EDIT: For anybody coming across the same issue, USB was the problem. I was trying to avoid having 12" of radio equipment sticking directly out of my laptop where it could easily be snapped off, so I used a little USB power meter as a short extension cord to let it bend. I don't know if it was the power meter or my laptop, but when I replaced the power meter with a short USB extension cord wrapped a few times around an ferrite E-core, the issue went away. I only got noise at the extreme horizons, and touching the antenna made no change.
I've ordered a USB extension cord with ferrite integrated to use in the future. $8 on Amazon.
I'm working on a V-dipole to pick up 137.5MHz NOAA APT transmissions. I found that when lowering the antenna closer to ground than my design, performance improved. I believe this is because the pavement in front of my house is not a great conductor, so my reflecting ground plane is actually a few inches below the surface.
The thing I can't explain is why I get such a dramatic improvement in performance when I use my finger to touch just the arm of the dipole connected to the center conductor of the coax. The difference is night and day. The surrounding noise in the signal drops to the point that it’s inaudible, but the radio signal is relatively unaffected. Touching the shield conductor does nothing.
I picked up a cheap VNA and was able to determine that touching the antenna does slightly de-tune it, but the impact is the same for both arms. With the configuration I have, the SWR is something like 1.07 before touching it, and it rises to around 1.2 when I do.
I've heard about surface currents that can cause problems, but those only occur on the outside of the shield conductor, and I have a ferrite choke right at the feed point anyway.
What's going on here?
EDIT: to add that I'm using a Nooelec SDR with a SAW filter tuned to NOAA frequencies as well as a terrestrial AM blocker. This is connected to a Framework laptop on battery power. I'm curious if this could be an electrical issue coming from my laptop.
Yeah, I already adjusted the length and tested it with a VNA. I was pretty close to spot on resonance with my original configuration after I lowered the antenna to account for my poor asphalt ground plane.
With the VNA connected, touching either side of the antenna changes the resonant point slightly, but not much. In testing during satellite passes, only touching the center conductor makes a change, so I don’t think it’s the tuning that’s the issue.
I also used the VNA to test my cables and they’re all at 50ohm and maybe -1dB loss.
My next step is going to be pointing to the SDR itself. Nooelec equipment is all case-shielded, so I don’t think it’s coupling environmental noise, but I’m using a removable USB-A port on my Framework laptop. I suspect maybe some noise is coupling in from the digital side.
Next pass I’m going to try a bunch of combinations of different USB cables, ferrites, etc.