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#41




Dr. Slick wrote:
"I don`t have that book." I don`t have Kraus either and I miss it. Space has a magnetic permeability and a dielectric constant. The square root of their ratio is the characteristic resistance of space. It is 376.7 ohms = 120 pi ohms. The reciprocal of the square root of the product of the permeability and dielectric consrtant of space is the velocity of EM radiation propagation. It is 300 million m/sec. The above is courtesy of King, Mimno, and Wing, in "Transmission Lines, Antennas, and Wave Guides", on page 73. The authors must have considered the information important as they repeated it on page 117. They followed the repetition with a discussion of the radiation resistance and input resistance of an antenna. They note that radiation resistance can`t be measured between two terminals in a circuit. The I squared R of the antenna power does not conveniently compute as might be expected with circuit terminals, as current is a variable along an antenna in most cases. Best regards, Richard Harrison, KB5WZI 
#42




Dr. Slick wrote:
But an antenna must be performing some sort of transformer action. Not quite  but there is a word for what it does: it's a transducer. A transducer is any gadget that converts energy from one form into a *different* form. Examples include a loudspeaker (electrical energy to sound/mechanical energy), a microphone (the reverse), a light bulb and a photocell. From that point of view, a resistor is a transducer that converts electrical energy into heat energy... but it also has some useful electrical properties :) An antenna is a transducer that converts electrical energy into E and H fields, and the reverse. You'll also notice that all practical transducers convert some of their input energy into heat energy. It's a useful word for a useful idea. (Cecil  can your IEEE Dictionary help us with a formal definition?) On the other hand, if you insist on using the word "transformer", you'll keep on believing you can work out new facts about antennas from what you already know about transformers: If an antenna is not a transformer of some type, then why is it affected by it's surroundings so much? They obviously are, just like the primary's impedance is affected by what the secondary sees in a transformer. That's a perfect example of the trap, because in reality it's not "just like". An antenna also has Efield interactions with its environment that a transformer doesn't have, so any resemblance will literally be only halftrue.  73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek 
#43




Dr. Slick wrote:
"What does it say?" I don`t have Kraus, unfortunately. I do have Arnold B. Bailey`s "TV and Other Receiving Antennas". Bailey covers more antenna territory than most, and does an excellent job of it. Bailey also includes a catalog of antenna types, all sized for 200 MHz for easy comparison. Bailey says the surge impedance of an antenna is inversely proportional to the capacitance per unit length. Reminds one of a transmission line. This is nonuniform, so Bailey has an empirical equation which says the larger the periphery of the rod, ther smaller the average surge impedance. The ratio of the electric field to the magnetic field surrounding an antenna must be related to the ratio of volts to amps in the antenna wire (the surge impedance). The surge impedance of a thinwire 1/2wave dipole from page 500 is 610 ohms (average). The surge impedance of a fatcylinder 1/2wave dipole from page 502 is 240 ohms (average). Pattern and gain are identical for both antennas. But, Dr. Slick may be on to something after all. The bandwidth of the fat antenna is about 3X that that of the thin. In antennas, bandwidth is often an indicator of match. Best regards, Richard Harrison, KB5WZI 
#44





#45




Ian, G3SEK wrote:
"Examples include a loudspeaker." Good transducer example. Its problem is abysmal efficiency, even if better than the usual incandescent lamp. The loudspeaker`s efficiency can be improved by a better match to its medium. The usual loudspeaker is small in terms of wavelength. A result is that it is capable of exerting much force on a small area of a very compliant medium, air. Air could better accept power exerted over a much larger area, especially at low frequencies, with less force required to make the air move.. We have a highZ source and a lowZ sink in the loudspeaker and air. Conversion from electric power to mechanical power can be more efficient through better impedance matching. Two solutions are often used for a better match, a larger loudspeaker or a horn between the loudspeaker and its air load. The larger speaker is directly a better match. The horn is an acoustic transformer. They both improve energy conversion efficiency. Best regards, Richard Harrison, KB5WZI 
#46




W5DXP wrote in message ...
Dr. Slick wrote: But a Black Box to me implies you have limited information from it. Black boxes radiate heat very well. :) Only if they're dummy loads! Slick 
#47




Roy Lewallen wrote in message ...
I'd be one of the people arguing. Radiation resistance fits every definition of resistance. There's no rule that a resistance has to dissipate power. The late Mr. Carr was quite apparently confusing resistance with a resistor, a common mistake. Your point has been well taken, Roy. But you have to admit that radiation resistance is not a easily understood concept (which is why it may be a common mistake), so for someone to call it a "fictitious" resistance can make sense, in the sense that it is not a dissipated resistance. After all, "Imaginary" numbers are well accepted. And from an arguing sematics point of view (which is unfortunately necessary sometimes), even you call it "radiation resistance", which means that it is obviously not the same thing as a dissipative resistance like a 50 Ohm resistor. That being said, rest assured, Roy, that you have convinced me! Slick 
#48




Roy Lewallen wrote in message ...
I'd be one of the people arguing. Radiation resistance fits every definition of resistance. There's no rule that a resistance has to dissipate power. The late Mr. Carr was quite apparently confusing resistance with a resistor, a common mistake. BTW, did Joseph Carr really pass away? Sad, his book is very practical. Slick 
#49




On Wed, 16 Jul 2003 18:20:22 0700, Roy Lewallen
wrote: An antenna can reasonably be viewed as a transducer. It converts the electrical energy entering it into electromagnetic energy  fields. As is the case for any transducer, the stuff coming out is different than the stuff going in. Think in terms of an audio speaker, which converts electrical energy into sound waves, and you'll be on the right track. Roy: Great analogy! The characteristic acoustic impedance of air (standard temp & pressure) is about 413 Rayleighs (or PascalSeconds/cubic meter). Do we worry about matching 8 ohms of electrical speaker impedance to 413 Rayleighs? C.f. Paul Klipsch and the Horn speaker. I wonder if much of the antenna radiation resitance/Tline impedance/reflection/intrisnic impedance of free space confusion stems from use of the same words to describe things that may be modeled mathmetically identically, but have different physical modalities? In heat sink calculations, for example, we use "thermal resistance" and an Ohm's law model but few would confuse ohms of resistance with degrees C/watt. Jack K8ZOA 
#50




Dilon Earl wrote:
Where does the loss occur? If you have 3 db of mismatch loss, is it in the coax, tank circuit? The loss in "mismatch loss" refers only to the fact that the power delivered by the generator to the load is less than it would be if the load resistance were the same value as the generator resistance, in other words if the load and generator were "matched". The best way to get a handle on this subject is to draw a diagram of a generator with voltage V=10, an internal resistance of 50 ohms, and a load resistor of R ohms. Let R vary from 1 ohm to 100 ohms and calculate the power dissipated in the generator resistance (50 ohms), the power in the load resistance (R), and the total power. Plot a graph of the three quantities. The load power goes through a maximum when R=50 ohms. The maximum power dissipated in the generator resistance is 10^2/50=2 W, which occurs when R=0 ohms. The minimum power dissipated in the generator resistance is 3.33^2/50=0.22 W which occurs when R=100 ohms. When R=50 ohms, the load power is 5^2/50=0.5 W (the maximum value), the dissipation in the generator resistance is 5^2/50=0.5 W and the total power is 10^2/100=1 W. Bill W0IYH 
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