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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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