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#61
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Dan Richardson wrote:
"The fact is resonance or not is not the determining factor." Resonance of the antenna system is the determining factor in the performance of a standing-wave antenna. This is an amateur group, so you may check the "ARRL Antenna Book". My 19th edition has resonant antennas on page 9-2. Fig 2 is a series RLC circuit representation of the typical standing-wave antenna. Ohm`s law should be noncontroversial (I=E/Z). To maximize I with a given voltage, Z must be minimized. Z in the series circuit is the phasor sum of R and X. R has probably been established firmly in an antenna by its construction and placement but we can tune the antenna system to make it resonant so that we eliminate X to get maximum current into the antenna and to thereby get maximum performance out of the antenna. Best regards, Richard Harrison, KB5WZI |
#62
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Art, KB9MZ wrote:
"You can`t ever discard the factor Q in any discussion with respect to antenna efficiency or any calculation for that matter." Kraus writes in his 1950 edition of "Antennas" on page 299: "The Q of an antenna, like the Q of any resonant circuit, is proportional to the ratio of the energy stored to the energy lost (in heat or radiation) per cycle." (in heat or radiation) are Kraus` words, not mine. It means the R of the antenna used in the Q operations is formed of the sum of Rr+Rloss. Efficiency is Rr/Rr+Rloss The Q=X/R, where R is the sum of Rr+Rloss. If R is heavily weighted toward Rr, the antenna is efficient. If R is heavily weighted toward Rloss, the antenna is inefficient. Q as an indicator of efficiency is baloney. Best regards, Richard Harrison, KB5WZI |
#64
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#65
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Dan it is wise to remember that Richard "cherry picks" statements from
assorted books and disregards the chapter heading and what is discussed in that chapter. He only looks for a statement that suits his present state of mind and suggests authority and ignores the underlying subject of the thread. In that same chapter it is discussed how the losses mount up as you move away from the resonant point and shows where the losses could amount to 5dB no less. There is a proviso in that the antenna books that I have precede the one that he is refering to so there may be some evidence that somebody has pushed the envelope beyond Maxwell and others. I think the actual chapter refers to broadband antennas rather than those of a narrow bandwidth so it is always advisably to check so called "book quotes" rather than accept what people interprete what is relavent in their own minds. Regards Art "Dan Richardson @mendolink.com" ChangeThisToCallSign wrote in message ... Let me try this one more time. You had posted earlier and I commented on this: "A receiving antenna must be resonant to enable full acceptance of available energy, and it must be matched to avoid re-radiation of more than 50% of the energy it is able to grab." I commented on the first portion of your statement (above). My only point is that it makes no difference if an antenna's is resonate or not in determining how much energy it grabs. That's it, nothing more. 73 Danny, K6MHE On Mon, 23 Feb 2004 13:43:55 -0600 (CST), (Richard Harrison) wrote: Dan Richardson wrote: "The fact is resonance or not is not the determining factor." Resonance of the antenna system is the determining factor in the performance of a standing-wave antenna. This is an amateur group, so you may check the "ARRL Antenna Book". My 19th edition has resonant antennas on page 9-2. Fig 2 is a series RLC circuit representation of the typical standing-wave antenna. Ohm`s law should be noncontroversial (I=E/Z). To maximize I with a given voltage, Z must be minimized. Z in the series circuit is the phasor sum of R and X. R has probably been established firmly in an antenna by its construction and placement but we can tune the antenna system to make it resonant so that we eliminate X to get maximum current into the antenna and to thereby get maximum performance out of the antenna. Best regards, Richard Harrison, KB5WZI |
#66
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Dan, K6MHE wrote:
"My only point is that it makes no difference if an antenna`s is resonate or not in determining how much energy it grabs." Not in the "grand scheme of things", perhaps, but a resonant antenna extracts more energy when swept by a traveling wave than an off-resonance conductor does at the same distance from the transmitter. On some occasion you may have adjusted the antenna trimmer on a receiver and found a signal peak. That adjustment balanced out the reactance in the antenna system leaving only the antenna resistance to oppose current flow in the antenna. Proper adjustment maximizes antenna current induced by the wave. The mechanism is simple. The antenna is a series RLC circuit. Current in the antenna is a function of the field strength divided by the impedance of the RLC circuit. Impedance to antenna current is least when all of the reactance has been eliminated. You have probably experienced peaking the signal this way with your ears, a meter, or an oscilloscope. Let`s look at an oft repeated type of true story by John E. Cunningham in his "Complete Broadcast Antenna Handbook": "Not every tall structure will affect the pattern of a broadcast station. The effect on the pattern depends on the height of the structure. In one case, a microwave tower was erected, one section at a time, near a 4-tower directional antenna system. As construction progressed, the current in one tower began to drop as each new tower section was added to the microwave tower. The current continued to drop until it nearly reached zero. But, as more sections were added to the microwave tower, the current began to rise. When the microwave tower reached its final height, all of the tower currents of the broadcast antenna, as well as the pattern, were normal. Resonance, or lack of resonance, makes all the difference! Best regards, Richard Harrison, KB5WZI |
#67
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Art, KB9MZ wrote:
"Dan it is wise to remember that Richard "cherry picks" statements from assorted books and disregards the chapter heading and what is discussed in the chapter." Forget the books! Surely some time in your long career you`ve peaked an antenna trimmer and it worked. Why? Best regards, Richard Harrison, KB5WZI |
#68
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#69
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Steve Nosko wrote:
"First, my "class C" model is as follows: I liken it to digital or "switched modes"." I do too. "----Second, it is the RMS current through the tube which will waste power, so that is what we must be concerned with." I don`t believe current through a Class C amplifier consists of an ordinary sine wave. I think it consists of short unidirectional pulses. The tuned "tank circuit" is the source of sine waves. RMS is the effective value, not the average value, of an a-c ampere. It is defined as 0.707X the peak value of the waveform. It is derived from the average of the squared current over a half cycle, as the heating value of an ampere is proportional to the current squared. Speaking inversely, the ratio of maximum to effective value for a sine wave is 1.414, which is the square root of 2. Ordinarily, with nonsinusoidal currents, the ratio of maximum to effective value is not the square root of 2. Best regards, Richard Harrison, KB5WZI |
#70
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Dan Richardson wrote:
So according to you, as an example, a resonate 1/4 or 1/2 monopole will extract more energy than a non-resonate 5/8-wave monopole. A non-resonant length physical 5/8WL monopole is made into an electrically resonant monopole with the addition of the base loading coil. Without that resonating coil at the base, not much energy would be delivered to a 50 ohm receiver or accepted from a 50 ohm transmitter. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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