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Author Topic: Hands on speaker lowers RFI??  (Read 5400 times)
KF5LWX
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« on: June 18, 2011, 12:37:54 PM »

An odd discovery I made today trying to limit the RFI in my shack has perplexed me. Hardware in question is an Icom 756 Pro III with an external JRC NVA-319 speaker. On a low 40 meter band with no activity, my constant static floor was about 2 db. When I touch the speaker (standard metal casing) with 2 hands, I can LOWER the noise floor according to my ears and the S meter by 1 full point. Take my hands off and it rises back to 2 db. Unplugging the audio cable from the back of the 756 Pro III to use the internal speaker, the S meter actually RISES +1 db. Some bands appear unaffected; but 20 meters also shows the same thing.

Any ideas as to what could be going on here?
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KF5LWX
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« Reply #1 on: June 18, 2011, 01:58:48 PM »

...As a follow up, the speaker audio is plugged straight into the Pro III, but the 2nd speaker input goes to a Timewave DSP-599zx (which is turned off in this case). Still, I noticed that if I put my hands on the Timewave, the S meter goes down a point just like the speaker is doing. If I remove the Timewave completely from the chain, hands on the speaker yields a 0.5 S meter drop instead of 1 point. If I unplug the speaker from the Pro III with the Timewave out of the chain, then the S meter remains unchanged instead of rising a point like it did with the Timewave in the mix. So it's looking like the Timewave is playing some role in this but not 100% since I can still affect the S meter by touching the speaker chassis even then without the Timewave.
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N0SYA
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« Reply #2 on: June 18, 2011, 06:03:56 PM »

This sounds like a ground loop issue. This is where grounds that should be the same potential are not, giving rise to rf noise voltages. It may be that the dsp box and/or power source is rf noisy. Also, this might be indicative of connections acting as antennas when they should not. try some ferrite cores on the connecting cables and see if that helps. Ferrite cores on all connections are seldom a bad idea.

http://en.wikipedia.org/wiki/Ground_loop_(electricity)
http://www.aa5au.com/gettingstarted/rtty_rfi.htm
http://www.audiosystemsgroup.com/RFI-Ham.pdf
« Last Edit: June 18, 2011, 06:16:36 PM by N0SYA » Logged

If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
KF5LWX
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« Reply #3 on: June 18, 2011, 09:32:08 PM »

Thanks, I have some torroids on cables, but not on the ones leading to the devices in question so I'll be ordering some more for sure.

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K1CJS
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« Reply #4 on: June 19, 2011, 08:03:49 AM »

Rather than a ground loop, it sounds like a lack of adequate grounding of the equipment in your station or your antenna system.  The lack of a ground is being offset by your touching the speaker case--or the speaker leads being connected to the rig.

Is your antenna system a balance system?  If not, reflected power may be the cause of all your woes.
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KF5LWX
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« Reply #5 on: June 19, 2011, 08:02:23 PM »

Indeed it appears inadequate grounding may have been the case. One of my projects this weekend was bonding my ground rod to the house mains which wasn't in place at the time I did the test (just the ground rod). A new test today setting my hands on the speaker chassis shows no discernible difference.

Now on to the next adventure, like why my noise floor has risen about 1.5 dB AFTER bonding the ground rod. It never ends does it.
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K5LXP
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« Reply #6 on: June 20, 2011, 07:33:32 AM »


> The lack of a ground is being offset by your touching the speaker case

What it tells me is his chassis ground is part of his antenna.  Adding additional grounds may improve the symptom, but in effect he'd also be grounding part of his antenna.


Quote
> If not, reflected power may be the cause of all your woes.

I think you mean common mode.  Antenna match/reflection shouldn't have any bearing on noise ingress.

I see two possible points of improvement - a combination of better antenna isolation, and minimizing the noise source.


Mark K5LXP
Albuquerque, NM

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K1CJS
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« Reply #7 on: June 20, 2011, 01:34:18 PM »

What it tells me is his chassis ground is part of his antenna.  Adding additional grounds may improve the symptom, but in effect he'd also be grounding part of his antenna.

Usually that is the case, depending on the antenna system.  The SO-239 antenna connector is usually mounted on the chassis and the coax should be grounded anyway.  A balanced antenna system should be connected to the coax through a balun, and the coax shield should still be grounded.

Quote
Quote
> If not, reflected power may be the cause of all your woes.

I think you mean common mode.  Antenna match/reflection shouldn't have any bearing on noise ingress.

Reflected power results in common mode currents.  Different name for the same effect--BUT--yours is the more precise term.  You can try to stop the common mode currents from getting into the shack with a balun, but sometimes that will not solve the noise problem.

Excessive noise can result from the common mode current as you pointed out.  But I see it as an inadequate ground.
_______________

Theo, welcome to the wonderful world of ham radio experimentation.  You started out right by keeping the ground conductor to the ground rod short, but then, by bonding the shack ground rod to the house electrical ground rod, you've given yourself that better ground--and you've also made the cardinal mistake of too long a ground cable from your station to the good ground--making the ground cable an antenna.  That may well be the cause of increased noise reception. 

Try a simple test.  If you can pull your shack ground rod out of the ground using nothing but your hands, you need a better ground.  Get some plain table salt, dig a hole around the ground rod and pour the salt in.  Pour water into the hole around the rod and then tamp the dirt down with a hammer and a wood board a couple inches wide.  Be careful you don't flood the area or the salt may well kill the grass there.  Continue to put water into the hole until it is saturated.  Do this a few times (the salt and water treatment) and it may improve the grounding of the rod, then you'll be all set.  A newly driven ground rod may not be in good contact with the earth around it.  Water and tamping will make the contact better, and time will make it even better than that.

If that isn't possible, try getting a longer ground rod (a sectional one) and drive that in, or getting a couple more ground rods driven in and bonded to the first.  Eight foot ground rods spread out sixteen feet from each other will give you a better ground system.

You can troubleshoot the cause and try remedies, but the only real way to get rid of the problem is by trial and error.



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K5LXP
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« Reply #8 on: June 20, 2011, 02:15:18 PM »

Reflected power results in common mode currents.

I can't seem to find a reference that supports this.  Do you know of one?


Mark K5LXP
Albuquerque, NM
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KF5LWX
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« Reply #9 on: June 20, 2011, 03:15:17 PM »

Well I do think that bonding the antenna ground rod to the mains with a long cable has caused the increased noise, to the tune of 3 dB on 40 meters when I checked this morning and 1.5 dB on 80 meters. At 20 meters and above I don't hear a change.

I live in the NM high desert so the soil is very dry and an insufficient ground rod could be suspect. Are you suggesting that additional ground rods connected to my one could actually lower the added noise I'm currently getting, even with a long bonding cable to the mains in place?
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K1CJS
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« Reply #10 on: June 20, 2011, 05:18:54 PM »

If you increase the conductivity of the soil around the ground rod, you increase the effectiveness of the ground at that rod.  But since you indicate that you've got dry soil there, maybe the increasing the length of that ground rod may be more helpful.

Then, with the increased ground efficiency there, the bonding cable between the two becomes a safety connection to equalize the ground potential between the rods than an actual path for the grounding of your station.

Additional ground rods may help, then again they may not.  The real trick is to get the ground rod down into moist, conductive soil, or as close to it as possible.
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K5LXP
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« Reply #11 on: June 21, 2011, 02:20:37 PM »

I think the discussion of ground rods is a distraction.  Ground rods are part of the AC safety and surge protection system.  They're not a black hole in which all energy connected to them magically dissipates, especially RF.  Where do you put the ground rods if this problem existed on the 22nd floor of a high rise?  Or an airplane?  The two key issues with any RFI problem are the noise source, and an ingress path.  Sometimes you get lucky fixing RFI when connecting grounds to equipment.  You shift high and low impedance nodes around and sometimes mitigate the problem, or move it from one place to another.  A combination of minimizing the noise source (possibly by more effective bonding, shielding, isolation or even replacement of the offender) and improving the isolation of the antenna are the real fixes.

You never did mention what's being used for an antenna.  Consider that if your configuration allows for noise on your chassis ground to be picked up in your receiver, any even miniscule noise source near, or directly connected to, chassis ground will be picked up.


Mark K5LXP
Albuquerque, NM
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KF5LWX
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« Reply #12 on: June 21, 2011, 05:01:56 PM »

Antenna is an Alpha Delta DX-SWL (60') sloper connected with 100' of RG8X. My noise floor is reasonably quiet and I really don't have an RF noise problem until the ground rod (which is bonded to the house mains) is connected also to the shack gear by way of the ground pin on the power supply. This was suggested by someone else on this forum, but in theory it would seem that creates a ground loop. Here's a diagram of my situation and what was suggested: http://www.darkatmospheres.com/temp/Layout_b.gif

...and this break in the connection between ground rod and equipment AC ground is what relieves the RFI: http://www.darkatmospheres.com/temp/Layout_a.gif

The question is, is the method in the 2nd chart adequate and/or correct from a safety perspective?
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K1CJS
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« Reply #13 on: June 23, 2011, 07:41:38 AM »

If you bonded your electrical ground rod to your shack ground rod, you don't have a need to connect the AC ground to your shack ground.  THAT is what causes the ground loop--and the increased noise.

If you've improved your shack ground so that the bonding of the shack ground to the electrical ground doesn't increase noise, it isn't necessary AT ALL to connect the AC ground to the shack ground inside the house.
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KF5LWX
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« Reply #14 on: June 23, 2011, 11:48:02 AM »

That's what I wanted to know, because when I DON'T connect the bonded shack ground to the equipment AC then everything sounds just fine.
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