Scary Anomaly

[Eldorado828]

Well, just spent the better part of the day sticking a J mount to the shed and hanging my old Sigma Venom on it with some cheap coax. Result?? (Drum roll...) NONE!!! Exact same noise, same tone, same strength. Everything identical. Now I know it's not my house or my wiring., my radio or any of my equipment.

Are you feeding it with the same coax you were using on the sirio?

 

[brandon7861]

If you have a small air variable, say 40pF, you can make a simple loop with nice nulls.

1/4 inch soft copper tubing from the hardware store, 18" diameter loop, should have an inductance of about 1.25uH. Doing the math on that, you will need about 27pF of capacitance for 27.2MHz, so a 10-40pF air variable would be perfect. Connect it to the loop ends and mount it on a PVC pipe. Add a coupling loop with a diameter of 1/5 the main loop (3.25 inches) and mount it near the main loop, but not touching it. I use the side of the loop opposite the capacitor for the coupling loop. Nulls are perpendicular to the loop plane. Connect to the HT and tune for the most noise then take 2 or 3 bearings from different locations. X marks the spot. Use the nulls to locate far away.

If you are interested in doing this, I will take a picture of the one I made when I get home from work. For added accuracy, you can transmit a test signal from a sig gen and practice locating that first.

Edit: when you get close, put a H-field probe (made from coax with a 1" loop) on the HT and start checking things for the offending noise. You basically take a piece of coax, strip the end, connect the center to the braid, then connect that shorted coax end to the shield a little further back making a 1" loop. Last step is to carefully split the shield in the middle of the loop without cutting the center conductor. It must have a split to let the magnetic field in. The shield will stop E-field signals and you will have to be right on the offending source to detect it. I trace wires through walls this way (with an oscillator driving the outlet neutral to ground - the connection between them in the panel provides a current path to produce the magnetic field needed to trace the wire), just remember the H-field loop has a null too, so the wire you are checking for noise needs to be along the loop plane, not perpendicular to it.

 

[Greg T]

Are you feeding it with the same coax you were using on the sirio?

Nope. Using good LMR-240 for the Sirio, and cheap RG58 for the Sigma. I am looking for change. I also played with a ground wire in different spots to try to get the signal to change. It's always the same. Sirio is 30' to the feed point and about 5 or so feet from the power lines. The Sigma is 8' to the feed point and 20 feet from the power lines. witching back and forth between antennas shows nothing.

If you have a small air variable, say 40pF, you can make a simple loop with nice nulls.

1/4 inch soft copper tubing from the hardware store, 18" diameter loop, should have an inductance of about 1.25uH. Doing the math on that, you will need about 27pF of capacitance for 27.2MHz, so a 10-40pF air variable would be perfect. Connect it to the loop ends and mount it on a PVC pipe. Add a coupling loop with a diameter of 1/5 the main loop (3.25 inches) and mount it near the main loop, but not touching it. I use the side of the loop opposite the capacitor for the coupling loop. Nulls are perpendicular to the loop plane. Connect to the HT and tune for the most noise then take 2 or 3 bearings from different locations. X marks the spot. Use the nulls to locate far away.

If you are interested in doing this, I will take a picture of the one I made when I get home from work. For added accuracy, you can transmit a test signal from a sig gen and practice locating that first.

Edit: when you get close, put a H-field probe (made from coax with a 1" loop) on the HT and start checking things for the offending noise. You basically take a piece of coax, strip the end, connect the center to the braid, then connect that shorted coax end to the shield a little further back making a 1" loop. Last step is to carefully split the shield in the middle of the loop without cutting the center conductor. It must have a split to let the magnetic field in. The shield will stop E-field signals and you will have to be right on the offending source to detect it. I trace wires through walls this way (with an oscillator driving the outlet neutral to ground - the connection between them in the panel provides a current path to produce the magnetic field needed to trace the wire), just remember the H-field loop has a null too, so the wire you are checking for noise needs to be along the loop plane, not perpendicular to it.

That sounds like it may be a way to go. Although, I'll wait til the city rebuilds my pole before I do anything else. I'd love to see a few pics of that setup tho. Appreciate it.

 

[brandon7861]

Sorry, its dark outside... This one was about 21" and 5", but its not that critical. Just resonate the big loop with the capacitor and make the small loop big enough to couple enough power but not so big it spoils the Q of the loop.
View attachment 20250610_015251.mp4

 

[Greg T]

Sorry, its dark outside... This one was about 21" and 5", but its not that critical. Just resonate the big loop with the capacitor and make the small loop big enough to couple enough power but not so big it spoils the Q of the loop.
View attachment 73189

That looks like a good idea! I did make some progress today, tho. A buddy was dropping something off and was chatting with someone as he pulled into my alley. As soon as he pulled in the hissing started and knocked the guy right out of his radio. The further he went south (towards my house) The worse it got. He said when he stopped at my place he couldn't even hear the guy's roger beep. When he left, he turned east, then north and the noise was gone in one block. This is encouraging.
Thanks for the vid. I think I'll begin creating such a monster, even tho some of it is over my head, it'll be a good chance to learn and research.

 

[SuperLid]

That looks like a good idea! I did make some progress today, tho. A buddy was dropping something off and was chatting with someone as he pulled into my alley. As soon as he pulled in the hissing started and knocked the guy right out of his radio. The further he went south (towards my house) The worse it got. He said when he stopped at my place he couldn't even hear the guy's roger beep. When he left, he turned east, then north and the noise was gone in one block. This is encouraging.
Thanks for the vid. I think I'll begin creating such a monster, even tho some of it is over my head, it'll be a good chance to learn and research.

I have my own share of power line RFI. I have used a homebrew loop, 2m Yagi, UHF Yagi, whip, attenuators and a few receivers. This thing here, was a game changer for me.

https://sites.google.com/site/portableflagantenna/

The reason is, this antenna is unidirectional, vs. the loop, that is bi-directional. When you have RFI traveling on the overhead, it can be confusing. The source could be tricky when you are not sure which direction it is coming from. With the flag antenna, you know which way to go. DXE sells a version of it if you don;t want to go the total homebrew route.

73,

SL