Re: [TowerTalk] CobWebb antennas - success stories?

[Jim Lux <jimlux@earthlink.net>]

At 04:26 PM 9/22/2004 -0700, Jim Lux wrote:
> At 11:45 PM 9/22/2004 +0100, John Tait wrote:
>
>
>
> > > Anyone have any success stories regarding the CobWebb antenna?
> > >
> > > http://www.g3tpw.ukgateway.net/
> > >
> > >
> > > I'd like to get one but it's about $500 to get one here after the exchange
> > > rate and shipping.  Just wondering if anyone had tried one of these before
> > > and had any opinions to share...  or if there's a better list to move the
> > > thread to i'd be happy to take it there.
> > >
> > > 73 Jason N1SU
> >
> >  It's a clever and effective antenna for people with little room, and 
> > with TVI problems.
> >  Basically it's a nest of FOLDED dipoles with a choke balun at the 
> > feedpoint. Ordinary dipoles in such a configuration would have a low 
> > feed impedance, and narrow bandwidth,
> > Using folded dipoles gets over both problems..
>
>
> I don't think they're folded, just wrapped. A folded dipole doesn't change 
> the bandwidth, just transforms the impedance (which would be nice in this 
> case, since it would transform the 10-12 ohms of the dipole in a square up 
> to 40-50 ohms..).  You'd still have the efficiency and bandwidth problems 
> though.  Can't get around that physical size Chu-Harrington thing.

I stand corrected... I just reread the section about construction of the 
CobWeb, and it IS a folded dipole.  Very clever, because the 4:1 
transformation gets the R right up to 50 ohms, by the time the losses 
factor in.

All those dipoles, though... adjusting the lengths must have been tedious 
the first time around! Another reason you want the ATU to drive it!

Jim, W6RMK 

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