Afghan Bridge Update and Sad News

200 Watts of power to

the 2 foot path. In

watts at a distance of 2

20 Log F (in mHz)

Does the power absorbed by the load increase by much when one uses a ~30 db gain parabolic antenna instead of that dipole?

Would'nt the ERP off the end of that feedhorn then be in the vicinity of say 120 KW?

--Winston

I worked in two-way radio/RF for most of my career. It really isn't all that easy to shield things from RF. What looks good to the eye can leak horribly and what you would swear could never work does...

But would RF deliver enough amps to cause detonation?

i

IF you got lucky and hit a resonate frequency, maybe, but I really doubt if this is useable. Remember these guys have their own communication equipment that can't be knocked off-line for any length of time either. If you make something powerful enough to detonate the IED's they will most likely wipe out their own communications. I wouldn't be surprised if on-board computers in vehicles went berserk and shut down too at the levels it would most likely take.

Most of out local commercial broadcast radio and TV stations are pumping out at least 50,000 watts, some more. Do these seem to have any noticeable effect on stuff that you've seen? Think about how much power it would take to do something like what you are considering when a 50,000 watt station does bother things all that much.

If I was going to try and make something it would look like a four wheel dumpster that you push down the road ahead of the vehicle. An angled heavy blast plate towards the vehicle to deflect blasts up/over and wheels that track the same as the vehicles. It should be heavy enough to trip the IED pressure plate. A quick disconnect hook-up so it could quickly released and jettisoned from the inside of the vehicle (shrug). KISS

Why do you think it would take so much power? How much power do you think it takes to initiate an electric cap?

Hint: U.S. military caps are specified to initiate with 16.3 millijoules of energy delivered in a period of not less than 10 milliseconds nor more than 24 milliseconds. That's a power level of between 0.67 and 1.63 watts.

This isn't to say that the notions of flails and heavy blast-proof non-manned vehicles aren't viable or even possibly best approaches, only to argue that while they seem to be obviously viable approaches they are not obviously the only viable approaches.

May be they did, may be they did not. But what they would definitely say - "get the hell out of this country, it's not yours".

C.

I don't know much about blasting caps Don, but if it was as easy as you say they would be going off all over the place prematurely. Fire up an RF spectrum analyzer and take a look around (preaching to the choir here :), the airwaves are filthy nowadays. A key tower site probably has 2 to 3+

50,000 watt transmitters, plus numerous (25-100) other transmitters. The RF fields around these sites is pretty impressive. Most of the people that used radios that I serviced never bothered to read the warning tags that were attached to the microphone.

I have read/know about the warning about using two-way radio equipment in "blasting zones", but have never heard any accounts of it touching off explosives. Have you heard/know of any reputable examples of this happening? I get a lot of Google hits on this and read some of the more promising ones, but the radio involved wasn't to blame in the ones I chose. I'm sure it has happened, otherwise there would be dire warnings concerning this.

I've heard a lot of interesting tales through the years and seen some stuff myself concerning radio. I'm not saying it can't be done, but I think it would take a lot of power and some luck. It would also probably wipe out/interfere with their own communications too. Crap, just the onboard vehicle computers used to wipe out a radios receiver at times.

I wonder if they still use blasting caps like you described today. The use of radio type equipment has skyrocketed in the Armed Forces. I would be concerned about them setting off their own stuff prematurely.

Wires to caps are usually twisted. It's pretty hard to couple much RF into twisted pair. They're kept shorted at the user end until just before detonation.

In an IED, the wires must spread enough to at least accomodate a battery. That isn't much, but very short wavelength could do it, like

10 to 20 GHz. Very directional antennae of managable size are possible (and usual) at those wavelengths.

Yup. I took those numbers from a 2005 spec.

The biggest problem I see with trying to do this is jamming up your own equipment, causing either radio or vehicle malfunction. How long do you think it will take for the creator to use this to their advantage, ie plant the IED knowing fully well that the Military will help detonate it for you with a transmitting device.

Trying to create a tool to work against a morphing target won't be simple, if not impossible. I would be curious to know too if the majority of the IED's even use a blasting cap or some other sort of detonator. Are they buried in a dirt road, paved, culvert? Not knowing some of these questions makes it even harder to suggest remedies.

I can think of lots of ways to defeat what we are trying to create, I suspect the bad guys can too (shrug).

Excuse me for jumping in at this late date, but what are you guys trying to do, transmit a lot of radio-wave energy to IEDs? If so, you're going to have to get 'way up into the microwave region to transmit any significant energy, right? E=hf or E=hv, where h is Planck's constant and all that.

If I'm missing the point, then never mind. d8-)

-- Ed Huntress

(...)

You got it Ed.

I figure a 200 W microwave amplifier should result in ca 100 KW ERP at the feed horn of a 30" diameter parabolic dish. After allowing for space attenuation and shielding, would it reduce the IED to carbon quickly enough to make it safe?

--Winston

Practical microwaves are not my department; I only have brushed with theoretical stuff, from long ago when I was studying for a radar endorsement.

You may want to look at the millimeter-wave (95 GHz) Active Denial system, V-MADS, to see if it provides any clues. They don't seem to transmit much power, just enough to heat up the target (human) to an uncomfortable degree. And the systems tested for missile defense run around 4 trillion Watts: a

12-16 million Amp pulse with a rise time of 400 nanoseconds. These are powered by big flux compressors.

I guess the needs here fall somewhere in between. d8-) There's a lot of comment that's been published on microwave weapons, so it should be researchable.

-- Ed Huntress

(...)

Thanks Ed.

I am sure there is enough microwave talent on this NG to produce a reasonable guess as to practicability, frequency and power level.

--Winston

You may be confusing ERP and presumably ERIP (power relative to that radiated by a dipole or an isotropic radiator respectively) with raw power. ERP is the amount of power you would have to feed to a dipole to get the same energy that the directional antenna delivers to a given area. A high gain antenna does not generate power; it puts a given amount of power into a smaller area. If you need 200 Watts of power coupled into something for some purpose, you will need to feed the antenna with a lot more than 200 Watts.

Kevin Gallimore

Yes, if you couple 200 W into a ~30 dbi parabolic antenna, it will radiate as much power (into a spot) as would a dipole driven by ~100 KW (into the same area, though the dipole is radiating isotropically).

(Power != field strength, always.)

Yes. Just the ticket for inducing eddy current and the resulting power dissipation in conductors within a limited, steerable aperture.

Yes. But the necessary power into the load (sans resonance) could be as little as say 2 W given Don's range of 0.67 W to 1.63 W. My question is: if we provide a 30" diameter field that has a power density of 2 W at 24" and we are radiating at the resonant frequency of the initiator, would that reliably disarm or failing that, trigger the device?

--Winston

The problem is more fundamental. While high-power microwaves (HPMs) can and have set ordinance off, it's very random as it depends on accidental resonances and shielding flaws and is in any event quite inefficient. Safety regulations are designed to make a low rate essentially zero, to cut down on fratricide. However, the natural low rate is really too low to be militarily effective.

Nor is the enemy stupid - if we start neutralizing too many IEDs with HPM, pretty soon IEDs will have improved shielding. Shielding is far cheaper than deployment of sufficient HPM gear, making the economic tradeoff unattractive. Some other way will be found.

Joe Gwinn

Funny you should mention this. I just read an article in the paper last night about this, gives me the creeps. They have some portable units now (if you call a Humvee portable) and are just itching to deploy them in Iraq. See:

There are plenty more articles around if you care to search.

I don't think that it will produce the results that they are hoping for...

But they could soup them up a little bit and they'd have a novel way to cook a turkey. d8-)

-- Ed Huntress

I'm already wondering if another parabolic dish could reflect this right back at them or other "good guys". Also what type of surface or objects may do the same. Interesting to ponder and I'm sure others are thinking along the same lines.