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Short antenna = reduced power
Quoting from Electromagnetism
By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... |
Short antenna = reduced power
"gareth" wrote in message ... Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Damned typo! ISBN 0 19 851801 3 |
Short antenna = reduced power
"gareth" wrote in news:m1g1n8$39o$1@dont-
email.me: (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. Ok, but again, doesn't this just mean the system, as in taking into account feeding it? I'm not up to the maths of it, I'm just imaging a kind of logical extreme where you have a tiddly bit of wire stub out of the end of a coax instead of a 9m tall vertical whip. It seems obvious to me that to get the same efficiency, same power, you have a vastly increased energy density, so even without the maths I have no problem seeing the relevance of comments like Jim's (Jeff's?) allusion to room temperature superconductors and such. In other words, any actual reduction is based on practical limits, not theory itself. It's not so different with laser diodes, in that a diffraction limited spot may be obtained easily with a simple aspheric lens from any size apeture so long as it's a signle lattidutinal mode emitter, but try actually MAKING an emitter that size. Theory says sure, no problem, energy density and nature of materials says otherwise. |
Short antenna = reduced power
"Jeff" wrote in message ... Gareth, please have a look around the web and find a copy of Kraus to download; then read, in particular, chapters 3 and 5. In particular note the following in relation to short dipoles: "Assuming no losses it [the power radiated] is also equal to the power delivered to the [short] dipole" "The maximum effective aperture of the 1/2 wavelength antenna is about 10% greater than that of the short dipole" The gains of a short and 1/2 wave dipole is also quoted as 1.76 and 2.14dBi respectively. So can we now put this to bed, the short dipole radiates well it is the practicabilities that make it a poor antenna. Jeff And along the same lines, antennas are often described in terms of isotropic (point) antennas. With radiation being related to length, isotropic antennas would not radiate. Also with effective aperture, the 10% greater you mention is a result of orientation of the aperture with respect to the maximum part of the individual antenna pattern. Considering the entire pattern of both antennas, reciprocity is maintained. |
Short antenna = reduced power
On 13/10/14 12:34, Jeff wrote:
On 13/10/2014 09:15, gareth wrote: Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... That makes no sense, at least quoted out of context, as it would imply that the power radiated was independent of the power applied. So an infinitely long antenna would radiate infinite power !!!! Jeff His problem is he is not considering the Radiation Resistance, Loss Resistance, and reactive element which determine the eff., and Zo. (The reactive element represents the energy 'stored' in the field around the antenna- just like the energy store in an inductor or capacitor, both reactive components.) A short dipole, for example, will be a poor match but RRLR. Provided the feeder loss is low, either by good matching or the use of low loss feeder (assuming the PA is 'happy') then the overall losses are low and the RF only has one place to go, to be radiated. A short dipole has other issues, in particular if matching is used to overcome the issue of the Zo, then the matching network plus antenna will have a very narrow bandwidth (compared to a full sized dipole) and adjustment will be essential to maintain efficiency if the frequency of operation is changed. Remember, the use of 'standard' Zo of 50 or 75 ohm is not essential, nor is maintaining a feeder SWR of 1.5, provided the PA can cope and feeder loss can be tolerated/reduced (eg by using open wire feeder). No doubt he will dismiss this with his usual tirade of abuse etc, but that is his normal response when corrected. |
Short antenna = reduced power
"Brian Reay" wrote in message
... His problem is he is not considering the Radiation Resistance, On the same page, the radiation resistance is defined, also including that term, for the radiation resistance is derived from the firat equation quoted No doubt he will dismiss this with his usual tirade of abuse etc, but that is his normal response when corrected. As usual, the only origination of abuse comes from you. |
Short antenna = reduced power
On 10/13/2014 11:00 AM, Wayne wrote:
"Jeff" wrote in message ... Gareth, please have a look around the web and find a copy of Kraus to download; then read, in particular, chapters 3 and 5. In particular note the following in relation to short dipoles: "Assuming no losses it [the power radiated] is also equal to the power delivered to the [short] dipole" "The maximum effective aperture of the 1/2 wavelength antenna is about 10% greater than that of the short dipole" The gains of a short and 1/2 wave dipole is also quoted as 1.76 and 2.14dBi respectively. So can we now put this to bed, the short dipole radiates well it is the practicabilities that make it a poor antenna. Jeff And along the same lines, antennas are often described in terms of isotropic (point) antennas. With radiation being related to length, isotropic antennas would not radiate. Also with effective aperture, the 10% greater you mention is a result of orientation of the aperture with respect to the maximum part of the individual antenna pattern. Considering the entire pattern of both antennas, reciprocity is maintained. Yes, but an isotropic source is an imaginary tool used for comparisons. It obviously cannot exist in the real world, but it's spherical radiation pattern can be used as a standard for comparisons. Similar to an inductor or capacitor with no resistance - only reactance. They don't exist in the real world, but are used to simplify calculations. Once you get an answer, you can tweak the results for the resistance. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Short antenna = reduced power
gareth wrote:
Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... You do that and while you are at it take note of the fact that the expression you give is unitless and can not be power. You will also find that the total power radiated by an antenna is the surface integral of the average Poynting vector over a surface enclosing the antenna. The surface usually chosen is a sphere in the far field to keep the equations "simple". -- Jim Pennino |
Short antenna = reduced power
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