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Antenna Theory Question
As you mentioned, Thevenin does apply, and your effective area formula
is correct (gain w.r.t. isotropic radiator). So a thin halve wave dipole antenna has an internal resistance of about 70 Ohms, and half the power is lost, but not as heat in the antenna. If I may add just a simple observation to the exhaustive Wim's explanation, I would say that we generally tend to consider resistors as things that necessarily dissipate power, turning it into heat. That is not always the case. As a matter of fact, having current in phase with voltage (what we usually call resistance) signify the transformation of electrical power into any form of power, not just heat. For instance: - an ideal electrical engine (no ohmic loss, no friction) fully transforming the applied electrical power into mechanical power. It looks like a resistor, but no heat is generated anywhere - an ideal antenna (no ohmic loss) fully transforming the applied electrical power into electromagnetic power. Again it looks like a resistor, but no heat is generated anywhere .. 73 Tony I0JX |
#2
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Antenna Theory Question
Antonio Vernucci wrote:
- an ideal electrical engine (no ohmic loss, no friction) fully transforming the applied electrical power into mechanical power. It looks like a resistor, but no heat is generated anywhere No, it looks like a resistance, not a resistor. There is one IEEE definition for resistor. There are two IEEE definitions for resistance. A resistor with current dissipates power. A resistance may or may not dissipate power. One definition of resistance in the IEEE describes a dissipationless resistance. There is no such thing as a dissipationless resistor. -- 73, Cecil http://www.w5dxp.com |
#3
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Antenna Theory Question
- an ideal electrical engine (no ohmic loss, no friction) fully
transforming the applied electrical power into mechanical power. It looks like a resistor, but no heat is generated anywhere No, it looks like a resistance, not a resistor. There is one IEEE definition for resistor. There are two IEEE definitions for resistance. A resistor with current dissipates power. A resistance may or may not dissipate power. One definition of resistance in the IEEE describes a dissipationless resistance. There is no such thing as a dissipationless resistor. It IS a resistance, but it LOOKS like a resistor because, from the exterior, one has no means to determine whether energy gets dissipated or transformed in some other form. 73 Tony I0JX |
#4
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Antenna Theory Question
Antonio Vernucci wrote:
It IS a resistance, but it LOOKS like a resistor because, from the exterior, one has no means to determine whether energy gets dissipated or transformed in some other form. Appearances can be deceiving. It doesn't look like a resistor to me until I hold it in my hand. -- 73, Cecil http://www.w5dxp.com |
#5
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Antenna Theory Question
Cecil Moore wrote: There is no such thing as a dissipationless resistor. -- 73, Cecil http://www.w5dxp.com Andy comments: Not to make too fine a point on this, Cec, but I , personally, would consider a perfect 52 ohm antenna to be a dissipationless resistor......... although, in the cosmic sense, the power that is fed into it will eventually reside as "heat" in the bowels of the cosmos.....somewhere...... Actually, .... "heat" is simply EM at a different frequency.... ..... Hmmmmm... I see a deep, philosophical discussion brewing here..... .. ... Me brain hoits......!!!! Andy W4OAH |
#6
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Antenna Theory Question
On 1 Apr 2007 09:16:46 -0700, "AndyS" wrote:
I see a deep, philosophical discussion brewing here.... Only if it starts out with: "Who's on first?" |
#7
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Antenna Theory Question
AndyS wrote:
Not to make too fine a point on this, Cec, but I , personally, would consider a perfect 52 ohm antenna to be a dissipationless resistor......... although, in the cosmic sense, the power that is fed into it will eventually reside as "heat" in the bowels of the cosmos.....somewhere...... Yep, a common misconception. The feedpoint impedance of a standing-wave antenna, like a 1/2WL dipole, is a *virtual* impedance equal to (Vfor+Vref)/(Ifor+Iref). It may be a resistance, but it is not a resistor. It is essentially the same entity as the impedance looking into a stub. For a resonant 1/2WL dipole the equation becomes: Feedpoint resistance = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) With Vfor at a reference angle of zero, Vref is at 180 degrees. With Ifor at zero degrees, Iref is also at zero degrees. Thus the ability to add and subtract magnitudes directly. Some guru once challenged me to make a measurement at the shack and tell the difference between a 50 ohm dipole and a 50 ohm resistor. Told him all I needed was a field strength meter. :-) The words "impedor" and "resistor" are reserved for real physical devices. The words "impedance" and "resistance" have two meanings and can mean either real devices or virtual V/I's. So says the IEEE Dictionary. -- 73, Cecil http://www.w5dxp.com |
#8
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Antenna Theory Question
On Sun, 01 Apr 2007 18:24:33 GMT, Cecil Moore wrote:
AndyS wrote: Not to make too fine a point on this, Cec, but I , personally, would consider a perfect 52 ohm antenna to be a dissipationless resistor......... although, in the cosmic sense, the power that is fed into it will eventually reside as "heat" in the bowels of the cosmos.....somewhere...... Yep, a common misconception. The feedpoint impedance of a standing-wave antenna, like a 1/2WL dipole, is a *virtual* impedance equal to (Vfor+Vref)/(Ifor+Iref). It may be a resistance, but it is not a resistor. It is essentially the same entity as the impedance looking into a stub. For a resonant 1/2WL dipole the equation becomes: Feedpoint resistance = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) With Vfor at a reference angle of zero, Vref is at 180 degrees. With Ifor at zero degrees, Iref is also at zero degrees. Thus the ability to add and subtract magnitudes directly. Some guru once challenged me to make a measurement at the shack and tell the difference between a 50 ohm dipole and a 50 ohm resistor. Told him all I needed was a field strength meter. :-) The words "impedor" and "resistor" are reserved for real physical devices. The words "impedance" and "resistance" have two meanings and can mean either real devices or virtual V/I's. So says the IEEE Dictionary. Amen to that, Cecil Walt, W2DU |
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