VLF radio noise from meteors

[andrew_t1000]

I recently read an article on SDC about low frequency radio emissions from meteors.

If I was going to build a purpose built receiver/antenna system, what frequency/s should I concentrate on?

I already have a lightning monitor, 2, 1 metre diameter loop antennas mounted at 90 degrees to each other. I am using a differential amplifier, then running the output into a detector. I can get a pretty good indication of distance and direction.
The loops are made from 50 way ribbon cable joined so that they are a 50 turn, 200 metre loop. I also have a simple long wire antenna using the same amplifier/detector setup.
It's pretty sensitive, whilst listening on a shortwave receiver tuned to an empty bit band space, I can hear the crackle and pop of lightning and watch the output from my crossed loops on a CRO.

Originally the lightning detector was built as a reference for a decimetre radio telescope for making Jupiter observations.
My reasoning was to make sure the lightning I was hearing whilst Jupiter was in the FOV of my antenna WAS from Jupiter and not terrestrial.
On that note, has anyone heard of decimetre emissions from Venus?
On a couple of occasions I have heard what sounds a lot like Jovian lightning while Venus was in the FOV of my antenna.

 

[billslugg]

There are a number of problems here.

It does not seem that meteors emit significant VLF radio waves. This article in SDC in 1999 refers to a balloon payload:

"NASA scientists and amateur radio enthusiasts at Marshall Space Flight Center in Alabama launched a helium balloon early Thursday morning. The payload included a video camera and a very low frequency radio transmitter."

"Lightning strikes within about 1,000 miles (1,609 kilometers) were picked by the VLF radio and relayed to the ground. Researchers also sought to pick up natural audio emissions from the meteors."

I believe there is an error in the article where it says "very low frequency radio transmitter." In order for VLF emissions from lightning to be picked up by the payload it would have to be a "receiver" not a "transmitter". Using a VLF transmitter to link to ground would be a very poor choice as it would limit the bit rate severely. I believe the VLF receiver was targeting emissions from lightning which is know to emit copious emissions in the VLF band,

Complicating the analysis is the mention of an attempt to detect audio emissions from meteors. Combining the items above I think you have come to the erroneous conclusion that meteors emit VLF radio waves.

That said, I have found several references to radio emissions from meteors. I can't find the url's right now, but I read of a study done in the 50's where an 18 foot dish was pointed straight up and receivers tuned to roughly 20, 250 and 450 mHz. It is necessary to point straight up so as to avoid the reception of reflected signals. Their conclusion was that some energy is emitted but that it is only slightly above the noise level.

 

[neilsox]

Definitions tend to blur in recent years, but HF was 3 to 30 megahertz, MF was 300 to 3000 kilohertz, LF was 30 kilohertz to 300 kilohertz, VLF was 3 to 30 kilohertz with a wave length in miles. An 18 foot dish is too small to be directional at any of these frequencies. Hundred year ago radio operators, with very long receiving antennas, reported an occasional whistling sound, which was thought to be caused by meteors.
A simple 5 tube broadcast band radio = MF = medium wave radio can be altered by putting a 5000 micro fared capacitor across the the input variable capacitor, and a 1500 microfarad capacitor across the local oscillator variable capacitor. The radio now tunes about 200 kilohertz plus or minus 10% instead of the wider range it tuned before. Bandpass is still about 20 kilohertz. A long antenna is needed. Harmonics of the local oscillator may still produce the 455 kilohertz IF = intermediate frequency from near by AM stations. There were no MF stations where I did this = Ankara, Turkey. I don't know if larger capacitors will allow operation at still lower frequencies, or if this modification is practical with more modern receivers. Neil

 

[billslugg]

neil
The study with the dish I mentioned was done in the multi megahertz range. I don't recall them working down in the LF range. The whistle noises may have been "whistlers" which can be made by the EMF from lightning strikes propagating along earth's magnetic field lines out into space then back down to earth. The "white noise" from the lightning is dispersed and arrives as a descending tone.

 

[neilsox]

Hi Bill: Should Andrew's title then be VHF instead of VLF = very low frequency? Perhaps the whistlers were more often thought to be lightning instead of meteors. The descending tone may have been a heterodyne which the regenerative receiver circuit tended to produce, with any signal that changed frequency slightly. These were common before the superheterodyne more than 80 years ago. You can build a one transistor regenerative receiver, for VLF to VHF, and feed the output into a modern high gain audio amplifier. Regenerative may not work for microwave frequencies. Neil

 

[billslugg]

Hi Bill: Should Andrew's title then be VHF instead of VLF = very low frequency?
I have found references to studies that have found EMF from meteors from the VLF all the way up to VHF. I takes an awfully bright one to register though. About visual magnitude -10.

Perhaps the whistlers were more often thought to be lightning instead of meteors. Lightning whistlers are common, I have heard them myself. You need to find a storm system opposite you on the earth. That is, it must have the same magnetic latitude and 180° away in magnetic longitude (+/- 500km).

There are lots of audio files on the net of lightning whistlers. If you have done a lot of short wave listening you will recognize the sound.

From what I have read, you need a fairly sensitive receiver to hear the EMF from a meteor.

 

[andrew_t1000]

I only thought I would need a VLF receiver due to the reports of people hearing meteors without electronics.
The SDC article I was referring to suggested people had heard an effect from dry, frizzy hair (I think that might be the wrong conditioner!)
Like I said, I was going to try and remove lightning as a possible source by listening to my lightning monitor.
Although thinking about it a bit more, lightning is DC to blue light.
But I would have thought meteor trails would "linger" a tad longer.