For a long wire it is all about length, it can be folded like an old clothes line. I remember a guy in Alabama years ago who had two power poles ~100' apart with crossarms in his back yard that had 4 lines between them. It was a folded long line. each of the crossing wires had ~2 feet between them so the entire crossarms were 6-8' long. Use a couple of boards with holes drilled to string insulators on.

 

No, not the PC, not the monitors, not a TV nearby, not a close by SONOS player, not any equipment in the workshop, not my UPS, not the WiFi extender I have here, not my cell phone, not the LED lighting in the workshop or any of my LED desk lamps.

Hold on...

 

OK I'm getting somewhere.

I have an LED lamp on my desk, it is powered through a small adapter.

If I touch the metal on the lamp the noise on the radio ceases, even when set to a high volume, the noise ceases and I can hear a faint - as expected - background hiss and a relatively minor 60Hz hum.

If I unplug (remove the power connector) for the lamp the noise remains whether the lamp is touched or not.

If I switch off the lamp (leaving it connected) the noise remains.

Only when the lamp is illuminated and I touch the metal (the limb connected to the base) does the noise cease.

The lamp is the one on the right:



Obviously it is not grounded.

This is very odd indeed...

 

Hello,

Perhaps ths ARRL book might interest you:
https://archive.org/details/fea_ARRL_Basic_Antennas

Bertus

 

I think this is a high-ish frequency signal on the house power line, radiating from the house wiring.

Somehow, me touching that lamp is diverting the noise to the physical ground, I'm acting as a "bypass" in some way.

But it makes no sense, the lamp is powered by a small AC adapter no doubt with a transformer in it so it doesn't really make sense to me.

I wonder if putting a ground wire to the lamp will do the same thing...

Nope, touching a wire from the house electrical ground to the lamp metal has no effect, the noise remains, only when I stand there and touch the lamp does the noise cease...

 

LED lamps are powered typically from SMPSs which are inherently noisy. I have a small pocket sized battery powered oscilliscope and put a 18" piece of wire in it's probe and wand around for noise. It can be found in some surprising places. I like to use my dedicated external ground rod for radio chassis grounding to help with noise reduction. Don't think the R75 has that option but wouldn't hurt to check.

 

Even more weird, whether touching the lamp kills the noise has been found to depend on the frequency the radio is tuned to!

If I set it around 5MHz and touch the lamp, the noise goes, if I set it to 10MHz and touch the lamp the noise seems unaffected.

The sound of the noise by the way is the same, it has a characteristic buzzing that I guess is several hundred Hz.

Man this is odd.

 

Oh, and the noise on my bench is from the SMPS for the signal generator. Also uses a wall wart supply and noisy even when turned off. Apparently OFF doesn't mean the SMPS. All plastic case with no shielding or grounding.

 

If I could understand what my body touching the lamp is doing, I'd be happier, but it makes no sense.

 

By the way the PSU on the radio itself is the original ICOM PSU.

 

Oh, and the noise on my bench is from the SMPS for the signal generator. Also uses a wall wart supply and noisy even when turned off. Apparently OFF doesn't mean the SMPS. All plastic case with no shielding or grounding.

So if all such devices were connected via an isolation transformer, you'd be good? would the transformer basically filter out the HF?

This electrical noise will make it impossible to do any DXing, so my antenna project is now on hold...

 

Perhaps its time to put the scope right onto the main 120v supply, get some idea of what is actually present on the house wiring...

 

If I could understand what my body touching the lamp is doing, I'd be happier, but it makes no sense.

Can you touch anything else, "earth ground, etc." and stop the noise also?

 

SMPS noise is not only line noise. It's also EMI released to the surrounding atmosphere. That's why wanding with just a short piece of wire attached to the probe is picking it up out of the air. Computers generate a good bit of EMI, but they are designed to contain it by shielding and not allow it to cause interference.

 

I still suggest switching off circuits one at a time, for just a few seconds each, to find the source of the noise. That is a chep and easy tet to do, and knowing what is making the noise is a handy part of the elimination process. There may be quite a few switcher supplies making noise,
And touching tha lamp may be just changing the noise generator frequency a bit.

 

I can't easily switch off the circuit breakers, this house has a a huge number of them and on two separate panels, always been a drag dealing with that because their labels are not really up to date or clear.

My touching the lamp also silences the noise if I do it capacitively, if I wrap paper around the metal and then my hand around the paper, it silences the noise too.

But if I touch the electrical ground to the lamp - no effect - noise just continues.

I think the receiver is picking this up via its antenna, if I disconnect the antenna there's no noise (or signals of course) but if this was carried into the radio over the supply I'd expect to hear it, besides the power supply for the radio is transformer based so likely filters out any high frequency noise.

Therefore I think the "house" or rather the network of electrical cabling within the walls, is radiating the noise.

Since the metal arm of the lamp is insulated from the DC supply feeding it, my touching it is just acting as some kind of route to the physical ground via a capacitive linkage.

If I wrap my hand around the wire carrying the DC to the lamp, there's no effect on the noise either.

The noise might be generated outside of my property in which case I can't ever shut it off.

I could shut off the entire house supply and see if the noise stops but I'd need a batter to power the radio and could still not be sure that the source wasn't outside my house, even if it were to cease it could be due me disconnecting the house from the street supply.

What is the best way for me to use the scope to try and see this noise on the electrical supply?

Set probes to X10, use AC coupling and just put the scope across the 120v wires?

 

Can you touch anything else, "earth ground, etc." and stop the noise also?

I'll try to see, touching the electrical ground with my body has no effect on the noise and attaching the electrical ground to the receiver chassis or antenna ground connection also has no effect, none, not even a slight change. This makes me wonder if the noise is present on the house ground wire too.

 

I can't easily switch off the circuit breakers, this house has a a huge number of them and on two separate panels, always been a drag dealing with that because their labels are not really up to date or clear.

My touching the lamp also silences the noise if I do it capacitively, if I wrap paper around the metal and then my hand around the paper, it silences the noise too.

…

Since the metal arm of the lamp is insulated from the DC supply feeding it, my touching it is just acting as some kind of route to the physical ground via a capacitive linkage.

Try connecting it to ground through a capacitor then.

Bob

 

You do NOT need to connect to the O'scope. Simply put a short ~12-18" piece of wire on the probe and it will pick up the EMI being radiated. The radio's power supply is adequately filtered for line noise. The type of noise you are describing is the reason to use coax for the incoming antenna feed to the radio.

 

OK I was slightly mistaken, if I do touch the ground wire to the lamp (a screw on the arm rather than just touching the arm) it has the same effect as my hand. Perhaps there's some kind of sheen or varnish on the metal that's misled me into thinking I was touching metal.

The when I go to the radio the noise has mostly gone but does reappear from time to time as I tune across the HF frequencies, however it is much better than it was and is all but absent except for a few places.

I also attached a scope input straight to the house ground and of course there's all kinds of noise in the mV range and clear trace of 60 Hz too, but also a huge mix of other frequencies.

Clearly there's a "tone" of like 500 Hz or so and that's being picked up as (AM) modulation across various frequencies.

My wife bought a handful of small LED based night lights for XMas, I wonder if one of these is the culprit, they plug right into the power outlets...