___Original Message_________________________________________
From: Jim Lux
Date: Mon, 24 Oct 2011 Time: 10:21:41

Say you want an antenna that covers both 10m and 15m. You start with a
10 meter resonant antenna. Now hook a trap which has very high
impedance at 10m on the end of it. Since it's high Z, not much current
flows, so it doesn't change the 10m behavior very much. But, you also
want it to work at 15m. So you figure out how long an additional
element you need to put "outboard" of the trap to get the system to
resonate. Typically, the trap is a parallel LC, so below resonance, it
looks somewhat inductive. Since a short element looks capacitive, the
added length for 15m will be shorter than it would be without the trap.


There's a whole raft of design approaches

Hi béo-master

Here is a simple trap antenna design tool that I have successfully used
a few times:

http://www.k7mem.150m.com/Electronic_Notebook/antennas/shortant.html

--
73
Ian, G3NRW

The AIM4170 Antenna Analyzer:
http://homepage.ntlworld.com/wadei/aim4170.htm