Slight digression here:
The radiated field from lightning has peaks around 80 MHz, likely due to the size of the "jumps" in the leader development and the current in the return stroke. This has been used to good effect by putting multiple VHF receivers out there and doing a sort of Time of Arrival mapping, and you can see the stroke develop within the cloud.  As you can imagine (time nuts alert!) this requires very good timing of the pulse detection. I first saw this at a conference in ~2000, so it's been around a while. 

The spectrum of the actual stroke is pretty broad, and depending on what's carrying it, may or may not radiate.  As PHK said, 2 microsecond rise time from 10-90%, 50 microsecond fall time to 50% is the standard "lightning impulse shape", in the form 1- exp(-alpha*t))exp(-betat)  (aka "double exponential"), with a series of pulses in one stroke.

Speaking of FFTs, that's one way they do analysis of antennas and systems with lightning is to use something like NEC to model the system at a variety of frequencies, and then combine them using an inverse FFT, weighted by the spectrum of the lightning. 

On Mon, 29 Sep 2025 07:31:30 +0000, Poul-Henning Kamp via time-nuts time-nuts@lists.febo.com wrote:

Bob Camp via time-nuts writes:

So this gets to choice of antenna.

The best, flattest wide-band antennas are indisputably "E-field
probes", basically a piece of conductor connected to an amplifier
with as high input impedance as you can make it..

It is almost trivial to build an E-field probe which is flat from
DC to north of a GHz. I'm personally partial to Chris Trask's
designs ("Complementary Push-Pull Amplifiers for Active Antennas:
A Critical Review") but there are many others.

But because they are wideband, they also pick up "static" and in
particular the insanely wide spectrum[1] of nearby lightning strikes,
which are the major cause of the big transients you talk about.

Below a MHz one can also use "M-field probes", which is a coil
attached to an amplifier with a balanced input, commonly known as
a loop-antenna.

The kind of noise spikes you talk about only happen in loop-antenna
if somebody quenches the superconducting magnet in the MR-scanner
next door.

A major difference between loop-antennas and e-field probes is
that loop-antennas have a figure-of-eight sensitivity pattern.

This is great if, like me, you have a hundreds of kW LF transmitter
in the next town over, but less great if you want to receive several
signals from all over at the same time.

Loop-antennas can also be tuned to a particular frequency band
by adding a capacitor in parallel to the coil, and you can get
amazing "amplification" by using a high impedance input amplifier
because it operates near-resonance. The downside is that it
takes forever for the resonance to die out again, which is
why it is almost only used in the "run forever on an AAA battery"
radio-controlled clocks, which only need a ~3Hz bandwidth.

I have experimented with both E-field and M-field probes in the VLF
band and I far prefer (untuned) M-field probes.

Poul-Henning

[1] Zero to many kA in less than 5 microsecond, you do the FFT.

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Bob Camp via time-nuts writes:

The three 12 bit, 500 ksps ADCs in a Rasberry Pico2 is plenty.

It comes with two ARM cores (or RiscV if you prefer), so you can
use one for the realtime processing and one for the interesting
stuff.

And I think they cost something like $5 qty=1 ?

It is probably fast enough that you can write the receiver software
in micropython instead of C :-)

Dont talk, just do it :-)

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.

Hi Paul
good points Paul.

I develop VLF underground proximity  systems professionally.. and these
are not simple single axis loop systems.
between 25kHz and 80 kHz.... so I have a reasonable handle on it.

Maybe people make the mistake of trying to acheive too high a Q antenna

----and smear the information that the LORANC signal is trying to impart.

I dont know much about LORANC, but I would expect that there is a direct
relationship between bandwidth and uncertainty for a specific number of
pulses observed.
Care to elaborate with some numbers on it ?
regards,
glen.

On 30/09/2025 19:42, Poul-Henning Kamp wrote:

glen english LIST writes:

Ideally a Loran-C antenna should pass 85-115 kHz with high flatness,
but in practice few did and 90-110kHz seems to have been the "normal".

As the bandwidth narrows it causes two problems:

1: It makes it harder to identify the 3rd positive zero-crossing

This is a serious problem for "delay and add" type Loran-C detectors[1]

If the receiver always get it wrong the same way, ie: always picks
the second or the fourth crossing, this is "mostly harmless":  The
second crossing has much worse S/N, the fourth may run into nightwave
problems, if you are very close to the transmitter.

If the receiver sometimes get one and sometimes get another, you
get a 10µs navigation error, roughly two nautical miles, which is
very noticeable near land but not so much on the high seas.

2: It changes the absolute timing of the 3rd positive zero-crossing.

This is only a problem for time-receivers, but if the antenna filters
are stable, it will be calibrated out with other electronic and
cable delays.

Serious Loran-C monitor or time receivers would often have from one
to five tunable high-Q notch filters to take out near-band CW
signals.

Poul-Henning

[1] If you add a 5 microsecond delayed copy of the Loran-C pulse
the sign of the result has a very characteristic 2-1-1-2 time
signature centered on the 3rd zero-crossing.  Most microprocessor
based Loran-C signals did this, because it eliminates the need for
fast ADC's and simplifies the RF part too.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hi

Since this seems to be headed off in a couple of directions:

Loran due to its very broad bandwidth pulse signal (check out any SDR near a Loran transmitter …) is a different signal than what WWVB / MSF put out. The why is pretty simple, the system was designed for navigation. To get accurate location you needed a good edge to lock on to. Its is not “spread spectrum”, but it has a lot of the same ideas lurking behind the design of the signal. The net result needed to be a measurement in the (sub) microsecond range.

Propagation changes a surprising amount in various VLF “bands”. What you see at 100KHz is not what you see at 60 KHz. Go to  20 KHz or 10 KHz it also changes. One could spend a lot of time wandering down the propagation rabbit hole. We don’t get to move the transmitter frequency. Let’s move on rather than go there right now.

Back in the day when this all was “how you do it” a lot of folks ran this sort of gear. Both WWVB and Loran devices showed up in the US. The normal conclusion was that WWVB was far more trouble / lower performance than Loran. Loran usually won that race by a couple orders of magnitude. For a “wall clock” time source, you used WWVB.

If you wanted to go crazy you could go one step further. NBS mailed out a monthly report mailed (as in what the mail man delivered) to you from NIST. After some looking at logs you could work out what had been going on weeks ago.

Did some of this gear use ferrite antennas at 60 KHz? It sure did. You might sit down and chat with the design guys at various companies. The answer they all gave you was: It’s more compact and some customers are after that. For the customer's application it works ok. It’s not the best choice stability wise.

If you decide to “roll your own” antenna, a horizontal wire simply does not work at these frequencies. That’s part of why those various internet SDR sites show very little “down there”. A vertical works ok, but it has a crazy high impedance at the base. Some sort of preamp is typically used simply to convert to a rational cable or twisted pair impedance. Air core loops come in a variety of configurations and each has its own “fun”.

Bob

Bob Camp writes:

Make up you mind Bob :-)

Loran-C is a spread spectrum signal by accident, even if that term
only got a firm definition a decade later.

The first thing was the power-bill.  Many of the early Loran-C
stations were powered by diesel generators, so they wanted
as much peak amplitude for as little power as possible.

That dictated a signel with brief pulses, spaced widely apart, A
typical dual-rated Lorsta would consume 50kW power and transmit 250
pulses per second, each pulse peaking between one and two megawatt.

For the hams: "Loran-splatter" happened because "Sidebands 20db
down" literally means "no more than 20kW" for a Loran-C transmitter.

The next problem was the "night-wave crash".

When you receive Loran-C at night, you will get both the direct
signal, and a reflection from the ionosphere.

If you watch this:

https://phk.freebsd.dk/AducLoran/animation2.gif

You can see the reflected night-wave dance around in the tail.

The path difference between the direct "ground-wave" and the reflected
"sky-wave" can be very, very small, and the skywave will often have
higher amplitude than the ground-wave.

See the plot under "6731" here for instance:

https://phk.freebsd.dk/loran-c/Chains/

But the "sky-wave" always travels further, and therefore arrive
later, and there is no way it can ever arrive at or before the 3rd
positive zero-crossing, which is therefore the official timing
reference.

In theory, they could have designed the signal to end after the
third positive zero-crossing, and they sort of did, but it is
not easy to implement in practice.

At these frequencies the only viable omnidirectional antenna is a
"top-hat-capacitor" where the antenna tower and a good fraction of
the "barduns" from the top to ground make up one electrode, and the
"counter-poise", a similar net of copperwire on the ground, is the
other electrode.

You dont make a big capacitor that way, so you need voltage measured
in tens of kilovolts, but you still get nowhere, unless you also add
a bit of inductance to get a resonance going.

So the "half-wave-generators" in the transmitter dump energy from
a high voltage capacitor into the antenna system at carefully timed
moments, to get a resonance going, and then as soon as they have
created the important 3rd positve half-cycle, their job is done.

The receiver still needs some way to tell which is the 3rd positive
zero-crossing, so in practice the signal amplitude is controlled
all the way to the top, so that the peak amplitudes can be used
to identify the 3rd positive zero crossing, but all the energy
is dumped into the antenna in the first 10 half-waves.

Because the signal is pulsed, the resulting spectrum consists of a
lot of closely spaced frequencies, and because the pulses have
"coded" polarities so you can tell them apart, the spectrum gets
even more complex when averaged over a GRI or FRI,

Theoretical plots here:

https://phk.freebsd.dk/loran-c/theoretical_spectrum/

This was a very important consideration for Loran-C:  They did
not want Loran-C to reach too far, because navigation would
not be useful and it would just interfere with other stations.
100kHz is not exactly "line of sight" but it does stop after
some hundreds of kilometers.

60kHz on goes halfway around the globe, which allows WWVB
to cover CONUS with a single transmitter.  (The first transtlantic
radio-telephone link used 60 kHz in one direction and 66kHz in the
other.)

Omega on the other hand had only 8 (9?) transmitters world wide,
so they wanted frequencies where the earth/ocean and ionosphere
forms a waveguide, that happens below 30-ish kHz.

And dont even get me started about the 76Hz and 86Hz radio
transmitters.  Yes: "Hz".  Not "kHz" :-)

Poul-Henning

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

That explaination by PHK about propagation at  LORAC freqs has a few
errors, but is mostly right, good enough for most people .

BTW , let's get our terminology right :
VLF is < 30kHz . LF is 30kHz-300kHz.
and 60kHz and 100kHz are actually not all that different in propagation.

AND, let's clarify we are talking about TIME problems here, not location
determination.

Ideal antennas for DCF77 and WWV and LORANC are different
Considering time :

• DCF77,   ~ 3kHz gives good fidelity of the PM component.  (a bit is
  120 cycles of carrier)
• LORANC -  ideally 30kHz of BW for best fidelity, likely difficult to
  acheive good SNRs over this full bandwidth over short periods in the
  suburbs .
  works with reduces bandwidth with increased smearing of those sharp pulses
   - WWVB   - PM service, ~ 4Hz will provide good fidelity.  (1 bit per
  second)

But what is good fidelity  must be defined by the user.

All these LF time signals will suffer degrees of multipath interference
due to the presence of the ionoosphere

There will be a crossover point where increasing the bandwidth which
reduces smearing of  the transitions of the data is undone by effects of
multipath and man man interference.

Narrow bandwidth is beneficial in that it  increases SNR (improving
precision) , and  reduces effects of off channel interference
Narrow bandwidth smears the data transitions (reduces precision)
Narrow bandwidth  from high Q antennas produces delays that need to be
removed. (reducing accuracy)
and then there may be some temperature and static magnetic field
variations that will perturb the narrowband tuned circuit .
There's some advantage here to having a ncie flat top response with the
double tuned circuit, but fundamentals like thermal drift, aging  etc of
components, stray fields all limit accuracy.
I would suggest a electrostatically shielded loop (to be able to null
out local dominant interferers and a dust core (low thermal
sensitivity)  in a double tuned circuit.

By all means use a wideband E field probe (to maximize bandwidth) if you
are in the middle of nowhere . I have to work with 3 x 50kW AM broadcast
transmitters 1 mile away.....

if the time signals were usable down under, I might consider making
something and publishing.

Poul, I dont consider LORANC-C spread spectrum at all .
It does not fit the accepted characterisation of being a direct sequence
modulated,  frequency hopped or chirp  waveform.

It's just a wide bandwidth pulse waveform.

-glen

On 1/10/2025 07:39, Poul-Henning Kamp wrote: