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