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#1
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Yuri, K3BU wrote:
"W8JI used this picture (Fig 10) to "see, it is constant". But that was only by specification. It`s the same as saying, "Let`s say the line is lossless". The text reads: "The loading coil acts as the lumped constant that it is, and disregarding losses and coil radiation, maintains the same current flow throughout." This says that in the impossible case of zero radiation and zero loss, the coil current is the same at both ends of the coil. This is close enough for a coil at 50 Hz, but unlikely at 5 MHz. A real loading coil such as a bug catcher, has a real length. The combination of incident and reflected waves at each point along the length of the antenna produces a different voltage, just as seen in a transmission line. This effect prevails in an antenna, too. Just as on a transmission line, the voltage variation represents an impedance variation. Impedance is high at the open-circuit end of the antenna , and it it is low 90-degrees back from that open circuit. Since some length is filled with the coil, there is a difference in volts at the ends of the coil due to the standing wave on the antenna. The feed paths to the coil are unbalanced as shown in Fig 6. That is not shown in Fig 10 which is meant to show the difference in antenna current above and below the coil, not what happens in the coil itself. The authors specify an idealized coil which has the same current in and out. This is only a declaration, not a real world situation. Best regards, Richard Harrison, KB5WZI |
#2
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Richard Harrison wrote:
This says that in the impossible case of zero radiation and zero loss, the coil current is the same at both ends of the coil. This would be true for traveling-wave antennas. But in a standing-wave antenna, the net current is the sum of the forward current and the reflected current. Even if the coil had zero radiation and zero loss, a real-world coil would have a delay through the coil. That delay changes the relative phase between the forward and reflected currents making the net current different at each end of the coil even for a coil with zero radiation and zero loss. The forward currents would be equal into and out of the coil. The reflected currents would be equal into and out of the coil. But their phasor sum would differ due to phasing. Assume the forward current and reflected current are in phase at zero degrees at the feedpoint. The net current is simply the algebraic sum of those two values. But 45 degrees out from the feedpoint, the forward current is at 45 degrees and less than at the feedpoint. The reflected current is at -45 degrees and greater than at the feedpoint and the sum of the two currents is the sum of two phasor currents 90 degrees apart. At 90 degrees, at the end of the antenna, the forward current and reflected current are equal and phasor sum to zero. -- 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! =----- |
#3
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Richard Harrison wrote:
Yuri, K3BU wrote: "W8JI used this picture (Fig 10) to "see, it is constant". But that was only by specification. It`s the same as saying, "Let`s say the line is lossless". I looked at the ARRL Antenna Book CD and it contains the same stuff. It also says: "This product is licensed under the terms of the License Agreement contained in the LICENSE.TXT file on the disk. Read it carefully before using the CD." :-) -- 73, Cecil, W5DXP |
#4
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Read it carefully before using the CD." :-)
I didn't use CD, just book 19th Ed. BUmmer |
#5
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Cecil, W5DXP wrote:
"I looked at the ARRL Antenna Cook CD and it contains the same stuff." Declaring a coil to have zero size and loss does not make it so. Loss resistance alone does not delay anything. It kills electricity by converting it to heat instantly. It takes no prisoners. It has no electrical storage. Pure inductance delays current by exactly 90-degrees behind the applied a-c voltage. Resistance adds vectorially with inductive reactance to produce an impedance on some angle with the resistance between 0 and 90-degrees, depending upon the magnitudes of resistance and reactance. So, in any coil the current is delayed. Coax with a coiled center conductor is manufactured as delay line and is specified in microhenries per foot. Coils are made of conductors which suffer skin effect resistance. None escape loss, despite declarations. None occupy zero space. Assuming perfection is valuable for analysis, but should not be used as proof of performance. Best regards, Richard Harrison, KB5WZI |
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