentially damage your radio.

While it's only showing 0.4 volts on your meter, it's likely much higher than that.

 

[hd625b]

I only get the shock when the coax is disconnected from the radio. I have a Y adapter on the end of the coax so I can run two scanners. When I connect one scanner I do not see the voltage. When I disconnect both scanners I see the 0.4 volts. The radio is a BCD996XT and uses the standard two prong AC adapter. The AC adapter is plugged into a Tripp.Lite surge suppressor which is plugged into the wall outlet. I checked the voltage between the ground stake and the ground of a 120 volt electrical outlet and see about 0.4 volts.
As mmckenna suggested, I thought it might be static charge as well but I see the 0.4 volts at all times of the day and night.

 

[profiledescent]

I saw this when I lived in Colorado. I've also seen an arc occur between the center conductor and threaded sleve on a PL259. That was during a storm, with the disconnected coax laying on the floor.

 

[popnokick]

As mmckenna wrote, chances are what you are experiencing is coming from static build up on the antenna. Sounds incredible, I know. But I used to see it all the time in the tower installations I worked with. Try this: disconnect the coax from the radio and bring the tip (center conductor) of the coax slowly close to a grounded surface, like the ground connection in your shack or a cold water pipe. You should see / hear a spark jump. Even more so if it's a little dark. I used to be able to do it every time. Wait a few minutes, and you'll be able to get another discharge spark from the static buildup to jump the gap.
Pretty scary, huh? Most people are amazed when I show them this. Puts a whole new wrinkle on grounding and protection from static buildup. Try it, and write to us here what happened.

 

[LtDoc]

You also get the same sort of shock from skidding your feet across carpet and touching something metal, or another person. That spark isn't exactly something to be afraid of. Cautious sure. The likelihood of it damaging your radio is not exactly that great. Use that voltmeter connected to the PL-259 on the end of your feed line through a diode. There can be some fairly high voltages present depending on weather or wind.
- 'Doc

 

[WA0CBW]

This may not be from static buildup. Once dissipated it should take some time for the static to build up again. If the shield is grounded correctly and you still get a shock I would be looking at your electrical system grounding. I would suspect that your main electrical entrance ground is defective or ALL your external grounds (including ground rods) are not connected to your main electrical ground. It is possible that the hot and neutral could be reversed. You could have an incorrect bond in a sub-panel. You may have a problem with either of your scanners power supply. If you are not comfortable with trouble shooting it yourself please have an electrical contractor check it out.
BB

 

[wtp]

ahhh no

if you could feel a .4 volt difference something else is going on. a 9 volt battery could kill you (kidding)
if you feel it, you are a high state of resistance it reads .4 when you conduct the voltage to ground through the meter
in other words it could be 50+ volts with a mega ohm resistor connected (you)
but the meter conducts the higher voltage to ground and .4 is the stable voltage

 

[hd625b]

I got a grounding block and grounded the shield of the coax to the steel stake that is driven into the ground but that did not eliminate the .4 VAC. I tried to move the coax cable close to the house ground but got no spark so I don't think static is the problem. The mast and grounding block are not connected to the house ground only to the steel stake. I checked the shield of the coax to the house ground and there was no voltage. When I checked the center pin of the coax to the house ground the DVM showed about 58 Volt AC. Something tells me, 58 volts is not a good thing.
Should I run another ground wire from the mast to the house ground or a cold water pipe?
Could I just run a ground wire from the radio to the house ground?
Thanks.

 

[popnokick]

Whoa! 58 VAC is NOT static electricity! I take back everything I wrote previously about the cause being static, and am now in agreement with WA0CBW. You need to check everything suggested by WA0CBW. This is something bad waiting to get worse in the correct conditions. Get your multimeter out and start disconnecting coax and measuring point to point and end to end to see if you can get it to stop showing 58 VAC at the center conductor of your coax. Better yet... get help from an electrical contractor as WA0CBW suggested.

 

[rescue161]

This may not be from static buildup. Once dissipated it should take some time for the static to build up again. If the shield is grounded correctly and you still get a shock I would be looking at your electrical system grounding. I would suspect that your main electrical entrance ground is defective or ALL your external grounds (including ground rods) are not connected to your main electrical ground. It is possible that the hot and neutral could be reversed. You could have an incorrect bond in a sub-panel. You may have a problem with either of your scanners power supply. If you are not comfortable with trouble shooting it yourself please have an electrical contractor check it out.
BB

^^^^
This (in bold).

I have lived in houses where one or more outlets were wired backwards, i.e., hot/neutral wires reversed. It is not really a problem as long as a 2-prong plug is plugged into the outlet. But if a 3-prong plug is used, then the hot is fed to ground via whatever is plugged into that outlet. When we moved into our current house, I went through with a meter and checked each outlet and found one with reversed polarity. I promptly corrected that outlet.

 

[fourthhorseman]

Glad to hear you got that one before it got YOU!..

 

[ramal121]

I had a remote radio site that was AC powered by a cable run up the hill. It had hot, neutral and ground but no ground rod at the site so basically it was running on a long "extension cord". Whenever I disconnected the the coax, held it in my hand and then grabbed the equipment rack I got bit real well. The antennas were on a metal mast with the bottom resting or slightly into the ground. I had to complain to the electricians something was really wrong up at the site. Believe they found the power line, after a number of years, was being covered by dirt and duff as it was laying on the ground and was suffering from water intrusion creating a leakage current into the ground. By disconnecting the coax I was the middleman between the leakage and the ground in the cable.

The point I want to make is you can have a separate ground system for your antenna and it will probably be perfectly safe and suffer no ill effects, but when a problem arises with the AC power and the gremlins emerge, that is when you put yourself at risk. Bonding your antenna ground to the house ground is the prudent thing to do. And since you are experiencing shocks at this point I'd say there is an abnormality that exists somewhere and the counsel of a qualified electrician is advised.

 

[LIScanner101]

Isn't this a normal phenomenon? I say this because a long piece of coax is like a very long capacitor, and a capacitor laying around is likely to develop a charge.

I work with high voltage power supplies, and I know that a long x-ray cable which has its ends unterminated on both ends will pick up enough of a static charge to knock you on your ***.

 

[Thayne]

It could be a grounding problem either with your house or another fairly close electrical service. I have seen a neutral from the utility to a structure sometimes be loose or disconnected and this can cause many weird things as the earth is the only return path for whatever load is should be going thru the bad neutral. All unbalanced current between the 2 hot phases will show up trying to use the earth to return---
Have the electric supplier check around both ends of service drops or laterals.

 

[prcguy]

This is one of the reasons the electrical code (NEC) requires you bond all ground rods to the house electrical ground with #6 or larger copper wire.

The reason for the difference in potential can be from your antenna ground or ground rod being far away from your house ground but maybe closer to your neighbors ground that is fed from a different phase or feed in your neighborhood. I had 90 volts once between a lone ground rod and my house ground and it really hurt when disconnecting the particular antenna on that ground. Read up on NEC article 810 and proper antenna grounding to avoid the problem.
prcguy

 

[Randdon]

The one thing that got my attention in all this was it was the center pin that had the charge on it! That means the voltage is coming through one of the pieces of equipment! Although in the right conditions, it is quite possible to have a continual potential at the center pin if it were directly connected to the antenna. It does happen, tower technicians are quite aware of this phenomenon. (Where they are using coax to feed an antenna) Take your shop vac, take out the filter, reassemble the vac and turn it on and hold the filter in one hand while using the vac hose to try and clean the shmutz of the filter ... you'll be dropping the filter in a hurry, or if you a sadist, you'll keep holding on and be repeatedly zapped!

Although in this case it sounds like the neutral or more correct the identified conductor sounds like it is not grounded at the panel and so everything is at a raise potential to ground being there is no return path to ground in the panel (Ground wire to the system ground broken or if grounded to a water service and the incoming service was changed over to plastic!?).

IF a wire was taken from the house system ground in the panel and then touched to the tower, I am betting there would be a fair spark and a bang!