béo-master wrote in
. fr:

HI,

I picked up two coils that could be a trap for the CWA-840 COMET
antenna. (They have two stickers "CL-840").
The manual of the CWA-840 says : 40/80m dipole.

I found amazing that there isn't any capacitor in // of these coils...
These coils measure 76uH.

That is a rather simplistic characterisation of the trap. If you were to
measure its impedance over a wide range of frequency, the data would
give a better idea of the extent of any parallel capacitance (and there
has to be some, even if only stray capacitance. Somethimes the
capacitance is between some metal tubes that make the end supports for
the trap.

Note that measuring the impedance of traps is very challenging.

Could someone tell me how they works ?

The trap introduces some reactance and resistance that is intended with
appropriate conductor lengths, to give a modestly low VSWR at the
frequencies of interest.

The traditional explanation is that they are resonant at one of the
frequencies of operation, and act like a switch, cutting off current to
the outboard legs. Of course, current would flow in the outboard legs
eve if you used a physical switch.

That explanation is appealing, but limits your options. There are
advantages is making the resonant frequency of a trap fall outside the
bands of interest.

In the manual of the CWA-840, each leg of the 7MHz section, is
11.1meters in lenght , wich, it seems to me, that it is rather
longer that 1/4 wave. (10.1m).
Why ?

See above.

Thanks a lot for your answers

CWA-840:

-----[ ]-------------ooo--//--
2.3m coil 11.1m balun

The trap is probably self resonant below 40m... but that is just a guess
from experience of having designed such things.

Owen