Discussion in 'General Technical Questions and Answers' started by N9ZDZ, Mar 30, 2019.
Here is a video I made of what's going on at my QTH.
The voltage you're measuring is most likely because you're running a DC powered rig from a power supply that floats with respect to earth ground. That in and of itself is not a big deal as many double insulated power supplies don't tie the negative lead to earth ground. If your supply is supposed to be grounded then your basement floor (and the metal desk standing on it) is floating with respect to ground which suggests your home is not well grounded. That could certainly be an issue and something I'd look into if indeed your power supply negative output is designed to be tied to earth ground.
Rather than test the voltage from your coax shield to your desk top, have you compared the voltage between a home outlet grounding connection (the third prong on the AC socket) and your basement floor (desk)? It's not great if your basement floor floats well above ground potential though I doubt that's the root of your noise troubles.
In terms of the noise, the best approach would be to temporarily power your rig off a 12V battery and turn off the main breaker to your home. If the noise level drops dramatically then start turning on household branch circuits one at a time until the noise returns. Check every device on that branch circuit especially looking at things like wall wart power supplies, battery chargers, computer supplies and the like as those are common culprits for wideband RFI. I'd also remove the computer from your operating position to see if that leads to lower noise as computers and PC power supplies can also be noisy.
Some devices like laptop computers and some internet router and perhaps alarm devices have internal batteries and may not turn off when you power down the house so it's possible that even with all the breakers off you could still have a noise source inside the home but powering down the house is a good way to start searching for noise generators. If nothing inside the home is generating noise then start looking outdoors for things like lighting sources (e.g. mercury vapor lights or fluorescent lights) or electric fences or other common noise generators.
Well I guess I should of stated that my first test was hooking my radio to a 12v battery then killing the main on the breaker and still have +9 noise level . I have ran radios with built in power supply and had same results . My power supply is a astron RS-20M
The RS-20M has its negative terminal tied to AC service ground so your basement floor (and metal desk) float a bit with respect to ground. You could likely turn off and unplug all the ham gear and measure the same voltage differential between your AC socket ground pin and your basement floor (or metal desk). Some small ground differential isn't unusual but I'd check to make sure your AC service is well grounded at the AC service drop. IOW, make sure your connections to the ground rod is secure and that the wire is well bonded back to the main breaker panel. I doubt this has anything to do with your excessive noise levels, especially if you measured the same high noise levels with your AC service turned off and with the rig locally powered.
I'd start looking outdoors for noise problems and perhaps looking at any battery operated devices like laptop computers near your operating station. Are there local power lines in an alley or near your antennas? How about outdoor lighting or electric fences? What does the noise sound like (couldn't really tell from the video you posted) and does it change as you tune the rig up and down the various HF bands?
I had the power company come out for 3 weeks checking everything and they stated they couldn't find nothing wrong with my house ground or with in a 6 blocks . wounding if the noise is traveling down there lines . and yes my antennas are close about 15 feet away but can get around that. The noise is only the complete bands of 75/80 all other bands are fine even 160 meter is good.
It's certainly possible that RFI is radiated and conducted by the power company distribution lines. In my experience the utility company won't track down the origin of that noise and it could be many blocks or even a mile or more from your home if it is conducted by the lines.
I've had good luck tracking down noisy power poles using an AM aircraft band receiver. I've done quick testing by turning on the AM radio in my car and driving around noting where noise jumps up but a VHF/UHF receiver makes it easier to pinpoint the exact power pole where the noise is generated as RFI tends to fall off quickly with distance when using a VHF/UHF receiver and tends to span wide areas at about the same level when using a broadcast band AM receiver.
Once I've been able to point utility company folks to a specific power pole they tend to fix it quickly but they won't go out and do the sleuthing for me.
Don't overlook other potential noise sources like leaky CATV lines or DSL lines to the home which often run parallel to AC service entrance lines. If your antenna is near those lines that could be a problem as well.
desktop to center lead of antenna ac DMM measurement. Well that certainly is a new one for me!
My plan would be: disconnect the PL259 from radio; if noise goes away, then I'd plan to roam outside and look for the offensive emitter, using VHF or UHF am equipment. Lots of published ideas how.
It's not unusual to see "ghost" or "stray" voltages under some conditions between different conductors or metal objects when using a high impedance voltmeter, which result just from normal AC power system operation. If you put a 10 k Ohm resistor between the antenna and the desk, does the voltage disappear?
(I can measure a ghost voltage of 5 Vac with a 10 megohm meter between one metal frame table and another sitting right next to each other in the shack. It's induction from various equipment sitting on them and their proximity to power cables, etc.)
Does the dipole run parallel to any power lines? That can easily induce a ghost voltage into the ungrounded side of the antenna, in other words, the center conductor of the coax.
What frequency bands are involved?
Please post a clear video or sound file of the noise, using AM mode on the radio.
What, exactly, is your antenna?
When noise peaks in one band and is much lower on the other bands can be an indication that your antenna works best on that band (with all the noise) and isn't as effective on the other bands, so everything will be weaker: Signals, and noise.
If it's a "one band" issue, sometimes using a separate receiving antenna like a small loop which you can steer to null the noise works really well. Very small loops intercept a lot less signal and a lot less noise, but often are followed by a preamplifier to bring the signal levels back up -- and even then, the noise can be reduced because you have something with a sharp steerable null. The sharp, steerable null can also help find the actual noise source (sometimes).
I would say a resounding "YES"
Electrical hash is propagating along the power lines and your antenna is simply picking it up. That's not particularly unusual and is normally expected. So the antenna is too close and the solution is to relocate the antenna farther away from the power lines, or try installing a separate receiving antenna. A separate receiving antenna doesn't have to be very large and they can fit into a small space somewhere else on the property. Even something like a mobile ham stick antenna will do.
One of the easier and effective receiving antennas is simply 22 feet of junk wire wrapped around a 10' tall section of large diameter PVC pipe. Larger diameter the better. Maximize space between the wire coils along the length. Works nicely on 75/80m.