I believe EZNEC will model this but any modeling software is going to
require a good deal good deal of tinkering when ported to real life
with so many lumped circuit variables.

Placing the coils at the end of an otherwise resonate section seems
like a good idea but it just is not that simple. The 40 meter
operation will essentially be a center loaded dipole.

If you choose to go this direction I would think it best to do one
dipole pair at a time. Also, study the commercial products carefully.

My preference would be four dipoles (one for each frequency) in a fan
arrangement.

de W8CCW John

On 2 Jul 2006 11:28:38 -0700, wrote:

Here are a couple messages I posted on eHam net. Any comments would be
appreciated.

Thanks,

Doug VE3XDB

--
MESSAGE #1:

Hi everyone,

I am thinking of building a center loaded fan dipole. One wire dipole
to cover 20 and 40 metres, and a second wire diple, connected at the
same feedpoint, to cover 30 and 17 metres. Coils would be placed at the
resonant point of the higher frequency, acting as both an RF choke and
a loading coil. My question is this. I would like to have the wire
beyond the coil as long as possible, but would still like the coil to
have enough inductance to act as an effective RF choke for the higher
band.

Other than through experimentation, how would one calculate or
determine this mix of inductance/length?

Best regards,

Doug VE3XDB
John Ferrell W8CCW