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These loading coils theories
Excuse me for my ignorance of antenna modelling and EZNEC models.
I've been following this lumped coils stuff and bugcatcher loading coils, as many others, just to learn on the theory. So far, I've learned on computer models and on Kraus theories. Please light my candle: Something I am missing: I'm thinking about how a trap antenna works: The trap is resonant isn't it? On the frequency of interest: Say 75m. On a trap dipole, there is a reduction of current flowing on the other side of the trap ? Because it is a resonnant circuit made of L and C. If someone would put current meters on each side of the trap, what would he measure ? A reduction of the current ? On a Bugcatcher coil, isn't this is what is happening? The coil is usually tuned after the antenna is installed ? The Bugcatcher coil is acting ilke a trap ? The capacitance of the trap is between the car body and the coil, isn't it? So would a properly adjusted Bugcatcher coil show a reduction of the current. Isn't that what we would like to achieve ? Stef |
The difference between the trap and loading coil is this:
Loading coil "shortens" the radiator physical length by replacing the portion of the radiator with inductance. Current on the ends of the coil is more-less as it would be if there was a wire that coil replaces. Trap typically resonates (by self resonance or with aid of parallel capacitor) on the operating frequency of the portion of radiator between the feedpoint and the the trap. It presents high impedance and practically isolates rest of the radiator beyond the trap. On other frequencies (lower) it becomes loading coil "connecting" the rest of the radiator and at the same time shortening its physical length. So the coil is a coil. In DC circuits behaves as flat earth believers want it to behave in AC circuits. In RF curcuits, the current distribution depends on the frequency and position of the coil along the radiator. Check my web page for the start of the argument and some explanations www.K3BU.us Yuri, K3BU |
Stef wrote:
On a Bugcatcher coil, isn't this is what is happening? The coil is usually tuned after the antenna is installed ? The Bugcatcher coil is acting ilke a trap ? A trap is operated at its resonant frequency. A bugcatcher coil is supposed to be operated far below its self-resonant frequency. So the bugcatcher is a lower impedance than a trap and doesn't impede the current as much as a trap. A trap operated at 1/2 its resonant frequency is primarily inductive, like a bugcatcher coil. -- 73, Cecil, W5DXP |
Traps are "resonant" loading coils are not.
The theory of a trap is as follows. Take an antenna for one band, say a half wave dipole on 40M. To add another band lower in freq, say 80M you need more length, right? If you add it, then the first band is off, but if you can add the length and remove it every time you are on the higher band you are ok again. A parallel resonant circuit is a high impedance at its resonant frequency...so put a parallel L & C, resonant at 40M on the end of the 40M dipole and add the length on the other side. Since the L/C is resonant = high Z, it is like an open circuit. On 80M the L/C is not resonant, so it is a "low" impedance and the added wire is now connected. Side note: since the Z of the antenna is pretty high at the ends, this L/C better have a really good Q (low loss & therefore a really high Z) so it effectively looks like an open (or nearly so) circuit and the extra wire has minimal effect on the 40M behavior. Since the trap is an L/C it has does have some Z on 80M (though we really didn't want this) and it will be the combination of the L&C which will be inductive. SO you have a coil at this point on your 80M antenna and will have to adjust the wire length appropriately. BTW, at this point, (even though this is not what we wanted) this coil happens to have a loading coil effect on the 80M band...see next. Theory of a loading coil. When we can't make an antenna long enough to get to the full resonant length (1/2 or 1/4 depending on the style) we have found that putting a coil/inductor (which looks inductive) into the antenna, we can find an inductance which gets the thing resonant and sometimes help the feed point resistance get closer to the desired 50 ohms. That's the basic difference -- Steve N, K,9;d, c. i My email has no u's.. "Stef" wrote in message ... Excuse me for my ignorance of antenna modelling and EZNEC models. I've been following this lumped coils stuff and bugcatcher loading coils, as many others, just to learn on the theory. So far, I've learned on computer models and on Kraus theories. Please light my candle: Something I am missing: I'm thinking about how a trap antenna works: The trap is resonant isn't it? On the frequency of interest: Say 75m. On a trap dipole, there is a reduction of current flowing on the other side of the trap ? Because it is a resonnant circuit made of L and C. If someone would put current meters on each side of the trap, what would he measure ? A reduction of the current ? On a Bugcatcher coil, isn't this is what is happening? The coil is usually tuned after the antenna is installed ? The Bugcatcher coil is acting ilke a trap ? The capacitance of the trap is between the car body and the coil, isn't it? So would a properly adjusted Bugcatcher coil show a reduction of the current. Isn't that what we would like to achieve ? Stef |
On Fri, 06 Feb 2004 12:05:09 -0500, Stef
wrote: If someone would put current meters on each side of the trap, what would he measure ? A reduction of the current ? On a Bugcatcher coil, isn't this is what is happening? The coil is usually tuned after the antenna is installed ? The Bugcatcher coil is acting ilke a trap ? Hi Stef, Your confusion arises from trying to learn via the internet where ALL answers are available (especially here). To avoid confusion, you need to establish a reference point, something you can trust. With modelers, you can start simple and progress to issues of more complexity. This instruction is inductive, which means (employing our EE term instead of the common English usage of the term) it can boost your understanding, or it can impede it. Thus it pays to find a good traditional source to enlarge the deductive side of understanding (usually a good book or correspondence/conversation). Philosophy aside, coils as parts of a radiating structure come in two flavors - you have already noted and described them above (if incompletely): traps and loads (which may also be traps). The controversy over coils that is currently raging (all puns intended) focus on the coil acting as a replacement length to a short antenna. These loads are not traps. Such loads, or loading coils, attempt to create a higher flow of current through them to achieve resonance. Traps imply through language an element that stops. Traps are parallel resonant elements that are high-Z in series to the adjacent sections of the antenna. They trap (stall, stop, impede) currents from proceeding through them to the other section on the farther side. Hence they are used with antennas that are larger than necessary. This makes them frequency selective, open switches that disconnect excess length from the resonant shorter sections. So there you have the duality of short-antenna/load and long-antenna/trap. Then we progress to where the trap may become a load (in the sense of lengthening a too short antenna). In this function, the coil/cap combination is no longer resonant alone, but within the parasitics of the larger structure (which also exhibits capacitance) it becomes resonant. In this sense, the trap becomes part of a series resonance and is no longer impeding flow. Hence there is another duality to consider: series-resonance/short and parallel-resonance/long. The use of an element (coil/capacitor) as trap (parallel resonant) AND load (series resonant) gives you opportunistic designs that allow one tuned element (still speaking of the coil/capacitor) to offer more than two band operation. Traditionally, the trap splits the radiating structure into two band operation; additionally, the sections that are thus physically split may also resonate (with the now incorrectly named trap) in series in a third band. This opportunistic arrangement may also reveal dual band operation of quarterwave sections driving halfwave sections (or other combinations) for single band, gain antennas. When you get into this kind of sophistication, it often turns on juggling many variables to achieve this legerdemain. You can either approach it cookbook style or through a modeler. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message
... On Fri, 06 Feb 2004 12:05:09 -0500, Stef wrote: If someone would put current meters on each side of the trap, what would he measure ? A reduction of the current ? On a Bugcatcher coil, isn't this is what is happening? The coil is usually tuned after the antenna is installed ? The Bugcatcher coil is acting ilke a trap ? Hi Stef, Your confusion arises from trying to learn via the internet where ALL answers are available (especially here). To avoid confusion, you need to establish a reference point, something you can trust. With modelers, you can start simple and progress to issues of more complexity. This instruction is inductive, which means (employing our EE term instead of the common English usage of the term) it can boost your understanding, or it can impede it. Thus it pays to find a good traditional source to enlarge the deductive side of understanding (usually a good book or correspondence/conversation). Philosophy aside, coils as parts of a radiating structure come in two flavors - you have already noted and described them above (if incompletely): traps and loads (which may also be traps). The controversy over coils that is currently raging (all puns intended) focus on the coil acting as a replacement length to a short antenna. These loads are not traps. Such loads, or loading coils, attempt to create a higher flow of current through them to achieve resonance. Traps imply through language an element that stops. Traps are parallel resonant elements that are high-Z in series to the adjacent sections of the antenna. They trap (stall, stop, impede) currents from proceeding through them to the other section on the farther side. Hence they are used with antennas that are larger than necessary. This makes them frequency selective, open switches that disconnect excess length from the resonant shorter sections. So there you have the duality of short-antenna/load and long-antenna/trap. Then we progress to where the trap may become a load (in the sense of lengthening a too short antenna). In this function, the coil/cap combination is no longer resonant alone, but within the parasitics of the larger structure (which also exhibits capacitance) it becomes resonant. In this sense, the trap becomes part of a series resonance and is no longer impeding flow. Hence there is another duality to consider: series-resonance/short and parallel-resonance/long. The use of an element (coil/capacitor) as trap (parallel resonant) AND load (series resonant) gives you opportunistic designs that allow one tuned element (still speaking of the coil/capacitor) to offer more than two band operation. Traditionally, the trap splits the radiating structure into two band operation; additionally, the sections that are thus physically split may also resonate (with the now incorrectly named trap) in series in a third band. This opportunistic arrangement may also reveal dual band operation of quarterwave sections driving halfwave sections (or other combinations) for single band, gain antennas. When you get into this kind of sophistication, it often turns on juggling many variables to achieve this legerdemain. You can either approach it cookbook style or through a modeler. 73's Richard Clark, KB7QHC Hello Richard "Your confusion arises from trying to learn via the internet where ALL answers are available (especially here)." ROTFLMAO And to Stef and the Assembled Multitudes: "You can either approach it cookbook style or through a modeler." I would add that you can derive a lifetime ...... (I'm near 50 years since my first crystal set.) .......of entertainment by building antennas and comparing them to one another. 73 H. NQ5H |
I would add that you can derive a lifetime ...... (I'm near 50 years since my first crystal set.) ......of entertainment by building antennas and comparing them to one another. That's my situation ! Thanks to all . Now I understand the loading coils on a bugcatcher are not resonnant, they're just loading coils. I've built traps before for an 80m / 40m dipole. I don't have the space for it here, but I'll try my hand on an 80m / 40m vertical with top hat soon. I've learned from books first, then by following the internet, discussions about the subjects, I find other ways of looking into things. It sure is fun to read what others are saying. |
On Sat, 07 Feb 2004 09:53:43 -0500, Stef
wrote: Now I understand the loading coils on a bugcatcher are not resonnant, they're just loading coils. Hi Steff, They ARE resonant, otherwise what's the point in using one? A short antenna exhibits a principally capacitive reactance that with the addition of a loading coil resonates the structure leaving only the radiation resistance behind. 73's Richard Clark, KB7QHC |
Now I understand the loading coils on a bugcatcher are not resonnant,
they're just loading coils. They ARE resonant, otherwise what's the point in using one? A short antenna exhibits a principally capacitive reactance that with the addition of a loading coil resonates the structure leaving only the radiation resistance behind. 73's Richard Clark, KB7QHC They are NOT resonant at the frequencies of interest - making antenna resonate on that frequency. If they were, they would act as a trap and "disconnect" rest of the whip beyond the coil. And that's the truth, fact, reality. Uh, huh? Yuri, K3BU.us |
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On 07 Feb 2004 18:43:31 GMT, oUsama (Yuri Blanarovich) wrote: They are NOT resonant at the frequencies of interest That is like saying the coil in a tank is not resonant by looking at the coil alone. Coil in a tank (with Cap) is not resonant on the same frequencu as "naked" coil. Big difference where it resonates. What a myopic way of looking at antenna design. My what? Loading coils are SERIES RESONANT, traps are PARALLEL RESONANT. Uh, huh! Bulsheet! If the COIL is self resonant (on frequency of interest), it becomes trap. The only difference is where the capacitor is. For a loading coil, the capacitor is made up of the reactance of a too short antenna. For a trap, there is a literal capacitor, AND the reactance of the antenna. And there is huge difference how the coil works in a circuit!!!! It could be a loading coil, or it could be a trap. That's why there are different names for it. If the antenna had no reactance, there would be no need for the loading coil (unless it is your next patent-applied for snow removal device). More W8JI rhetoric? Maybe time to grow up? 73's Richard Clark, KB7QHC Using your jabs Richard, I would say that you are off the wall, rocker, tree, you name it. You are displaying lack of proficiency, understanding, basics, etc. He was talking about the COIL. Coil without "capacitor" made of antenna or rest of the radiator. LOADING COIL is a COIL - that's it. Coil will resonate rest of the antenna with its whatever reactance. But coil by itself will resonate somewhere else, depending on its internal capacitance. But definitely not on the resonant frequency of the ANTENNA. If it did then it would be a TRAP. And that is the difference, and that was what was asked. Loading COIL is not reasonant on the frequency (of interest) that we want antenna to be resonant on (unless using it as helical antenna), otherwise it would act as a TRAP and insulate the remaining portion of the radiator and make the portion of radiator up to the trap (coil) resonant on frequency corresponding to its (electrical) quarter wave length. If you can not comprehend the difference between the LOADING COIL and TRAP than rest of the discussions here are pointless and you should go back and re-read some books (how do you like that? :-) Yuri, K3BU.us Warning! I bite back! |
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Uh-huh And if the Pope were Presbyterian he wouldn't be Agnostic....
Uh-Huh, and you are full of it and displaying conversion to W8JI ridiculing cult followers, good luck and enjoy it. Joining the freaktal team? 73 la - la |
On 07 Feb 2004 22:05:42 GMT, oUsama (Yuri Blanarovich)
wrote: Joining the freaktal team? Do you guys still hang out in robes at airports? |
Richard Clark wrote:
On Sat, 07 Feb 2004 09:53:43 -0500, Stef wrote: Now I understand the loading coils on a bugcatcher are not resonnant, they're just loading coils. They ARE resonant, otherwise what's the point in using one? The *antenna* is resonant, Richard, the *coil* is not resonant. If the coil was resonant, you wouldn't need the rest of the antenna. -- 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! =----- |
On Sat, 07 Feb 2004 17:27:52 -0600, Cecil Moore
wrote: If the coil was resonant, you wouldn't need the rest of the antenna. Uh-huh, makes a trap an antenna? |
Richard Clark wrote:
Loading coils are SERIES RESONANT, traps are PARALLEL RESONANT. BS, Richard. If they were series resonant how could they possibly supply any inductive reactance which is what is needed to neutralize the capacitive reactance of the short antenna? A "SERIES RESONANT loading coil" is purely resistive, by definition. -- 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! =----- |
C'mon guys! Self resonance is significantly different from circuit
resonance!! DD Richard Clark wrote: On 07 Feb 2004 21:15:57 GMT, oUsama (Yuri Blanarovich) wrote: That is like saying the coil in a tank is not resonant by looking at the coil alone. Coil in a tank (with Cap) is not resonant on the same frequencu as "naked" coil. Uh-huh And if the Pope were Presbyterian he wouldn't be Agnostic.... |
Yuri Blanarovich wrote:
Using your jabs Richard, I would say that you are off the wall, rocker, tree, you name it. You are displaying lack of proficiency, understanding, basics, etc. Not only that, Yuri, but he is desperately trying to change the subject away from the original. Have you noticed? That logical diversion, diverting the issue, is much older than Richard and won't work against people who recognize it. -- 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! =----- |
On Sat, 07 Feb 2004 17:36:09 -0600, Cecil Moore
wrote: BS, Richard. If they were series resonant how could they possibly supply any inductive reactance which is what is needed to neutralize the capacitive reactance of the short antenna? A "SERIES RESONANT loading coil" is purely resistive, by definition. and the cow jumped over the moon |
Richard Clark wrote:
wrote: If the coil was resonant, you wouldn't need the rest of the antenna. Uh-huh, makes a trap an antenna? A helical trap could be a 1/2WL antenna. But we are better off with a good ground plane and a 1/4WL helical antenna since it matches our 50 ohm transmitter better and has less loss. The ground plane furnishes half of the antenna. -- 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! =----- |
Richard Clark wrote:
wrote: BS, Richard. If they were series resonant how could they possibly supply any inductive reactance which is what is needed to neutralize the capacitive reactance of the short antenna? A "SERIES RESONANT loading coil" is purely resistive, by definition. and the cow jumped over the moon And Little Richard laughed to see such a sight, or maybe cried. -- 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! =----- |
C'mon Richard. Everybody who works in electronics knows that capacitors
are RESONANT, self resonant. Everybody who works in electronics knows that inductors are RESONANT, self resonant. At general RF frequencies the self resonances are significantly higher frequencies than the design frequency resonance. Get a Boonton 'Q' meter and go measure some. Then we can celebrate the funeral of this thread and really get into something interesting like 'Does the charge to mass ratio of an electron remain constant as it approaches relativistic speed?' Pops! Wrong group. DD |
Not only that, Yuri, but he is desperately trying to change the subject away from the original. Have you noticed? That logical diversion, diverting the issue, is much older than Richard and won't work against people who recognize it. -- 73, Cecil http://www.qsl.net/w5dxp Yep, it's getting pathetic, looks like graduates from W8JI "university". Time to move on to some serious work with those who see the light. I want to explore inductopacitive loads. WX is warming up, the white sh1t is melting, I will get out soon, and do some more 'sperimenting. We are trying to have mobile ant shootout here in the spring. Yuri, K3BU.us |
Richard Clark wrote:
"Uh-huh, makes a trap an antenna?" A trap usually isn`t the best radiator. A 1/4-wave shorted stub is electrically equivalent to a parallel resonant circuit, so they are interchangeable. You can use a parallel resonant circuit to replace a 1/4-wave shorted stub or vice versa. VHF collinear antenna arrays often use 1/4-wave shorted stubs to invert the phase at 1/2-wave intervals to keep the current going in the same direction in all elements. The Franklin antenna is a vertical collinear array used by some medium wave broadcasters. A 1/4-wave stub would be inconvenient for the medium wave broadcaster, so he sometimes uses a parallel resonant circuit for his phase inverter between 1/2-wave collinear elements. You might think the parallel resonant circuit or 1/4-wave shorted stub would function as a trap. Traps are used in multiband antennas. But the parallel resonant circuit does not function as a trap in collinear arrays. The length of the element beyond the tuned circuit is 1/2-wave resonant and readily accepts power through and around the tuned circuit. Not so with a trap. The length of the element beyond the trap is not resonant and accepts very little power. The trap is an isolator which keeps power out of the extension beyond the trap. The phase inverter is a coupler which conveys energy with a phase reversal produced by the inverter. Best regards, Richard Harrison, KB5WZI |
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