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Although not an all-round solution, here is my starting point - a shield for the car key RFID tags As the frequency goes up, you will need more shielding. The good news for passive tag shielding is that shields work "twice as well", forcing anyone trying to work through it to both generate higher powers to transmit in through the shield, and read even smaller return signals mostly blocked by the shield.

If you have a card and a reader, you are in a good position to experiment. Perhaps a simple foil lining for your exisiting wallet will be sufficient. This could constitute an unusual DoS attack, particularly if you have a clustered population of users that depends on the RFID system. On weather: at the frequencies and distances for normal RFID the attenuation from weather should be insignificant. It might begin to matter for active RFID.


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On the other hand, antenna flaws and shorts from wet antennas, multi-path reflection, etc. These are too complex to answer without real situations to analyze.

On terrain: the They do not reflect off the ionosphere like HF. They do not have a ground wave component. They do difract a little around barriers, but not much. They do reflect off metal reflectors and the like, but that probably doesn't help with use in mountains. They are attenuated by rain and slightly by fog. Losses are up to a few db per kilometer of rain. They penetrate wood and most dry non-conducting materials adequately for their purpose as office networks. Conducting metal structures confuse everything with absorption, retransmission, and multi-path reflections. Terrain is a major obstacle.

The team said they believed their setup could theoretically reach miles but they were limited by line-of-sight fresnel zone issues. For example, hot surfaces can distort radio waves in a manner similar to light waves and water absorbs waves in the 2. And that's not to mention that Rain scatter is a significant problem at higher frequencies as is any water vapour - so a humid day will have a higher path loss than a very dry day. If the received signal is below the minimum discernable signal mds level at the receiver you can't demodulate decode it.

The MDS can also be affected by noise interference either generated by man made devices or from outer space solar radiation, etc. Any active device in the signal path will increase the noise as well as the signal, and so the 2 easiest places to increase the received signal are the antenna s e and the feed line.

A higher gain more directional antenna reduces the path loss by aiming the signal in one direction in the same way as a reflector in a car headlight. Better coax or wave guide reduces the losses between the antenna and transceiver. Other questions; 1 At these frequencies signals are line of sight so beaming over a mountain is not going to work. Silver foil heat insulation would be the worst offenders along with corrugated iron although I can testify that 3ft of granite, blocks signals too. Wet wood would also be bad and concrete especially reinforced whereas dry lining plasterboard shouldn't have too much effect.

I appreciate the info re: terrain and weather. The next time some yahoo tells me he can give me WiFi coverage in across the mountains of Montana without a repeater, I'll be armed with the knowledge to prove that he's up in the night. We've been able to go for good distances in relatively flat land nowhere near the record - but as you mentioned, line of sight is the name of the game.

Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Most molecules are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and is therefore twisted to and fro as it tries to align itself with the alternating electric field induced by the microwave beam. This molecular movement creates heat. Microwave heating is most efficient on liquid water, and much less so on fats, sugars, and frozen water. Microwave heating is sometimes incorrectly explained as resonance of water molecules, but this occurs only at much higher frequencies, in the tens of gigahertz.

RFID tags come is self-powered and externally-powered flavors. Making conclusions about RFID range is stupid if you don't know what kind you are dealing with. It isn't hard to trigger one from a distance.

A Mobil SpeedPass is externally-powered and operates at kiloHertz. I defy you to trigger one from more than a few inches. All this crap about RFID is just that For those interested in science rather than conjecture, baloney and the like, here are a few facts: The wavelength corresponding to 2. Therefore a dipole is in the order of a couple of inches in length. That's a lot bigger than a water molecule. The major absorption lines in the microwave spectrum are around 26 GHz and 60 GHz, corresponding to the water vapor line and the oxygen absorption line.

Absorption at 2. Most of the older satellite links are in the S-band GHz region. If there were a lot of absorption, they wouldn't be there, would they? As to range of radio waves, nothing is going to beat the laws fo physics. Comtech Telecommunications Corp. The V3 also operates in the 4. Both versions meet full military specifications for tactical deployment. They may be shipped by military or commercial transport.

The terminals are shelter mounted, can be operated on their transport vehicles and site preparation is generally not necessary. Emplacement or march-order pack-up does not involve special tools, and the time required for set up ranges from 1 to 5 hours. High wind and lightning protection kits can be supplied. Heating and cooling of the equipment is independent of environmental control of the personnel area and permits system operation over the full range of climatic conditions likely to be encountered in worldwide deployment.

Still, would be interesting to see what could be done with The WiFi record set is building on interesting stuff that many practice plus, of course, the team's wonderful work! I tend to believe things that "ECMpukes" say about what is doable and what is not - at least, I think that they have the experience, gear, motivation, funding, and franchise that most of us lack in the realm of "signals" Troposcatter is pretty inefficient.

The main advantage is that the scattering centers are always there. Hams have found that aiming at mutually visible thunderstorms can be much more effective over s of km -- but you need the storm to be there. People also use aurora and meteor trails for long distance scattering. These communications are of pretty low quality for data. There is lots of multipath, which is bad news for wideband data. Troposcatter is, as correctly stated, very inefficient, but pretty reliable.

You will need loads of power to make an acceptable link work. You can propagate WiFi over the horizon using meteor scatter There are lots of anomalous propagation modes for microwaves including tropospheric ducting, scatter, meteor scatter, e-layer ionospheric propagation, temperature-inversion effects They were investigated in an effort to understand some out-of-ordinary behavior of radar.

There really isn't much about radio propagation that is not already known and exploited. But you can aways believe in Santa Claus and the Easter Bunny. Meanwhile, reality will prevail.

Radio Propagation 101

Funny you should say that That being said, most people here myself included were talking about microwave ovens based on a radio wave frequency of roughly MHz, which clearly demonstrates that energy is in fact "absorbed" by water, fats and sugars. Or, if you'd prefer, the radio waves agitate the water molecules, which bump into the other molecules, which create heat from friction While the vapor won't absorb as much energy as a giant bowl of soup or huge slice of meatloaf in the sky I think moisture is still a factor to consider if you're operating at 2.

But don't take my word for it. I don't know of any cards which are battery-powered. My understanding is that they make use of the Wiegand effect, or similar proximity properties from the realm of physics. The specs from HID offer solutions which can easily be read at 2. Essentially TEMPEST is about bandwidth and energy limiting, a usefull signal will escape you if there is sufficient of either and an appropriate conducting medium to carry it.

This applies to all form of energy radiation sound, gravity not just RF. As far as the physics go with RF the limiting factor is the noise floor at your receiver input which is around db for a 1 hertz bandwidth. An antenna improves the sensitivity, the feed line decreases it a bit like trying to focus the sun into a light pipe ;.

The antennas must capture Voyager information from a signal so weak that the power striking the antenna is only 10 exponent watts 1 part in 10 quadrillion. A modern-day electronic digital watch operates at a power level 20 billion times greater than this feeble level.


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  4. Interestingly all transmission lines also have an upper frequency of operatiion for a desired mode see stuff on Wave guides and G Wires for more info. If your signal couples into say overhead power lines it might travel thousands of Km without dificulty. Conductors also act as reflectors and it is well known that when conductors are placed in favourable positions a signal will be focused in a direction Yagi TV antennas are an example of this. The atmosphear is a poorly conducting medium at the earths surface and improves with hight, as the preasure dropes it is more easily ionized, and starts to look like a fairly good transmission line ducting.

    This is why you get bad telivision interferance at certain times of the year. Front Matter Pages The Photochemistry of the Mesosphere. Pages Ionizing Radiation Affecting the Lower Ionosphere. Lower Ionosphere Ion Chemistry. Coupling Between the Neutral and the Ionized Atmosphere.