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Author Topic: RFI from thunderstorms  (Read 13236 times)
AD5KS
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Posts: 4




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« on: September 27, 2013, 11:36:07 AM »

Ok. This is something I have never experienced before and would like to know what was happening and if I can replicate it at some point:

On my way home from work I had to travel through a particularly strong, low precip thunderstorm. I had a 10M mobile (in this case accidentally in AM mode) on as I always did but with the squelch up. The closer I got to the strongest part of the storm the squelch would break and there was a wide band noise that would increase in volume and frequency until there was a lightening strike.  As soon as the strike would occur there would be the typical static crash on the radio and it would go quiet again. An indeterminate amount of time later the process would start again followed by a strike. The sound is hard to describe in detail but I will give it a try: It started at a low volume and low S-Meter level. The noise was somewhat like audio pink noise. It definitely had a center frequency that would increase and was tracked by the noise on either side. The noise was smooth like what would accompany "snow" on the old analog televisions when they were tuned to a channel that had no broadcast signal on it.  But again that noise had "color" hence my reference to pink noise. I assume that the BFO was not the source of the "pink" component because the radio was in AM mode - albeit unintentionally. Stupid me sold the radio before the next major storm and it never occurred to me to see if I could get it to do that again.  The effect was awesome and a bit spooky as it happened at night. What gives?  How can I get a rig to do that again?

I remember some years ago a demonstration by a physicist using a modified portable AM radio. From what I understood something odd had been done with the IF so that it was VERY wide. Would like to figure out how to do that again as a project for my kids. We get some nasty lightening here in Oklahoma. Can anyone here help?
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AD5KS
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« Reply #1 on: September 27, 2013, 11:43:41 AM »

Forgot to mention that the volume and S-Meter reading would increase along with the audio freq...they tracked together.
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AA4PB
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« Reply #2 on: September 27, 2013, 12:12:27 PM »

It's probably precipitation static caused by the antenna moving through the rain. It starts out low and continues to increase in strength until the voltage builds up enough to arc over to something. Then you hear a "snap" as it discharges and then it all begins again.
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AD5KS
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« Reply #3 on: September 27, 2013, 01:31:30 PM »

Believe it or not I have observed that type of static build up before and this was nothing like that. In that instance the arc occurred in the shack a few times before I decided to stop listening. That one happened in the fall during a steady rain with no lightening. My push up pole ground had become disconnected. The pole was strapped to a wood post for the extra height. I would have expected that the post being wet would drain the charge to ground but the noise and snap didn't return after I fixed the ground.

In the car the noise terminated at the instant there was a lightening stoke. No lightening between buildups. I would have considered another source if it didn't appear to be coupled to the lightening. Not to say it couldn't be something else but it happened a lot for about 20 minutes. Until I got clear of the core of the storm even cloud to cloud would drop the signal level but an arc that got to the ground would drop the noise back below squelch.  Would have fiddled with it but the storm and the odd noise and the fact that I was on a desolate strip of Hwy 33 kept me from thinking about fiddling with the knobs on the radio. 
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WB6DGN
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« Reply #4 on: September 27, 2013, 08:08:19 PM »

Quote
On my way home from work I had to travel through a particularly strong, low precip thunderstorm. I had a 10M mobile (in this case accidentally in AM mode) on as I always did but with the squelch up. The closer I got to the strongest part of the storm the squelch would break and there was a wide band noise that would increase in volume and frequency until there was a lightening strike.  As soon as the strike would occur there would be the typical static crash on the radio and it would go quiet again.

You just discovered the principle of the "StormScope" which has made many pilots feel somewhat safer, and actually done a pretty good job of making that feeling a reality as well, and, in the process,  made 3M even more millions than they already had.  Since avoiding a thunderstorm cell is one of a pilot's primary duties, this device took that "fingerprint" of a thunderstorm that you so adeptly described and integrated it into a rather clever instrument that quite effectively detected a thunder cell not only visually hidden from a pilot but also hidden from his radar by heavy precip. between him and the dangerous cell.  While, obviously, not 100% effective (what is?) it, coupled with good judgement, went a long way toward keeping many aircraft in one piece to fly again.
Just a fascinating bit of trivia.
Tom
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K0ZN
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« Reply #5 on: September 27, 2013, 08:37:01 PM »

Hi,

I have heard that type of thing before on both HF and VHF. I am no expert on the exact mechanics of lightning, but you can pretty easily conclude that you were very NEAR the point that was developing a serious charge. i.e. you were in a high risk situation. I guess it could also be some type of cloud-to-cloud charge and discharge, but I think
that you were just "In the wrong place, at the right time.".....  Clearly, there was a very high electrical potential either around you or very near you.....and the
lightning did what it is supposed to do.....allow the charges to equalize.

73,  K0ZN
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KD0REQ
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« Reply #6 on: September 28, 2013, 11:17:40 AM »

the old trick of jamming an AM radio between stations and when it becomes suffused with static, get off the hill on another band.  weathermen have advised this for outdoor activities for decades.

it is possible to hear "whistlers" if you are way too close, which would be the actual RF emissions of the air as it was energized.
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AD5KS
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« Reply #7 on: September 30, 2013, 03:26:22 PM »

You are correct in stating that I was in the wrong place at the right time.  The lightening was intense and close enough for a good 10-20 mins that I hear it over the road noise and there was very little delay between when the flash and bang.
 
Again, I did see a show where this physicist was using a "modified portable AM radio" (that's what he called it) to hear the static field surrounding a balloon that he rubbed some cloth on.  It was a long, drawn out hiss and worked from several feet away.  Would like to know what he did to that radio.  Could replacing the loop stick with a plate(s) work?  I know it's part of the receiver's front end but I am not interested in preserving broadcast band reception just in hearing the field.  Did find some plans for a very simple FET and LED arrangement but I want to be able to hear it not see it. Could be interesting to set one up and plot noise over the long term. Just to see what is going on around the QTH. Might be helpful in advance warning of storms...could us an Arduino to set off an alarm or some such. Maybe a personal alarm to let you know when someone is sneaking up behind you intent on "zapping" you with some static. Any ideas?
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KB4QAA
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« Reply #8 on: September 30, 2013, 08:48:46 PM »

You described P-static build up, which then dissipated with the lightning discharge.  Lather, rinse, repeat.

I was crossing the continental divide in Wyoming once in a lowering rain storm and had a buildup of static on a CB radio, that ended when a diode in the AGC section blew out.

This phenomenon is well known to aviators.  Bonding the body sections of your car and adding static wicks can help dramatically.    Alan has much info on this at his website, www.k0bg.com
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K1DA
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Posts: 473




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« Reply #9 on: October 31, 2013, 09:11:33 AM »

I've heard it on BDC AM crossing a local (tall) bridge over salt water during thunderstorm activity.  I've also seen the limiter current in my vhf radio dance around with noise which would not open the squelch. 
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