Z0 = sqrt (L/C) 
Z0=100 Ohm
C=180pF

Re: Coaxial Marx Generator for EMP
Proud Mary, Fri Apr 04 2008, 04:04PM


Much of the groundwork on UWB 'impulse radiating antennas' driven by Marxes was done by J. R. Mayes, W. J. Carey, Larry Altgilbers, and Everett G. Farr

They started off with a TEM horn, and graduated to a kind of centre-fed dipole centred in a folding umbrella-like parabolic reflector which could be carted about.

The flat sheet metal bow-tie driven elements were driven from the centre, and terminated where they met the rim of the parabolic with hefty resistors, of, as I remember 600 ohms.

The whole thing is described in United States Patent 6340956 here:



I understand that the invention was intended for use in disrupting electronic systems - such as motor vehicles - at a distance.

Searching for "UWB antenna" and "impulse radiating antenna" will get you what you want to know.

Re: Coaxial Marx Generator for EMP
Dr. Shark, Fri Apr 04 2008, 05:06PM

Hey, thanks for your reply! I really don't know much about antenna theory, so if you could elaborate a little bit on what the dish does, that would be great. Does it somehow 'focus' the dipole radiation pattern of the antenna to give you more gain?

A related question concerns the impedance of antennas: I understand that free space has a characteristic impedance of about 380 Ohm, so I wonder why most antennas are around 100Ohm. Does an antenna contain some kind of implicit transformer?

One more thing I came up with regarding the design of the Marx generator: We have the frequency omega^-2 =LC and the impedance Z^2=L/C so for a given pulse width and target impedance we can exactly calculate the required capacitance and inductance. E.g. if we want energy at 1GHz, and Z=100Ohm, we get L=20nH and C=2pF. This is completely and utterly ridiculous, so I wonder where I am going wrong?

Re: Coaxial Marx Generator for EMP
Proud Mary, Fri Apr 04 2008, 05:51PM

Cold water for a shark, Suomi!

It is a long time since I looked at this material, so I forgot that Everett Farr has a business called Farr Research Inc which has a lot of its research papers published online here:



I understand that a lot of this work has been paid for by the little known US Air Force "Directed Energy Directorate." Uses of this technology are said to include UWB high definition radar, the jamming of missile control systems, and the remote disruption of motor vehicle electronics to create 'exclusion zones.' Whether it can be made to work reliably, or is like the robotic self-propelled Tesla coil that hopelessly failed its field trials as an exploder of 'terror' devices in Afghanistan is another matter...

Very detailed photos and descriptions of the antenna I first told you about, both the folding version and one with a hard non-folding parabolic reflector, are shown in this Farr paper here:



Now for the next bit of your question!

The "impedance of free space" is a physical constant that relates the magnitude of the magnetic and electric components of a wave as it flies through infinite space, thus: Zo = E/H. Happily, it has limited use in practical antenna design, and cannot be measured directly.

The thing to think about is the radiation resistance of the antenna, which is that portion of an antenna's input/feed point impedance caused by the radiation reaction of conduction electrons in the antenna itself.

It is to the radiation resistance of an antenna that a transmission line must be matched. The most famous radiation resistance is that of the half-wave dipole antenna, which has a characteristic impedance of 73 ohms - usually rounded up to 75 ohms for practical purposes - which is why RF co-axial cables are commonly made to have a characteristic impedance of 75 ohms.

And now for the last bit! When Marx generators are used as high power Ultra Wide Band (UWB) generators, the designer tries to make the rising front of the output pulse as nearly vertical as he can. He wants the leading edge to go straight up in zero seconds like a pure square wave, so it will contain all possible frequencies, and thereby be 'ultra wide band.' So there is always a lot of research on how to make the rise-time as short as possible to be as near the perfect vertical rise as possible.

Attempts to shorten output pulse rise times will often include the use of low value charging resistors (50 ohms or so), low inductance capacitors, synchronized simultaneous triggering of all gaps, pre-ionization of gaps, use of pseudo-spark switches, krytrons or sprytrons, and combination of the marx generator with co-axial pulse forming lines and Blumlein circuits.

That's enough for now!

Re: Coaxial Marx Generator for EMP
jpsmith123, Fri Apr 04 2008, 11:59PM

I'm also interested in fast high voltage pulse generation, but for a different application.

One major issue with regard to creating very-fast-rise-time-pulses, whether you're using a blumlein or a Marx generator, is the design and construction of the switches (usually a spark gap).

If you're talking about a few hundred kv into 100 ohms, with a rise time on the order of a nanosecond or so, then you're talking about a switch di/dt on the order of 10^12 A/Sec or so.

According to what I've read, getting this type of fast switching means operating the spark gaps at very high electric fields, which means using short gaps with high pressure gas insulation or alternatively using water switches.

Re: Coaxial Marx Generator for EMP
Proud Mary, Sat Apr 05 2008, 12:39AM


The references I've cited above in my reply to Dr Shark look at techniques for reducing output pulse rise time in exhaustive practical detail.

Re: Coaxial Marx Generator for EMP
HV Enthusiast, Sat Apr 05 2008, 04:12AM

Hi Dr. Shark. Love the name!!!

I did get your PM and unfortunately, due to obvious reasons (ITAR etc...) i can't discuss the information you requested.
However, there should be numerous public domain IEEE articles on marx generation design which is available, some on coaxial transmission line design.

Re: Coaxial Marx Generator for EMP
Dr. Shark, Sat Apr 05 2008, 09:08AM

Hi Dr. Shock, can you guess how I came up with that? I was actually half expecting you to PM me to change my name back to joe since "impersonating authorities" is against the rules

I respect your professional obligations, but obviously I am not asking for detailed plans but rather trying to develop an understanding of the general principles involved.
But I probably got a step closer to that anyway: I found another IEEE paper (which happens to be available to the general public) where there is a short mention of a "wave erection" Marx.
This seems to be a particular example of the principle Harry mentioned: Get as fast a rising edge as possible. With wave erection, which is maybe somewhat related to the Blümlein circuit, a rising edge on the order of 100s of ps can be obtained.
It seems that on the old 4HV someone knew more about this . Unfortunately the link he gives is dead, but I tracked down his stuff here . Still I wonder if this is a feasible concept for an amateur construction?

Harry, thanks also for your crash-course in antenna theory, very much appreciated!

Re: Coaxial Marx Generator for EMP
Proud Mary, Sat Apr 05 2008, 09:57AM

'Wave erection' is not itself related to the Blumlein circuit, but concerns the sequential order of switch closure in a Marx:

Wave Erection Marx

The general Marx circuit does not force switches to close in succession. Therefore, switches can fire randomly, adding temporal jitter to the system’s performance.

"The transient wave Marx insures that after closure of the first switch, stray capacitances hold one side of each successive switch near ground until the switch fires. A voltage wave propagates toward the load with increasing intensity, triggering successive switches faster and faster until it reaches the load with sub-ns risetime and low jitter for output voltages of several hundred kV at moderate per pulse energies." See:

Re: Coaxial Marx Generator for EMP
jpsmith123, Sat Apr 05 2008, 02:15PM

To start out with, you might be better off with a stacked blumlein instead of a marx generator, say a stacked coaxial blumlein using coax cable charged to 30 or 40 kv or so, and
only using one switch.

There are several papers available on-line regarding this type of generator.

I have some papers somewhere dealing with design of switches, blumleins and UWB antennas. If I get a chance later I'll try to find them.

Re: Coaxial Marx Generator for EMP
Dr. Shark, Sun Apr 06 2008, 02:13PM

Yet another entertaining post by Dan about his Marx:

I killed a new FLUKE 87 handheld multimeter with my Marx Generator. I had the multimeter attached to the NST i was using to power a CW multiplier which then charged the Marx Generator to 40kV per stage. The multimeter was attached to the Primary side of the NST and was completely isolated from Marx Generator and charge circuitry. The Marx Generator even had its own isolated grounding.

Anyways, a discharge took out the multimeter.

Actually, I've killed quite a bit with that Marx Generator. My house alarm system, my house digital thermostat, a TV, and most recently, a nice multimeter. You'd think i learn, right?

It's a shame that he won't join the discussion, but I am glad I found two other people who are interested in this topic.


Now I am not really prepared to throw over the idea of a Marx generator in favor of a Blümlein circuit, since that would mean getting all new parts, doing all the reading again etc., and I'd rather see some sparks soon

Harry, that paper you cited from what exactly what I had linked in my post above, did you notice that? I was just referring to the wave-erection as being Blümlein-like since it involves a wavefront traveling at the speed of light. I don't know a whole lot about the particularities of that circuit apart from it's alleged use in TEA nitrogen lasers.

Anyway I said I wanted to get this going on the experimental side, so here we are. This is a bit of an "artist impression" of what I want individual stages to look like.



The spark gap (only one electrode shown, the other of course is connected to the next stage), is exactly in the middle, and there is a hole drilled down the center so all the gaps can "see" each other. The arrangement with the three capacitors in parallel is taken from the work of Mayes et al., although they don't say where the sparkgap is, and I presume it is not down the middle. I thought it was a nice idea though, because is makes the Marx even more coaxial
The whole assembly is going to slip in an acrylic tube, which will be sealed to the PCB disks with some kind of goo. I want to keep the distance between stages as short as possible to have a short discharge path.
The right photo also shows that I started putting the capacitors in blocks of 6, so I will have only 5 stages of 60kV each. My hope is that this way I will have less timing jitter and a faster pulse. With some clever design I might be able to make the whole 300kV generator as short as 30cm (12").

Machining the disks is a total PITA for me, look at my ghetto setup:



just a little bit too much feed in my "lathe" - which is a drill press and a chisel in a vice, really - and I lost an inch of plexiglass to shards. Whooops!
The PCB segments are even worse in their own way, the fiberglass dulls my cutting bit in about a second. I guess I need tungsten carbide bits here, maybe I'll grind down a drill bit for one.

Re: Coaxial Marx Generator for EMP
Proud Mary, Sun Apr 06 2008, 03:07PM

If the gaps are accurately aligned, front surface mirrors at top and bottom of the stack should increase the amount of UV available for pre-ionization of the subsequent gaps after the first gap has fired.

So far as I can see, the fast Marx designs of Mayes and others are often arranged so as to be fitted into a pressurized (very portable) cylinder, which also provides the stray capacitance said to be needed for the sequential gap firing in the 'wave erection' mode.

There is an interesting paper Development of a 300-kV Marx generator and its application to drive a relativistic electron beam which contains lots of useful practical detail of home made parts, which you may find helpful:

Re: Coaxial Marx Generator for EMP
Dr. Shark, Mon Apr 07 2008, 06:33PM

Harry,
that last paper you cited is part of my printed paper collection too. Seems that there is only a limited amount of publications available to the "general public" (with IEEE access, that is), so the papers I have read are likely to greatly overlap with what you and others have read.

I'd like to get back to the issue of pulse rise time once more. I am happy with the fact that, regardless of inductance, you can theoretically get a voltage rise time as short as you want, e.g. in the picoseconds. But since the power delivered to the load is P=I*V, the current has to rise fast, too. This is, in my understanding, where the inductance of the generator comes to play, and I would like to go back to my example at this point:

We have the frequency omega^-2 =LC and the impedance Z^2=L/C so for a given pulse width and target impedance we can exactly calculate the required capacitance and inductance. E.g. if we want energy at 1GHz, and Z=100Ohm, we get L=20nH and C=2pF. This is completely and utterly ridiculous, so I wonder where I am going wrong?

This example may be rather over-the-top, but remember that with my proposed 10-stage generator, we would be looking at inductances in the uH(!!!), which would basically mean deliberately placing inductors in the discharge path, thereby considerably slowing down the discharge, just to get the impedance right. There must be somethings wrong here, but right now I am completely blind to what it is.

Re: Coaxial Marx Generator for EMP
Proud Mary, Mon Apr 07 2008, 07:13PM

I am not clear what you're getting at here, old fellow.

Rise time is always limited by bandwidth, since an ideal square wave would require an infinite bandwidth.

The output impedance of the conventional Marx is capacitive and this can make matching to a specific load difficult. Generally speaking, the erected Marx appears as a single capacitor to the load.

Ultra-fast Marxes are powerful UWB sources, but are not suitable for the generation of a single frequency such as 1000MHz as you have suggested.

Re: Coaxial Marx Generator for EMP
Dr. Shark, Mon Apr 07 2008, 07:32PM

Sorry if I have been unclear, I'll try and rephrase the question. I certainly agree with everything you have said above; the point I was trying to make is the following.
Say we have given erect capacitance, like 200pF for the particular configuration I was proposing for my capacitors. Now to match this to a 100Ohm load, we wish to make the characteristic impedance equal to 100Ohm. In this case that would mean introducing a total inductance of 2uH, since
Z0 = sqrt (L / C) = sqrt (2uH / 200pF) = 100 Ohm
now 2uH is a rather big inductance, and even with very sloppy design I would probably not get up to that value, to I would have to add a series inductor to the Marx generator to make it look like a 100Ohm line. This does not make sense to me since a 2uH inductance would severely limit the bandwidth, and probably confine the output to frequencies below 10MHz.

Re: Coaxial Marx Generator for EMP
Proud Mary, Mon Apr 07 2008, 09:06PM

Now I get you, Dr Shark! You need a Pulse Forming Line to match the Marx to the load.

According to Mayes:

Traditional Marx designs utilize a pulse-forming-line
(PFL) to deliver energy to the load at a matched
impedance. Because of the physical length needed to store
a useable amount of energy and deliver a pulse from the
ten to hundreds of nanoseconds, PFLs can range from
several feet to several dozen feet in length depending on
the dielectric used. These dielectrics also serve the dual
purpose of providing voltage hold-off between
conductors, and consequently are also used inside of the
Marx itself. As a result of the insulating media (typically
transformer oil) and the large capacitance necessary to
rapidly charge the transmission line, the system becomes
extraordinarily bulky and difficult to calibrate and control.

Re: Coaxial Marx Generator for EMP
Dr. Shark, Tue Apr 08 2008, 04:11PM

Harry, I really appreciate your efforts here, but it seems that I still didn't make myself quite clear. What you cited above is about Traditional Marx designs, i.e. a huge multi-kJ beast, whereas I am interested in a compact low energy system (I already spend $110 on the capacitors, for only 8J energy), that can be directly coupled to the antenna. I belief that this is also what both Dan and Mayes have in mind.

I'll just bite the bullet and build something, I won't be able to do any precise tuning anyway. My new scope (yay, Tek 222) only does 1MS/s single shot, I would need at least 1GS/s for some proper diagnostics. If it does not work for generating UWB radiation, I'll just use it for flash x-ray or some other easy-to-drive load.


Regardless of the application, and if this whole thing is ever going to work, I am still curious about the surge impedance though. Would it at all be valid to link the condition of Z_load=Z_0, where the highest possible amount of power is delivered to the load, to a critically damped discharge? I am trying to link the reflected wave traveling back and forth down the line to the oscillations in an underdamped RLC circuit. This would fit in with the definition of Q = Z_0/R_load, but it wouldn't it also mean that the speed of light is decreased by something like 10 orders of magnitude in a typical circuit?
Looks like I have some reading to do...

Re: Coaxial Marx Generator for EMP
Proud Mary, Tue Apr 08 2008, 06:54PM


Mayes doesn't mean that at all. Mayes is a business-minded scientist, and Farr a science-minded American businessman. By 'conventional,' Mayes means in comparison with his 'wave erection Marx,' which is the thin basis for his academic claim. Farr also has to promote his technology as new to sell it to his military sponsors.

Did you notice that Farr was not able to stop the transmission line to his 'impulse radiating antenna' from flashing over? That's because he couldn't match the impedance, isn't it?

Now, it seems to me that you could use a Marx pulse to excite a tuned circuit, but not with any great efficiency - as with Farr's transmission line flash-over. If you decide to do this, I would suggest a lumped constant tuned circuit such as a cavity resonator filled with oil, and immersed in oil, as a good place to start.

Notice that Farr has dumped most of his antenna energy into a network of terminating resistors, but even then the system would still flash over.

I know of no other technology but Pulse Forming Lines for impedance matching of the Marx output, and I cannot think of another matching technology that would not flash over and/or be destroyed by the output pulse.

Anyway, I shall be most interested to see how you get on with your experiment.

Re: Coaxial Marx Generator for EMP
Dr. Shark, Thu Apr 24 2008, 02:20PM

After I finally obtained (hopefully) all of the needed materials, construction of the generator is underway, so I am starting to put some consideration into the antenna structure. I have been browsing through the Radar Handbook, a true gold mine for all things RF, and I got interested in Phased Array Antennas, more specifically planar arrays. From my (naive) point of view this would solve two problems at the same time:

• Virtually arbitrary impedance by serial / parallel connections
• Very high gain if the array is made large enough

The second point is really a question of size constraints, at the very low frequencies the generator is radiating, anything remotely directional would be prohibitively large. But the impedance matching point may be more interesting: My generator is largely capacitive, so it looks like a 10 Ohm source rather than a 100 Ohm source. So if I have an array with 10 antennas in parallel fed by individual 100 Ohm coax cables, presto, I have perfect impedance matching.

Now the question remains, how do I split the 10 Ohm output into ten 100 Ohm outputs? This is probably not trivial, right?

Re: Coaxial Marx Generator for EMP
Proud Mary, Thu Apr 24 2008, 09:20PM

Why don't you connect a TEM UWB horn made out of sheet metal or PCB board directly to the Marx ouput terminal, Sharky?

Re: Coaxial Marx Generator for EMP
Adam Munich, Mon Jan 23 2012, 06:21AM

Ever finish this project Dr. Shark?

Re: Coaxial Marx Generator for EMP
jpsmith123, Mon Jan 23 2012, 06:16PM

It'd be nice to see someone come up with a decent EMP generator. The way things are (unfortunately) going, something like that might come in handy some day - as a method of self-defense against drones, which seem destined to fill our skies.

Re: Coaxial Marx Generator for EMP
haxor5354, Sun Jan 29 2012, 02:09PM

how does EMP generators really work? does it just induce a current in the wires and circuit board traces to fry the electronics? so its like a long ranged version on an induction heater?