Topic: 1346 On the air

[va3mw]

I have been sitting on my detectors since the last red shipment back last year.

So, this week, I decided to build it.  It took about 3 hours to build/solder up both boards and initial tests were fine.  I was trying to watch a hockey game that went for 6 periods at the same time. 

I then needed an antenna, so I decided to spend about an hour building the 1M loop as it seemed the simplest without ordering more things. 

I used the power supply from one of my Garmin devices, likely from a Forerunner that was sitting in a box of USB power supplies.

I then mounted (placed) the amplifier in a water proof box I got from Canadian Tire for $7 and the controller is just sitting by a window in our boathouse for the time being.  The antenna is hanging in a Cedar tree about 10ft off the ground.

I ran about 50ft of CAT5 (the cheapest I had lying around) from the controller to the amplifier.

Other than that, it pretty much worked right out of the box.  I need to do more reading and watching things, but I'm pretty happy so far.

I'm an RF guy (like most hams) and I wasn't too concerned with my noise floor (just yet).  Let's see how the next week goes.

I think I have an external GPS antenna at home that came off some project and that will allow me to mount the control board in a better location.

Mike va3mw

[Cutty Sark Sailor]

Might keep in mind:  This is not RF.   This is not even 'audio' or 'supersonic'... we have to think a bit differently... lightning strokes are "impulses"... phase and frequency energy... for me, it is a bit more profitable to think of the signals I see as 'audio', but that's not quite correct, of course.  This system, for display, actually constructs a waveform from the 'energies' in the impulse, so that human eyes have some perception of what is actually occurring...  this is 'wideband',... not 'tuned',...  reception.  Likewise, the 'interference' and 'noise' we encounter at some locations has to be approached with an 'off the wall' approach in some cases, since it 'mixes' with the 'impulse' energy we're after. At the same time, the system 'knows' the difference between a 'stroke impulse' and 'noise' to a large degree... .
Enjoy!
Cheers!'
Mike

[va3mw]

Hi Mike

I am going to disagree.  It is RF and in someways no different that Spark Gap was in the Marconi days.  This is why you can hear lightning on AM radio.  You just can't hear it as far in the Broadcast band as you can at VLF.

We are using RF designed antennas with resonant frequencies in the 20-50Khz range.  The energy we detect is no different than the energy we detect at FM radio.  It is just the wavelengths are much longer (1Meter for FM broadcast so to speak).  Of course wavelength is 300/freq in Mhz or E (speed of light) / frequency in Hz.  That is why it is called VLF (Very Low Frequency).

The techniques being used here are no different than the military uses for communicating with submarines or the ham's use to communicate at 136Khz.  http://en.wikipedia.org/wiki/2200-meter_band.

Personally, I use Loop antennas for 1.8Mhz and 3.6Mhz reception during times of extreme atmospheric noise.  Those same antennas we use the same techniques we are using here and have to deal with  both H Field and E Field.

Make no mistake about it.  This is RF energy and we are using an RF receiver to deal with it.

When I get back up to my cottage, I will run the VNA over my Loop and see if it has any resonance to deal with (something we want to avoid).  The resonance issue is described on the manual very well and explains why we  want a broadband antenna since we are listening for.

Of course, since I left the site, my performance has gotten worse.    I have a lot more investigating to do with this before it is optimized.

Regards,

Mike

[dfroula]

Just to weigh in here, the lightning signals we are detecting are certainly radio frequency EM waves that obey the same laws as any other far-field EM wave - strength diminishes as square of the distance and the H and E components are in-phase. Neither the H nor E component can be individually removed or filtered by shielding.

However, local interference is often a near-field inductively-coupled magnetic source, or a capacitively-coupled electric source. These near-field interferers do NOT follow the inverse-square law. Their electric and magnetic components from an interference source are out of phase. Individual H and E components CAN be filtered, as we do when constructing electric field shields that bleed near-field electrical interference to ground while allowing the RF EM wave to pass (if a gap is left in the shield). With the exception of VLF stations operating within the pass band of our amplifiers, most interference discussed on the boards is this near-field type. It is not RF EM interference (except perhaps in the case of a spark-induced EM wave noise source, like the spark gap transmitter Mike mentioned. My 5-foot Tesla coil is certainly generating far-field EM waves at the resonant frequency of the coil!)

The antennas used for BO must NOT have any self-capacitance (or intentionally added capacitance) that would tune the antenna to a narrow part of the band of interest. The output of the tuned circuit would "ring" like a classic tank circuit and completely obscure the details of the lightning signal that are used by the BO system to determine the group arrival time for any individual station. You can get great strike rates with a tuned antenna, but none of the detected strikes will be used by the servers for location calculations. I've seen such signals where some smart guy decides to add a capacitor to his antenna to improve the detected strikes.

Best regards,

Don F.
WD9DMP

[dfroula]

I like Mike's comparison of the lightning signal to FM modulation.

If one were to look at the plot of an oscillator-generated unmodulated VLF carrier signal, it would appear as a sinusoidal wave repeating at very precise intervals. If frequency modulation is added to the carrier, the repetition rate of the carrier wave will vary with the audio frequency applied. However the carrier will still be sinusoidal.

The RF generated from a lightning strike is not a sinusoidal carrier wave, like one would see from an oscillator. Rather, it is a very short-duration non-sinusoidal series of non-repetitive AC pulses that looks like the lightning signal we are familiar with on the BO scopes. A conventional radio receiver would be tuned to the repetitive carrier wave and would either detect the presence or absence of the signal, or extract the modulation from the carrier, with a tuned circuit. A conventional radio receiver is not concerned with characterizing the amplitude and frequency of the individual cycles of the received signal, as we are with the BO receiver.

The BO receiver works differently from a conventional radio receiver in that it extracts the waveform of the individual "cycles" of the very short lightning waveform. Because the period between the individual cycles of the lightning pulse vary considerably, the pulse can not be said to be at one particular frequency, like a radio station would be. What can be said is the the lightning signal has a series of frequency components that can be detected with a receiver with a very wide bandwidth and fast analog to digital converter. The BO receiver is more akin to a digital oscilloscope than a conventional radio receiver. It is the low-frequency nature of the signal that makes this possible with modest hardware.

The frequency plot of a BO strike on the lightningmaps.org web site makes this clear. A single strike has a multitude of frequency components over its duration, with a peak around a certain frequency range.

Trying to use a conventional narrow-band tuned circuit would efficiently detect the presence of a lightning pulse. This would work well if all one wished to do is flash a light or sound a buzzer on every detected pulse. However, we need to chop the waveform more finely to get an accurate TOA determination. This is complicated by propagation characteristics of the signal, which may add both ground and sky wave components. A tuned circuit would obscure the details of the signal. Imagine a digital oscilloscope that was NOT linear and broad-band over a large frequency range. It would be useless for its intended purpose. People pay big dollars for a linear and wide-band amplifier and A-to-D in their 'scopes.

Best regards,

Don F.
WD9DMP

[dfroula]

I ran about 50ft of CAT5 (the cheapest I had lying around) from the controller to the amplifier.

You really should use SHIELDED CAT5/6 for the controller to amp connection for to eliminate noise pickup, as the connection is unbalanced and subject to common-mode noise pickup. This is per the manual.

Best,

Don F.
WD9DMP

[Cutty Sark Sailor]

My response was relative to this 'if you think in terms of RF reception", you are going to be mislead, especially as algoriithms change on the server.. we need to think in terms of 'impulse' with varying frequencies, signal phase,  and strengths... ditto if thinking in 'audo' frequencies,,,,,  this is an 'impulse' of energy' which varies in freq content and strenghth... as Don implies, the frequencies 'die out' with distance and information... we must think in terms of 'signal shape' compared to 'demodulation' as is more conventient...  the 'signal' you'll see is a 'reconstruction' based on 'freq' and 'energies' contained... if you consider this RF you are not understanding the 'stroke' pheneonema, which includes 'predischarge', 'discharge, after 'discharge',,, "bipolars" etc...  While it is "EMF"and"RF" the information we need is 'non-specfic' if you will... Don F has alluded to folks who decide to 'modify' antennas', etc... we are NOT dealing wth an MSK signal or an intelligence modulated singnal.. we are dealing with a "bandpass'... shape recognition, per Phase, Time, Frequency, signal... an "energy impulse.'..   if you don't undestand that, then more research should be conducted. The rules and paradigms for "vlf receiving" do not necessarily  apply... think differently when observing the system signals...

Cheers!
Mike

[va3mw]

Hi Don

Yes, I agree on the not wanted a resonant antenna.  We do require a broad band receiving antenna and RF rules do apply (speed of light, functional operating range, etc.) with designing a broadband type antenna.  In our case it needs to receive equally poorly through its entire operation range.  If we could design an Isotropic antenna for VLF, that would be great, but there is no such thing and this is why we require 2 antennas to attempt to emulate an isotropic design.

For the feeline:

I might be wrong, but assuming that the signal being sent from the amplifier down to the controller are on the correct pair (I need to check the schematic), would that not be balanced as the wires are twisted?  CAT 5 cable pairs have a impedance of approximately 100 ohms.  The QRP guys use it for balanced feedlines all the time using a 2:1 balun (not unun) at the radio end.  The phone guys (as in hard wired telephone) used twisted pairs for a hundred years as the currents are balanced.  UTP cable is a balanced line.

I was concerned about common mode currents, so I did had some RF chokes (mix unknown so who knows if they are actually functional). 

There are a few reasons I didn't run shield CAT 5.  One, was I didn't have any handy and I needed to run about 20m to get the antennas away from the main structure.   

The other was under the 'lets see if I can get away with it knowing that CAT 5 is a balanced feed.   In my line of work I often see requirements overstressed by 'engineers' who don't have any real world experience.  I am not saying that is the case here (again, all I had was UTP cable).

However, please consider this part of a test and a learning experience.  I am also in a very quiet electronic area and not a suburban location.  It is the home of my HF remote base and there is a reason while it is there. 

I am a big fan of trying something and see what happens.  Record your results and see what happens.  That is what I am doing now.

Since I am down to 1 antenna at the moment, you can see I do have some ripple at about 5mv on my good channel (red).

As will like bury the CAT 5 at some point in the very near future, that does a pretty good job of handling the common mode currents.  However, I might have to shield that only to prevent issues with the ground surge from a nearby lightning strike.

I have been working in RF for about 40 years now.  I am a very active 'contesting' ham and a hard core geek.    Yep, I do break things, but I learn lots doing it.  I often have to prove things for my self.   

BTW, I am not out to improve anything.  I fully understand the design and how the signals are sampled and decoded.

Don, if you read QST, I was one of the first who used a Drone (UAV) to move lines in place for wire antennas.  I also had the article on 3D printing a few years ago.

Mike va3mw
http://www.qrz.com/db/va3mw

[dfroula]

Mike, good info.

I've been a telecom engineer for 35 years now, 30 years with Motorola, 5 with Nokia.

You are correct that CAT5 is balanced twisted pair in construction and is suitable for POTS telephone use, where circuits are carefully balanced to avoid common-mode noise pickup. Ever hear a phone line with loud 60Hz hum on it? That's a line where one of the two wires has ground leakage because of water in the line or some other condition that unbalances the line and allows common mode pickup of AC hum radiated by power lines.

However, if you check the schematic, you will see that the CAT5 connections between the controller and amp are not using the conductors in balanced mode. There are 7 CAT5 connections for the H-field amp - +5vdc, Gnd, Amp-A Out, Amp-B Out, and two serial data lines for gain control/reporting, Amp-TX and Amp-RX. The analog signals are on Amp-A Out and Amp-B Out. Both are referenced to ground and are unbalanced, hence the requirement for the shielded cable.

Snap-on common mode ferrite chokes work well at HF and VHF/UHF, but do almost nothing for power line interference. We are concerned here with the fundamental line frequency of 60 Hz and harmonic distortion of the line frequency due to non-linear loads on the power line. These manifest as integer multiples of the line frequency that are radiated from the power line inductively or through capacitive coupling. These are the non-EM "near field" interference sources I mentioned. The shielded CAT5 does a good job of eliminating those sources, as well as stray RF interference.

I love the drone idea for stringing wire antennas. I used to use a bow and arrow!

I guess we all build a set of "mental models" to conceptualize how we think about complex systems. Thinking about lightning signals as audio is not particularly helpful to me, as it limits understanding of how propagation, antennas, interference and shielding operate. The lightning impulse, along with precharge and all the other complexities is just a complex RF waveform that we can look at in detail due to the low frequency which makes capture by relatively slow A-D circuits and processors possible.

Best,

Don
WD9DMP

[miraculon]

The antennas used for BO must NOT have any self-capacitance (or intentionally added capacitance) that would tune the antenna to a narrow part of the band of interest. The output of the tuned circuit would "ring" like a classic tank circuit and completely obscure the details of the lightning signal that are used by the BO system to determine the group arrival time for any individual station. You can get great strike rates with a tuned antenna, but none of the detected strikes will be used by the servers for location calculations. I've seen such signals where some smart guy decides to add a capacitor to his antenna to improve the detected strikes.

Very true. I learned early on with the Green system that there was a lot of ringing present with the default design and ferrite rod antennas. I kept stray capacitance to a minimum, but the original input Z was 1M Ohm! I tried a variety of termination resistors, but eventually settled on 10K Ohm. With the ringing brought under control, I was able to open the bandwidth to the 34KHz filter jumper settings, instead of the 17KHz.

I was able to use Electronics Workbench where I worked and built a simulation model (on my own time, of course) of "Green's" amplifier circuit. I was able to see the result of the termination resistances and filters quite well in the simulation. Unfortunately, since I retired and the NI software is too much in terms of dinero for this retiree, I can't do it any more.. (my quest for a freebie/cheap simulation tool that compares has not met with success)

Greg H.

[Cutty Sark Sailor]

Down to brass tacks:  IF the system developers COULD determine what type antennas we all use, they'd 'require' identical antennas... that's where everything begins.  They may be moving toward that with the 'built in' E probe... this is NOT about how broad or how specific our reception is... it's about everybody sending 'indentical' data related to a stroke impulse.  The Cat 5, Cat 5E. CAT 6, shelded, is a very important issue.... whatever the 'theory' suggests may be appropriate for 'VLF" reception, "Elf" reception, the developers have spent a lot of time, personal finances, etc, to determine 'what works'... when they say "shielded'  we need to accept that.  Figure out the 'theory' later,... if you send a 'distorted' signal, it won't be used,... they design and process for certain 'standards'  and if you deviate, anything that 'distorts' the energy impulse messes with the signal... Follow the cookbook. A lot of folks are going to be perturbed later this year as the 'quality' paradigms are more tightly implemented...  when they suggest 'shielded' from Amp to Controller and 'unshielded' from Controller to Router, and 'cold water ground' controller only, it's simple... follow the 'suggestions'.... issues with this network occur because folks forget it's a 'network' and decide their 'station' can do it's own thing... that's not the way it works... so some folks are heading for a bit of frustration down the road...  argue, debate, nitpick all you will... This is a hobby, and experimentation is encouraged... but if experimentation begins without the 'common' baseline'  it is self-defeating. 

[dfroula]

There is a pretty good discussion of the difference between RF EM waves, inductive magnetic fields, and electric field here:

http://electronics.stackexchange.com/questions/64464/what-is-the-difference-between-electromagnetism-and-electromagnetic-radiation

So, signals in the VLF band of interest for lightning detection could be thought of in 4 ways. Take a 20 KHz sine wave generated by an oscillator:

1 - Feed the signal into a loud speaker. The cone of the speaker moves at the same rate as the electrical signal, compressing the air periodically. The compression air wave generated by the speaker is audio.

2 - Feed the output of the oscillator to a coil of wire. The field produced around the coil is a magnetic field. The field can be detected by a similar coil held hear it and connected to an amplifier. This is a transformer and the effect is called magnetic induction.

3 - Feed the output of the oscillator to two metallic plates. The signal can be received by two other plates held a short distance away and connected to an amplifier. This is capacitive coupling of the electrical field.

4 - Feed the oscillator to a very long wire 1/4 wavelength at 20 KHz, the other wire from the oscillator to ground. You will get some inductive and capacitive effects near the antenna, but an RF EM field will also be generated that can be detected far from the antenna and follows different rules.

The signal we detect in our BO receivers from lightning is the result of #4, although the system is also sensitive to nearby signals propagated by #2 and #3. It is completely insensitive to #1 . Shielding on our ferrite antennas protects against interference from signals received from #3. It offers no protection against #2 or #4.

Regards,

Don
WD9DMP

[dfroula]

To expound on this a bit more, what effect would various types of shielding have on the signals I mentioned?

Suppose you have a ferrite antenna in a tube completely surrounded by aluminum or copper tape with no gap. The shield will prevent #4 (EM radiation or RF) and #3 (capacitive coupling from an electric field). It will have no effect on a nearby magnetic field.

Suppose you now make a longitudinal slit in the shield. The shield will now block only #3. #4 can now pass since the currents induced by the EM radiation in the shield can flow on the outside and inside of the shield, where they are coupled to the ferrite core coil.

Suppose you make a shield from an iron pipe, no slit. Such a shield blocks all three propagation methods. This has been suggested here in the forums as a way of blocking reception completely from the antennas to determine if noise is being coupled in via the antennas or power supply. Good idea!

Suppose you used an iron pipe with a slit? You would shield against #2 and #3, but the lightning signal, (#4) would be passed.

The idea that the shield blocks the electrical component of the RF wave, while allowing the magnetic component to pass is NOT true. For an EM RF wave, the wave cannot exist without both components present. While a magnetic inductive field and electrical field usually are generated together one CAN happily exist wthout the other!

Regards,

Don F.
WD9DMP

[va3mw]

Well, my station has been on for about 2 weeks, and I have to say, I am very impressed by its performance.

The only cause for interference here is one of my Hobby King LIPO battery chargers I use for my model planes.  That will drive me right off the air so to speak.

As this is our cottage, the only other concern is when I when to power up the aerator for the septic system (big air pump).  It has about 50 years old and always works just fine.

Fortunately no VLF signal emissions to worry about and no addition to the noise floor. 

Onward and upward!

Mike

[Cutty Sark Sailor]

Don't worry about the sporadics... I think many of us deal with such... you might try your gains at a balanced 8*8 and see how it does... sometimes a 16*4 or similar can be woozy, I think with near fields... can't prove that, but some of have noticed better response with lower 1st stages than second... especially at certain times,  which doesn't agree necessarily with general paradigms...

[va3mw]

Just to update everyone, I have left mine in Auto mode and haven't touched it to be honest.  Like most weather stations I run, my goal was to just keep it up and running.  A quick peek of others shows that I am in ballpark. 

As a reminder, I am running with 2 1M loops (I do have to replace them with a few hockey sticks--it is a Canadian thing) fed with some CAT5.

About once a week I look at the actual station specs just to see if I am in interference mode. 

I'm pretty pleased just how well this is working 'right out of the box'.  But, then again, I am a big fan of the 80/20 rule.

Mike