Hum when coax is grounded

[tcg]

Hi,

I just installed an ST2 (scantenna) outdoors. It is on a mast 28' off the ground. The mast is grounded with #6 bare solid copper to an 8' copper ground rod. The bottom of the mast is 12' off the ground. It's mounted on a second story deck.

The RG6 coax runs 16' down the mast and is connected to a coax ground block right before it enters the house. That ground is a #8 insulated stranded copper wire. it follows the mast ground and attaches to the same ground rod.

Another 16' RG6 coax cable goes to the scanner, an old RS PRO-2030.

I run an audio cable from the scanner to my computer the hum is introduced at the computer.

If I disconnect the coax ground the hum stops.

Any thoughts?

Matt

 

[prcguy]

Use an isolation transformer between the scanner and sound card. Your antenna ground is at a different potential than the computer and the audio cable shield is the single item trying to equalize the difference. AC current will flow on the audio cable shield and induce hum into the audio.

NEC code requires you bond the antenna ground rod to your house ground with at least #6 copper wire for safety and it may have a positive effect on your hum problem.
prcguy

 

[tcg]

Use an isolation transformer between the scanner and sound card. Your antenna ground is at a different potential than the computer and the audio cable shield is the single item trying to equalize the difference. AC current will flow on the audio cable shield and induce hum into the audio.

NEC code requires you bond the antenna ground rod to your house ground with at least #6 copper wire for safety and it may have a positive effect on your hum problem.
prcguy

Bonding to the service ground was my last step in the install. I will do that and see if that solves the problem.

Matt

 

[smokeyjones666]

My first guess is that the ground potentials between the ground block on the outside of the house and the grounded outlet for your computer are different, my thought is you could test this theory by (temporarily) defeating the ground prong on the computer's power cord (if you have one of those two to three prong adapter dealies lying around that would make it easy and - I stress - temporary.)

Is that 8' ground rod for the grounding block and the antenna bonded to the ground at the electrical service entrance as well? If they arent and the two are close enough together, bonding them may be an easy fix to resolve your hum (as an added bonus it would also bring your installation up to code, plus in the event that lightning should strike your antenna installation and burn down your house that won't be a reason for the insurance company to deny your claim.)

Another cause could be a miswired electrical outlet, which I would probably check for first before bonding any grounds together (assuming they're already not bonded in the first place.)

Otherwise, and this is not my recommended course of action unless you've checked those first two things, a simple ground loop isolator available at any Radio Shack should remedy the symptoms of your problem. Make sure to buy any necessary adapters.

 

[tcg]

My first guess is that the ground potentials between the ground block on the outside of the house and the grounded outlet for your computer are different, my thought is you could test this theory by (temporarily) defeating the ground prong on the computer's power cord (if you have one of those two to three prong adapter dealies lying around that would make it easy and - I stress - temporary.)

Is that 8' ground rod for the grounding block and the antenna bonded to the ground at the electrical service entrance as well? If they arent and the two are close enough together, bonding them may be an easy fix to resolve your hum (as an added bonus it would also bring your installation up to code, plus in the event that lightning should strike your antenna installation and burn down your house that won't be a reason for the insurance company to deny your claim.)

Another cause could be a miswired electrical outlet, which I would probably check for first before bonding any grounds together (assuming they're already not bonded in the first place.)

Otherwise, and this is not my recommended course of action unless you've checked those first two things, a simple ground loop isolator available at any Radio Shack should remedy the symptoms of your problem. Make sure to buy any necessary adapters.

The grounds are not bonded and are about 25' apart.

I checked the outlet it does have a good ground. But the computer that the audio is plugged into is a laptop and plugs in with a two prong plug.

I will go get some wire and bond the two grounds and report the results.

Thanks
Matt

 

[tcg]

Thanks guys.

I bonded the antenna ground to the service ground. They are 37' apart. My little hardware store only had #8 solid bare copper. I will upgrade to #6 at a later date.

Problem solved.

One other question. If the bonding wire is not a trip hazard should it be buried or can I leave it exposed?

Anyone want to explain why this worked. Keep in mind I am not an electrician.

Thanks
Matt

 

[tcg]

I spoke too soon.

The hum is definitely less then it was before bonding to the service ground

It is still present but at a much lower volume. I didn't hear it until I turned up the volume.

Is the ground loop isolator my next step?

Matt

 

[prcguy]

Radio shack used to sell a 1:1 600ohm transformer, put that between the scanner and sound card and your problem should go away.
prcguy

I spoke too soon.

The hum is definitely less then it was before bonding to the service ground

It is still present but at a much lower volume. I didn't hear it until I turned up the volume.

Is the ground loop isolator my next step?

Matt

 

[tcg]

Radio Shack has this

1:1 Isolation Transformer - RadioShack.com

If that is what you are referring to how would I hook it up?

Thanks
Matt

Radio shack used to sell a 1:1 600ohm transformer, put that between the scanner and sound card and your problem should go away.
prcguy

 

[James04TJ]

In addition to increasing the conductor size of your bond between the site and service grounds, also consider installing an additional ground rod (or two) between the different grounds. Polyphaser has a very good writeup targeted at amateur and similar stations on their website.

 

[N_Jay]

Is a test, I would underground everything and just bond the radio to the computer.

You may have a power supply problem in one or the other.

When trouble shooting these issues it is always best to start with the simplest configuration and work from there.

 

[prcguy]

That's the one. You can cut the cable between the scanner and computer card an solder the black wire to the scanner side ground and center to the red, then the yellow/stripe wire to the ground of the computer side and white to center. I could be off on the colors but the transformer will come with a diagram. This will allow low level audio to pass between the scanner and sound card and completely isolate the grounds between the two devices.
prcguy

Radio Shack has this

1:1 Isolation Transformer - RadioShack.com

If that is what you are referring to how would I hook it up?

Thanks
Matt

 

[tcg]

Did that.

That helped a lot. The hum is low enough now as to not be a distraction You have to really listen to hear it.

Thanks
Matt

That's the one. You can cut the cable between the scanner and computer card an solder the black wire to the scanner side ground and center to the red, then the yellow/stripe wire to the ground of the computer side and white to center. I could be off on the colors but the transformer will come with a diagram. This will allow low level audio to pass between the scanner and sound card and completely isolate the grounds between the two devices.
prcguy

 

[tcg]

Ok the hum was still there and even more noticeable when I recorded audio. If I ran the audio through a 60Hz filter with software it would eliminate the hum.

I stopped at our local electronics store and bought one of these.
Xantech 634-00 Ground Breaker
Eliminates hum bars or audible hum
Isolates ground potential differences within an audio/video system

Now the hum is absolutely gone.

Matt

 

[N_Jay]

Now you just have to figure out why the antenna ground has some AC on it.

 

[tcg]

Now you just have to figure out why the antenna ground has some AC on it.

Should I be concerned with that? How would I trouble shoot it?

Matt

 

[N_Jay]

I would start with a meter and see what is there.

Then I would find all the places it is grounded and either lift them, or ground them better and look for a change.

 

[tcg]

I would start with a meter and see what is there.

Then I would find all the places it is grounded and either lift them, or ground them better and look for a change.

How would I use a meter. Am I checking the coax for AC?

What do you mean lift the ground?

The antenna mast is grounded to an 8' copper rod with #6 solid copper wire. The coax is grounded to the same rod with 1 #8 insulated stranded copper wire.

That rod is bonded with a #6 solid copper wire to the service ground. That wire is 37' and is not buried.

Matt

 

[breadtrk]

I would say, yes measure for AC, driven element to ground, braid to ground, ground to ground, radio chassis to ground, tower or mast to ground.

Lifting means un do the ground at each point to see if it help or gets worse.

Bottom line is that if you have to add things to get rid of the hum, you have a problem.

 

[breadtrk]

I might add, probably not......a funky ground could be deadly