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