question about filters for VLF/ELF/ULF

[KF5EGM]

okay, so color me ignorant but is there any reason I couldn't use audio filters (which i have because i have some audio synthesizers, and to be honest audio frequency filters are easier to find) for vlf/elf/ulf frequencies? I could do much more complex preselector filtering if so. They're in the same frequency range. Would it work?

 

[RFI-EMI-GUY]

No reason why not. However the noise floor of any active electronic filter may limit sensitivity. The other concern is out of band rejection. Your electronic filter may be great on paper, but MW signals may overload the system. There is no substitute for properly designed lumped copper coils and capacitors.

 

[KF5EGM]

I was actually thinking analogue filters, not digital.

 

[majoco]

Yes, a passive filter could easily be built - the coils would need to be big (relatively!) inductances but a selection of toroids should do that - not much more different than a crossover in a good speaker really! For VLF you really need to get out into the country to get away from the power line noises - but then you get electric fence clicks - well, you do in NZ!

Here's an interesting project.....

https://physicsopenlab.org/2020/05/07/vlf-receiver-for-sid-monitoring/

 

[Patch42]

Based on my experience with LW, I'd suggest starting with a low-pass filter that eliminates MW and above. There aren't terribly difficult to find. Getting rid of that "clutter" improved weak signal reception in LW. The same would likely be true for even lower frequencies.

 

[ratboy]

I've built a couple of pretty effective MW/BC killers over the years in cheap aluminum cases. The caps are the only thing that I had to buy, everything else was in my junk drawers. I'm so old and been messing with this stuff so long I have multiple junk drawers and boxes of stuff.
I wish I could point you to the exact circuit I used, but it's been a long time and they were potted, so I can't tell you what's in them, but I got them online like 15 years ago. I made both low pass and bandpass filters. I needed them just to listen to any lower SW because I had a nearby station that had ground issues and it literally got into all my HF stuff due to the crazy high signals that came from the high tension wires that passed close to the station's antenna, and across the street from me. Just me walking towards any of my radios without an antenna attached was enough to get an S-9 reading on the meters, and if I took my finger and got it within about 6", it would peg it at +60 over. Even my best receivers had country music playing in the background or worse, the beat of the music was like some sort of hash. I called and called to complain, but I had to wait until they redid the antenna setup and the signal dropped to "only" about +50 over on my slinky dipole. The filters took care of that level signal fine, no more "K100" on even my Yaesu FRG-8800.

 

[GB46]

This reminds me of WRSU, the student AM station at Rutgers U., where I studied during the sixties. They were broadcasting directly over the local power lines. The signal overloaded my AM radio, and was not only distorted, but loaded with AC hum, as well. The station's over-the-air range was supposed to be limited to 90 ft., but there were reports from off campus about hearing them, even in distant towns.

When I got my novice ham ticket and started transmitting on 80 meters CW at 15 watts input, the neighbors heard my key clicks over the phone lines.

 

[ratboy]

I had a neighbor with one of the old cordless phones that was on 1.7MHZ and 46MHZ and the base could be heard for many miles if there wasn't another phone close by. One night, I was listening to calls on 46MHZ and when my neighbor hung up, I heard someone ordering pizza from many miles away! I had a map and it was over 10 air miles away. I confirmed it several times, they ordered pizza a lot and a drive by proved it was where I thought it was. (I pass by that area a lot, I didn't make a special trip!)

 

[GB46]

I had a neighbor with one of the old cordless phones that was on 1.7MHZ and 46MHZ and the base could be heard for many miles if there wasn't another phone close by. One night, I was listening to calls on 46MHZ and when my neighbor hung up, I heard someone ordering pizza from many miles away! I had a map and it was over 10 air miles away. I confirmed it several times, they ordered pizza a lot and a drive by proved it was where I thought it was. (I pass by that area a lot, I didn't make a special trip!)

Some people are still using their old cordless phones. A year or so ago I picked up the base unit of one of them on 49 Mhz. A guy in an apartment block just down our street was complaining to his sister on the other end about late night disturbances from his upstairs neighbor. Apparently he has moved since then.

There was also a baby monitor in use on 49 Mhz, and judging from the voices in the background the living room was close by, but too far off mic to make out the conversations.

I used to hear that stuff on my R75, which covers up to 60 Mhz, but lately there's no activity at all in my area between 30 and 60 Mhz.

 

[ratboy]

My one neighbor had her old cordless until about 2017, when it died after a long life. She herself died soon afterwards. Connection?

 

[prcguy]

Is the intent here to use passive audio filters or active? For passive you have to consider the impedance as that is part of what determines the filters other parameters. RF filters are usually 50 ohms and audio filters could be anywhere from 200 ohms to 50k ohms depending on what they were originally designed for. Stick a low pass or band pass filter designed for a 10k ohm or higher system on a 50 ohm line and its not going to work the same.

An active filter will have OP amps inside and they have an operating range of voltages and are intended to be used in a closed system with known audio levels. Connect a really long piece of wire to one and you will probably overload the first OP amp rather quickly with high RF levels, static electricity, etc.