Television antenna voltage

If I read your post correctly, you are measuring 36V from the un-connected coax center conductor to ground through a suspended ceiling grid. If this is the case, whether the TV is on of off should not be a factor. I suspect that you have a pre-amp located on the antenna and the power module, located somewhere in the coax run and plugged into a walll outlet, is installed backwards. The 36V (open circuit voltage) should be going to the pre-amp not the TV. Also, what kind of accident did you have that made you notice the situation?

I guess it is hard to explain the situation, since I got the same response from a colleague. Here is the situation...I have a TV, which is plugged in to the household electrical suppply. Attached to the TV is a section of coax cable, let's call it end "T". The other end of the coax cable is not connected to anything, just hanging free in space, let's call it end "F". I was in my basement, standing on a ladder with my head between the ceiling tile gridwork, in which is mounted fluorescent light fixtures that are grounded to the household wiring system, trying to connect end "F" to a splitter mounted between the first floor joists. I felt a tingle on my cheek when I bumped against the ceiling grid, and so I began to disconnect all of the various pieces to check which of them was carrying the current. I found that the voltage could be measured when I tested between end "F" of the coax calbe and the ceiling grid, and also between end "F" of the coax cable and the grounding lug of my receptacles. I have exactly the same situation with another TV that I was going to connect to the splitter. The antenna itself is mounted outside on my chimney, and there is no voltage detectable between the outside antenna and ground. There are no pre-amps or other electrical devices in the system.

You have read my post correctly, but there is no pre-amp in the system. While I have not actually checked it (I will tonight), I assume that the voltage would be detectable at the antenna jack on the back of the TV.

Have you bothered to check where the ON/OFF switch is wired. It might be turning off the output instead of input power and the antenna might be in parallel or in series before the switch. Just a guess.

Thanks, but I have no idea what you are talking about. There is no on/off switch anywhere.

It appears that the voltage is being induced into the ungrounded ceiling grid, possibly because of magnetic fields generated by flourescent light inductors. The voltage is probably much higher than 36 volts because the voltmeter is a relatively low impedance in this situation. Connect a grounding wire to the ceiling grid through a 1000 ohm resistor. Measure the voltage at the ceiling grid side of the resistor before connecting the wire to electrical ground. The voltage should be 36 volts with the wire ungrounded and zero when the wire is grounded. If you measure a voltage on the ceiling grid side of the resistor when the wire is grounded, you might need an electrician. My guess is that the voltage will be there when the wire is ungrounded, and it will not be there when the wire is grounded.

Thanks, but we have gotten way off track here. The issue is a voltage between the TV antenna cable (disconnected from everything else) and ground. The mention of the grid was just how I discovered it.

It's possible that the TV antenna itself is not grounded. If not, use a good appropriate clamp and a minimum solid #16 copper wire and run to a nearby water pipe or house electrical ground.

I give! Apparently I have not been able to describe the problem adequately. Please, before you reply, go to the back of your own TV and measure to see if the TV coax connection, when DISCONNECTED FROM ALL EXTERNAL CONNECTIONS has a measurable voltage to ground.

No. But lets go back to your original. Do you have 36V from the outside to the center pin of the coaxial connector? Or is it from either to the other reference (ground)?

I have rechecked the TV. There is 36 volts between the coax connector on the back of the TV and ground, both the ground lug on my houshold receptacles, and to a grounded water line. This same voltage exists on the external portion of the connector an on the inner core of the connector.

A wire or cable suspended in space and not connected to anything will not normally have a voltage on it. There are many ways to make a voltage appear on the isolated cable, but your situation is probably a physical contact somewhere. Maybe you can connect the meter to the F end and start pulling the loose T end away from the TV through the walls or ceiling space until the voltage goes away.

Please recall that a GOOD ISOLATED conductor may be charged by ELECTROSTATICS through space LIGHTNING Discharges
--- OR ---
Radiations around the environment due to devices like Radio-Transmitters in Mobile-Phone etc etc.

Once charged, it remains long after it due to Hi-Insulation of Co-Ax.

2nd-ly
If you are using DMM or Hi-Sensitivity Analogue Multi-meter it may show this voltage in highly Radiation-Polluted environment [even higher voltage] on Flying-leads of DMM; or Analogue Multi-meter.

You may feel it if you leave your Multi-meter in such environment while working.

Ground the antenna mast as I suggested above. The problem will go away.

I agree, and was intending to do this anyway as a partial protection from lightning strikes. I am still curious as to whether TVs are supposed to produce a 36 volt differential as a matter of course, and why.

It's possible that the tv could be the problem. There are usually two capacitors from each side of the power supply AC line input. If either is leaky (in this case, it would be the "high or L side), one would see about 36 volts. Enough to tingle you.

Depending on whether the tv plug is polarized, you might try reversing its connection at the outlet. Problem goes away? It's the tv.

I should mention: if you have a vcr/dvd/cable box, disconnect them first to isolate the problem. Either of the other may also be the problem.

TVs are not grounded in my experience (I have forgotten the reason why!), they have a two wire cable only, therefore it is possible, due to poor design or age (or both) that the aerial socket gets a floating voltage on it relative to ground.

The only way I feel to get rid of this is either to place an isolation transformer between the TV and the supply or get the set repaired/replaced.....but even some new TVs show this from new.....

I suspect that if you measured the (milli-) amps, there would be almost nothing there, micro amps maybe..... (HOPEFULLY!!!)

tv's are not usally grounded for interference purposes.....if you get a new plasma and have gohst line take out the ground and watch them go away:)

I have also seen this phenomenon on coaxial in my 1950's era house. I found that there was a noticeable voltage between the coaxial cable and ground. I could feel it so it as a pulsing DC voltage through my finger tips and measured about 40vdc between ground and the shield. It was strange because It was not the coaxial that came from the cable TV into the house that generated the voltage. When I eliminated all the splitters, I found that the V source was a wire running down to a coaxial TV wall outlet. I grounded the splitter, as a stop gap measure, and eventually resolved the issue by replacing the coaxial cable. Induced voltage on an older style of coaxial cable, possibly?

Thanks for confirming the condition. As you can see, some of the replies do not really connect with event. Regardless, I have confirmed that the condition exists at the coax connector on the back of the TV when there are no connections to coax cables anywhere. This is the same for new and newer TVs in the house. The coax cables that run through the house are brand new, as I ran them myself this summer.

Were you replying to post #17 or #18?

Post # 18, like it says in the upper right hand corner of the post.

I know which post you answered to, its clearly marked in the top RH corner!

It was just that that post is a completely different problem, to the actual problem you are trying to identify, (that was induced voltage on wiring, not voltage on TV aerial connection!) therefore I thought that you had made a mistake - which was the reason for my question in the first place!

But if you are happy, then so am I !!

It did appear that the original question referred to a floating coax cable not connected to anything, which was confusing. It now appears that Bill ML #16 most likely has identified the problem and solution.

...I don't think so, read post #2 where it says:- " I have exactly the same situation with another TV that I was going to connect to the splitter."

I have personally never seen a TV with a 3 wire mains cable, (it may have a 3 pin plug if you live in certain areas, the UK for example, but that is because of mechanical interlocks in the sockets only!) that means that all the voltages inside a set are floating and not tied or referenced to ground!! So I would not be surprised if there is a voltage on the aerial pin.....

I am sure that it must be legally within certain limits and the amount of current is probably not even milliamps.....

It would be nice to have a fully qualified TV engineer on CR4 today.....