MAH replied the topic: Performance of Limiter with poor match on o/p

Ok, I will give the NL Capacitor a try.

Many thanks

madengr replied the topic: Performance of Limiter with poor match on o/p

Probably depends on the diode model, but all the models I'm looking at in MWO do not have a non-linear capacitance. That capacitance is not supposed to vary anyway in a PIN diode since the charges move too slow.

Maybe instead try a non-linear capacitor. Then using a DC+ 2-tone HB simulation, use the first tone as a drive signal, then the second as a probe signal, using it look at input impedance. It would have a negative resistance to cause the oscillation. The DC is needed since the phenomenon may not work if the swing goes below 0V and the capacitance curve has odd symmetry.

This is all just a guess on my part; may try it today if I find time.

MAH replied the topic: Performance of Limiter with poor match on o/p

Following on from Walshrj99's question. I am attempting to model the phenomina using a generic ADS PIN diode model with little success. Would the generic non-linear ADS pin diode model be expected to produce this effect?
I have simulated over a wide input power range with a load pull circuit on the output.
Many thanks

Walshrj99 replied the topic: Performance of Limiter with poor match on o/p

Thanks very much

madengr replied the topic: Performance of Limiter with poor match on o/p

It's a parametric oscillation. Your RF signal modulates the diode capacitance, generating a negative resistance, which allows oscillation determined by some other resonance in your circuitry, probably your sliding load. I have also seen them in limiters, sometimes generating a wide-band noise burst as the limiter turns on, then it settles down. Anyway, used to be a common method of amplification prior to GaAs FET.

Steve Cripps has a Microwave Bytes on it here:

ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7010391

Mason replied the topic: Help with Weaponized Microwaves

Thanks again, Dave. I expected the equation would be magnificently difficult. I wish I could find experiments of this nature, as that can make calculations much easier. Strangely, I found very little in the ways of building charge via radiation (only a couple at the tiniest scale).

Do you know of a member of the forum who may be experienced with this? I would like to PM them for advice.

I can't thank you enough for your help. I looked everywhere for the answers to my questions, and came up with almost nothing. Now, I have pretty well everything I need. I'll look into Maxwell's equation.

Thank you.

Dave replied the topic: Help with Weaponized Microwaves

Hi Mason,

Short version: yes.

Long version: it's strongly dependent on parameters like the target's shape, range, electrical size, and you may need Maxwell's equations to work it out. At this point, I bow to a number of people on this site whose knowledge on this topic far outstrips my own.

Mason replied the topic: Help with Weaponized Microwaves

That is a good point on the arcing. I think you'd make a pretty good scifi writer.

I would like to ask, as unfortunately I couldn't find the information. Do microwaves build up charge in metal, outside of a microwave oven? More specifically, I was hoping to find a formula to calculate the buildup of electric charge, so I can calculate how what sort of a tool, power and timeframe would be necessary to build up the charge.

Sorry, these details will no doubt be on the site. If the engineering challenges are steep or impossible today, it should be all right. The characters using these weapons are from an advanced alien civilization.

Thank you for your great help, Dave.

Dave replied the topic: Help with Weaponized Microwaves

Hi Mason,

One point to note: the field strength I picked is roughly enough that it might cause arcing. Most microwave components are far more sensitive than that, and will be blinded or destroyed at much lower power levels. So if your antagonists are only using "smart" weapons with guidance packages, that's a plausible solution.

Best frequency is a great question. Low frequency propagates better, and is easier to generate the power, but high frequency is better for penetrating the inevitable gaps in the structure.

You could, of course, go for some pseudo-scientific mumbo jumbo like this:
The pi-ratio was first discovered by Australian stealth researchers in the late 20th century. When the target is coherently illuminated with pure sinewaves at the two pi-frequencies (3.1415926 GHz and 31.4159265 GHz), it induces a high-Q resonant mode in any structure with radiused components. The result is to increase the effective power by a factor of 1000, instantly destroying any nearby electronics and causing the structure itself to disintegrate.

That's all pidooma, but might be plausible to some... that said, I'll stick with engineering and let the authors do the writing :)

Is it feasible to generate that much power at those frequencies in a single emitter? Honestly, probably not. Not with today's technology anyway. If it was, Boeing, Lockheed Martin, Raytheon etc would love to know about it.

Mason replied the topic: Help with Weaponized Microwaves

Thank you Dave! That is perfect! I see that the microwave radiation shield doesn't work out quite as hoped. You'd have to use something like 100GW to get any effective range. Even if you only covered half the area, things don't improve by much.

My friend and I came to the same conclusion as yourself. Better to go with more of a maser. It could adjust its power and spot size for a few purposes. Radar, jamming, maser against flesh and electronics, and an ionizing maser. The latter not being intended to destroy the target, but to make it vulnerable to an electro-laser or spark-coil.


If I may, I'd like to ask you about one other thing. The best frequencies for such a tool. And whether it is feasible to have one microwave emitter capable of producing all of them.

Thank you again, Dave. This has really helped with the story. Microwaves are quite interesting, so I hope to get them accurately.

Dave replied the topic: Help with Weaponized Microwaves

Hi Mason,

A couple of useful pages to get you started:
www.microwaves101.com/encyclopedias/power-handling
www.microwaves101.com/encyclopedias/atmospheric-breakdown

10 kW will cook some electronics, but for plausible arcing (e.g. the field is so strong that incoming weapons self-destruct as soon as they get close) consider bumping up your sources to MW levels. By way of comparison, using some example figures from the textbook Microwave Engineering, by D Pozar, coaxial cable breaks down at 520 kW and waveguide breaks down at around 2.3 MW.

From the charts on the power handling page linked above, it looks like you need a field strength of about 3 MV/m to cover a 100 mm gap at sea level (1 atmosphere). This drops with altitude, if the pressure was only 0.01 atmospheres then 30 kV/m would probably do it.

In terms of range, lets assume your shield is spherical around the spaceship. The relationship between field strength, power, and distance is:

E = sqrt(30*P)/r

Rearrange to solve for distance

r = sqrt(30*P)/E

So, to get 30 kV/m from a 10 kW source, you've got an effective range of 18 mm. Hmm, not much.

Bump up the source to 10 MW, the effective range is 577 mm. Still not great.

At 1 GW, the effective range is still only 5.77 m.

If it fits your story arc, it might be more plausible to suggest a smart system (more like your microwave laser) that sends a focused beam to each target. Give it a 50 dB gain antenna, which has around a degree beamwidth, and you can get that same 5.77 m range from a 10 kW source. A still plausible 10 MW source now gets you 180 m of range. 100 MW pushes that out to 577 m, and a 1 GW source with a 50 dB antenna will happily fry things at 1.8 km range.

Hope this is helpful, good luck with the story!

rzvc replied the topic: How to get a microstrip out from under a SMA connector?

Yeah, I guess I'll have to do that. I was hoping there was a standard way of doing it and I missed it.

Thanks.

tc replied the topic: How to get a microstrip out from under a SMA connector?

How is the connector oriented. Is it parallel to the substrate with the connector body touching the microstrip or is it vertical to the substrate, maybe on the backside with the inner conductor penetrating the substrate and then connected to the microstrip line metal? If it is a vertical connector, I would use a 3D EM simulator to work out the best configuration for a match.

rzvc replied the topic: How to get a microstrip out from under a SMA connector?

Hi,

There's soldermask between the connector's body and the microstrip. What I'm wondering is how to keep the 50 ohm impedance while routing that microstrip out of there.

When the microstrip is wide due to the height of the dielectric, there's going to be a bit of capacitance added because the body of the connector will be much much closer to the trace than the reference plane. I'm wondering how you would normally deal with that.

A different type of connector would definitely be a solution (edge mount for example), but let's say you can't do that and you have to use a through hole connector.

tc replied the topic: How to get a microstrip out from under a SMA connector?

I need a better understanding of the connection you are describing. As stated it seems as if the microstrip line on the substrate is touching the outer conductor of the SMA which means that the inner conductor is open(?) and the microstrip line is grounded. This is a bizarre connection. To start, I would use an exacto knife to cut the microstrip line away from the outer conductor of the SMA and then connect the inner conductor to the microstrip by a wire bond extending over the gap in the microstrip line you just created. For low frequencies this might be enough but at higher frequencies you would need to tune the inductance of the wirebond and the capacitance of the gap to try to get a better match. This is very simplistic but its a start.

sm4rzw replied the topic: waveguide cut-off and the attenuation below that

Yes, I 've learned that cut-off frequency doesn't mean totally stop, but rather like -20 loss. My first mistake. The steel ball, yes it was a small bearing ball, like 4mm.
I just took a SMA-T-coupling, (which I normally hate because people connect things without having a clue of the impedance), put a peice of .141 in one female port, soldered the shield to the threads, hooked it to the VNA and started with the file ;) It's great fun!
As for the smaller waveguide, that would have been WR112. But I needed the passband down to 5.7 GHz. But yes, it would probably worked better, it's easier to handle 10 dB loss in passband and -80 to kill the carrier power than 2dB loss in passband, and -20 to kill the carrier.. I guess it all cooks down to the meaning of Fco, repeating to myself " It doesn't mean totally silent, but -20 dB loss" Once more, thanks for helping

Desert Sage replied the topic: waveguide cut-off and the attenuation below that

Thank you for closing the loop and letting us know results. Couple of thoughts. I should have checked dimensions. You are operating only about 7.5% below cutoff. The next smaller guide would have worked better. The steel ball sounds too large. BB to pea sized I estimate. But your solution works. Thanks again.

sm4rzw replied the topic: waveguide cut-off and the attenuation below that

With 3" WR137 the attenuation improved from -17 to -28 dB. Still too much. Tried the trick with a steel ball and a magnet, but every position only made it worse... Desperately, I ac-hoc'ed a stub notch out of .141 semirigid and tuned to 4100 MHz. In cascade with the waveguide adapter filter it became perfect. The combination of a file and a network analyzer is great sometimes ;)

sm4rzw replied the topic: waveguide cut-off and the attenuation below that

Thanks for quick reply :) Not that used to waveguides, but I'll learn. Those wg-to-coax adapters are quite short, so the radiators inside them get's quite close, thats why I had that thought. Will try to track down a 3" or 4" WG on ebay and try it out.

Desert Sage replied the topic: waveguide cut-off and the attenuation below that

Yes, small length of waveguide should work. WG below cutoff is surprisingly effective as HPF. Surprised how little rejection you are getting. If I wasn't lazy I could calculate how much guide you need but you forgot to say how much rejection you were looking for. As a guess, a couple inches should give you plenty.