Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
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 |
#2
|
|||
|
|||
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. |
#3
|
|||
|
|||
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 =--- |
#4
|
|||
|
|||
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 |
#5
|
|||
|
|||
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 =--- |
#6
|
|||
|
|||
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 |
#7
|
|||
|
|||
Ian White, G3SEK wrote:
"An ideal loading inductance does "not" radiate." A loading coil does not need to radiate to produce significant difference between the current at its ends. The same power may be produced at various impedances. The product of the real voltage and the real current must be the same in all cases for the same power. A loading coil experiences a different voltage to current ratio (impedance) at every point along its length.. This is due to the combination of forward and reflected volts and amps. Because of the reflection, their forward and reflected vectors are rotating in oposite directions. This makes every spot along the paths of these vectors unique with its own voltage to current ratio (impedance). Cecil. W5DXP has already posted in some detail how the forward and reflected values encounter the incident values and their superposition produces a new impedance. That surely happens at the load end of a loading coil. If the phase between the incident and reflected waves, at the coil to whip junction, makes a different impedance than that at the feed end of the coil, it is likely that the coil output current will be different from the coil input current That`s expected. Best regards, Richard Harrison, KB5WZI. |
#8
|
|||
|
|||
Richard Harrison wrote:
Ian White, G3SEK wrote: "An ideal loading inductance does "not" radiate." A loading coil does not need to radiate to produce significant difference between the current at its ends. Those two paragraphs, one right after the other, say it all. You want to gallop straight on to talk about "loading coils". I want you to stop and think a moment, about how an IDEAL INDUCTANCE behaves in an antenna. (Sorry to shout, but every time I type "ideal inductance" quietly, you seem to read something else :-) Hopefully you will agree that an IDEAL INDUCTANCE does not ever have different currents at its two terminals, and does not radiate either. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#9
|
|||
|
|||
Ian White, G3SEK wrote:
I want you to stop and think a moment, about how an IDEAL INDUCTANCE behaves in an antenna. (Sorry to shout, but every time I type "ideal inductance" quietly, you seem to read something else :-) Ian, please take your own advice. It's pretty obvious that you are thinking about an IDEAL INDUCTANCE in terms of a lumped circuit analysis which is invalid when analyzing a STANDING-WAVE ANTENNA. The equations governing the behavior of a standing-wave antenna are similar to the equations governing the behavior of a lossy transmission line with reflections. In fact, just by looking at the equations, you cannot tell whether they apply to a transmission line with reflections or to a standing-wave antenna. Hint1: Write the equation for the total current on a standing-wave antenna that includes forward and reflected currents and "loss" due to radiation. Hint2: A real-world mobile loading coil acts like a section of transmission line where Z0=SQRT(L*C). It does NOT act like a lumped circuit inductance. Hint3: An IDEAL INDUCTANCE doesn't exist in reality. Lumped circuit inductances are a shortcut that doesn't exist in reality and surely does NOT apply to distributed networks like a standing-wave antenna. Hopefully you will agree that an IDEAL INDUCTANCE does not ever have different currents at its two terminals, and does not radiate either. Can't agree to that at all. In fact, here's a repeat from another posting that proves that the superposed forward and reflected currents at each end of a lossless inductance *cannot* be equal. Please don't use the copout excuse that an ideal lumped inductance doesn't have any phase shift through it. *ALL* real-world loading coils have a phase shift that can easily be measured. If it has any phase shift at all, the current magnitudes at each end of the coil *cannot* be equal unless a current min/max occurs in the middle of the coil which doesn't happen in a typical mobile antenna. P.S. How about discussing the technical issues instead of the personalities involved? There doesn't need to be a current drop through a coil for the total current to be different at each end. Assume a base-loaded mobile system. Assume the forward current through the coil is constant at 1.1 amp. Assume the reflected current through the coil is constant at 1.0 amp. Assume the phase shift through the coil is 45 degrees. If the forward current and reflected current are in phase at the base of the coil (feedpoint) the total current will be 1.1+1.0 = 2.1 amps of total current at the base of the coil. The total current at the top of the coil will be 1.1 amps at -45 degrees superposed with 1.0 amps at +45 degrees. 1.1*cos(-45) + 1.0*cos(45) = 1.48 amps. The coil is lossless and the component currents are absolutely constant through the coil yet the superposed total current at the top of the coil is only about 71% of the superposed total current at the bottom of the coil. No "technical jargon" involved. Using circuit analysis on a distributed network problem simply demonstrates ignorance of the problem. It's an easy mistake to make and a hard mistake to admit (especially for gurus :-). -- 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 =--- |
#10
|
|||
|
|||
Cecil Moore wrote:
I want you to stop and think a moment, about how an IDEAL INDUCTANCE behaves in an antenna. (Sorry to shout, but every time I type "ideal inductance" quietly, you seem to read something else :-) Ian, please take your own advice. It's pretty obvious that you are thinking about an IDEAL INDUCTANCE in terms of a lumped circuit analysis which is invalid when analyzing a STANDING-WAVE ANTENNA. It makes life easier to compartmentalize your scientific world-view in that way.... but it is deeply, fundamentally wrong. In reality, all true scientific knowledge joins up seamlessly - that's how we *know* it's true! If we can't see how it joins up, that means we still have work to do. Dividing it into compartments that don't join up is lazy and will always lead you false. A fundamental physical property like inductance doesn't change its behaviour depending on the situation it finds itself in. If you cut the antenna wire and insert an ideal, lumped inductance, that inductance will behave in exactly the same way as it does in any other circuit. If you really looked hard at the math of antennas considered as transmission lines, you would find there is no problem whatever about inserting an ideal inductance, with no difference in current between its two terminals. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Lumped Load Models v. Distributed Coils | Antenna | |||
Current in antenna loading coils controversy | Antenna | |||
Eznec modeling loading coils? | Antenna |