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John Smith wrote:
"And in what ARRL publication might I find this information?" It`s in ON4UN`s "Low-Band DXing", an ARRL publication. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
Tom Donaly, KA6RUH wrote: "There may be a difference in current along the coil, but it isn`t a current drop." Call it a decline if you don`t like the word drop. A wave traveling along an antenna induces current in the wire. This current causes radiation from the wire. A current traveling from "a" to "b" in the wire loses energy to radiation. The energy at "b" is less than the energy at "a" if the source is at "a". If the impedance at "a" is the same as the impedance at "b", the voltage and the current at "a" are larger than the voltage and current at "b". We don`t need energy to decline from "a" to "b" to have a current drop. We only need current to decline between "a" and "b". Yuri has demonstrated a "current drop" with r-f ammeters inserted at both ends of the loading coil. Analysis of the cause is not necessary to demonstrate a current drop. As straight wires are usually better radiators than the same wire in coils, I speculate that the current drop measured by Yuri is mostly due to the high impedance (High voltage, low current) on the output of the loading coil. Best regards, Richard Harrison, KB5WZI I would urge any young person who reads this and wants to understand electromagnetics to get a good book on the subject, read what the authors say, and forget what Richard just posted. He's all wrong. 73, Tom Donaly, KA6RUH |
Tom Donaly, KA6RUH wrote:
"There may be a difference in current along the coil, but it isn`t a current drop." He wrote that about four times, what's the point of it? Smart Alec attitude? If there is something you believe is wrong and you know better, than please correct us stupid hams. Thanks for your patience Richard, KB5WZI, you are one of few decent posters here. 73 Yuri, K3BU |
I would urge any young person who reads this and wants to understand electromagnetics to get a good book on the subject, read what the authors say, and forget what Richard just posted. He's all wrong. 73, Tom Donaly, KA6RUH See what I mean? I urge any young person to read the book about jerks. |
John Smith wrote:
"The loading coil acts as the lumped constant that it is, and disregarding losses and coil radiation, maintains the same current flow throughout. This statement is somewhat misleading. A standing-wave antenna is somewhat like a transmission line with standing waves. There are TWO component currents in a standing wave antenna, forward and reflected. The total current is the phasor sum of those two currents. Even if the forward current were constant and the reflected current were constant, the total current changes because of the phase shift between the forward current phase and reflected current phase as these two phases are changing in opposite directions. For instance, given a 1/4WL ground plane vertical, the forward current and reflected current are in phase at the feedpoint, thus resulting in high current. At the tip top of the 1/4WL ground plane, the reflected current is 180 degrees out of phase with the forward current and their phasor sum is zero at that point. -- 73, Cecil, W5DXP ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Tom Donaly, KA6RUH wrote:
"---forget what Richard just posted." Suits me. You recall the old story about leading a horse to water. Tom rejects measurements of r-f currents at both ends of a loading coil at work. The currents were clearly differing. Tom must be stuck with battereis and the rise and fall these produce in the current of an inductor. A loading coil is usually in the antenna field in our examples. The loading coil is subject to an incident wave and to a reflected wave. These waves combine in a continuously varying phase relation along the coil to make current, impedance, and voltage all functions of their positions along the coil. Every spot along the coil is different when both waves are sensed together. That`s the way SWR works, and we`re discussing standing-wave antennas. I have a 1982 ARRL Antenna Book. On page 13-3 there is a Fig. 6, "Improved Current Distribution Resulting From Center Loading". The loading coil clearly shows less current at its top than at its bottom. Best regards, Richard Harrison, KB5WZI |
Yuri Blanarovich wrote:
I would urge any young person who reads this and wants to understand electromagnetics to get a good book on the subject, read what the authors say, and forget what Richard just posted. He's all wrong. 73, Tom Donaly, KA6RUH See what I mean? I urge any young person to read the book about jerks. The key to understanding the total current in standing-wave antennas is in understanding the forward current and reflected current components and their superposition at different points along the antenna. The cosine distribution of standing-wave current in a 1/2WL dipole is the result of the superposition of forward-traveling current and rearward-traveling reflected current. An unterminated rhombic is a standing-wave antenna because the forward current gets reflected at the open end of the wire. The forward current causes radiation in the forward direction and the reflected current causes radiation in the rearward direction. The radiation "loss" causes the reflected current at the feedpoint to be a lower magnitude than the forward current at the feedpoint. There are standing waves all up and down an unterminated rhombic and the Vtot/Itot feedpoint impedance depends partially upon the phase between the forward current and reflected current and, of course, upon their magnitudes. Properly terminating a rhombic virtually eliminates reflections and turns the antenna into a traveling-wave antenna which radiates mostly in the forward direction. There are virtually no standing waves on such an antenna. These same ideas can be applied to other standing-wave antennas, including a 1/2WL inverted-V. In EZNEC, we can terminate the ends of such an antenna to ground through resistors that eliminate standing waves on the antenna. The feedpoint impedance of such an antenna is in the ballpark of 600 ohms. Where does the low feedpoint impedance of an unterminated 1/2WL inverted-V come from? It comes from the superposition of the forward current and the reflected current at the feedpoint. These two components are in phase and phasor-add to a large current. The two voltage components are 180 deg out of phase and add to a small voltage. small-voltage/large-current is a low feedpoint impedance. Using a minus sign for 180 degrees, the feedpoint impedance of an inverted-V is approximately (Vf-Vr)/(If+Ir). We can understand a standing-wave antenna by doing an analysis of a lossy piece of transmission line. If the losses in the transmission line approximately equal the radiation "loss" of the antenna, the feedpoint impedances will have approximately the same value. Once standing-wave antenna currents are understood, it is easy to see why the total superposed currents at each end of a 75m Bugcatcher coil are nowhere near equal even though the forward current and reflected current at each end of the coil are close to the same value. Circuit analysis works well when there is only one current flowing in a coil. Circuit analysis falls apart when forward and reflected currents are flowing in a coil and distributed network analysis is required when such coils are installed in standing-wave antennas. -- 73, Cecil, http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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Richard Harrison wrote:
I have a 1982 ARRL Antenna Book. On page 13-3 there is a Fig. 6, "Improved Current Distribution Resulting From Center Loading". The loading coil clearly shows less current at its top than at its bottom. My 1988 version shows the same thing on page 16-4, Fig. 7. Unfortunately, two pages later, in Fig. 10 it shows how the current at the top of a loading coil can be four times higher than would exist in an equal top length of a 90 degree antenna. Since V*I*cos(theta) is the power, does that mean that the voltage at the top of the coil is 1/4 the value of an equal top length of a 90 degree antenna? So the impedance looking into that 15 degrees of whip above the coil is 1/16 of the impedance looking into that same 15 degrees of that same whip mounted above 75 degrees of wire??? Doesn't sound reasonable, does it? Exactly how does that coil manage to change the impedance looking into 15 degrees of whip by a factor of 16 AT THE TOP OF THE COIL????? That's exactly what happens when one uses circuit analysis on a distributed network problem. If circuit analysis worked on such a problem, we wouldn't need distributed network analysis. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Tom Donaly wrote:
Richard Harrison wrote: Tom Donaly, KA6RUH wrote: "There may be a difference in current along the coil, but it isn`t a current drop." Call it a decline if you don`t like the word drop. A wave traveling along an antenna induces current in the wire. This current causes radiation from the wire. A current traveling from "a" to "b" in the wire loses energy to radiation. The energy at "b" is less than the energy at "a" if the source is at "a". If the impedance at "a" is the same as the impedance at "b", the voltage and the current at "a" are larger than the voltage and current at "b". We don`t need energy to decline from "a" to "b" to have a current drop. We only need current to decline between "a" and "b". Yuri has demonstrated a "current drop" with r-f ammeters inserted at both ends of the loading coil. Analysis of the cause is not necessary to demonstrate a current drop. As straight wires are usually better radiators than the same wire in coils, I speculate that the current drop measured by Yuri is mostly due to the high impedance (High voltage, low current) on the output of the loading coil. Best regards, Richard Harrison, KB5WZI I would urge any young person who reads this and wants to understand electromagnetics to get a good book on the subject, read what the authors say, and forget what Richard just posted. He's all wrong. Well, not entirely, if he follows through the logic of what he says. An ideal loading inductance does *not* radiate; therefore the current at its two terminals *must* be the same. I keep coming back to the same point: until someone correctly understands what pure unadulterated inductance does in an antenna, he can never *truly* understand how a real-life loading coil works. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
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