A common mode choke does not solve the issues of grounding the
shields of the coax in the K1WA design. The K1WA design relies
on the 3/8 wave open coax acting like an inductor to tune the
unused slopers as reflectors. Unfortunately, when the shield
is connected to ground, the ground connection adds capacitance
in parallel with the inductance and DETUNES the reflector.
I know of no commercial switch that opens both sides of the coax
and floats the shields. Someone building an antenna that uses
this principle needs to: 1 )build their own switch, 2) work with
a length of feedline (5/8 wave ?) that will yield the proper XL
when SHORTED or 3) use a half wave of feedline and coils. In
all three case you will still want an effective common mode choke
at the feedpoint.
73,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Roger (K8RI)
> Sent: Monday, April 20, 2009 8:37 PM
> To: jim Jarvis
> Cc: k4fj@k4fj.com; towertalk@contesting.com
> Subject: Re: [TowerTalk] sloping dipole arrays
>
>
>
>
> jim Jarvis wrote:
> > I have seen K1WA's excellent article in the Antenna
> Handbook for years
> > regarding sloping half-wave dipole arrays. His design
> switches both
> > the center conductor and braid for each feedline.
> >
> > Is it really important to switch the braid?
> > 73, Steve, K4FJ
> > -0-
> >
> > YES.
> >
> > The K8UR sloper array design, and the variants upon that, require
> > that the
> > reflectors not be connected to the driven element. If you use a
> > switch which
> > ties all shields together, you'll have half of your
> reflectors and
> > your driven element
> > fed together.
>
> That is why you install a choke/current balun on the feedline
> the proper
> distance from the antenna. Then the shields can remain at DC ground
> (for safety)
>
> 73
>
> Roger (K8RI)
> > God knows what pattern will result. Or, knowing
> > this audience,
> > only an adept NEC user with a lot of time will know what the
> > resulting pattern
> > will be.
> >
> > The technique of feeding each dipole via 3/8 lambda coax from the
> > switch is a particularly neat way to assure that the
> undriven elements
> > function as
> > reflectors.
> >
> > N2EA
> >
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
> >
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