> I am not discussing directivity loss or gain--I am just
> curious of whether the field in the vicinity of metal absorbs
> some of the field and somehow that causes the total field of
> the driven element to be reduced.
A changing electromagnetic field generates current in any
conductive element in that field. The amount of current is
proportional to the level of the field, the resistance in
the conductive element and how closely that element's resonant
frequency matches the frequency of the field. The induced
current sets up its own field that adds to or subtracts from
the field of the driven element.
If the conductive element is close to resonance, the current
induced in that element will be high. But if that conductive
element is lossy the field created by that (parasitic) element
will be small ... the energy drawn from the field generated by
the driven element will be dissipated in heat and not returned
as a radiation source that can be used to create directivity
(gain). In that sense, you can say that any electromagnetic
energy that is taken from the main field and not re-radiated
by the "parasitic" element causes the total field to be
reduced.
All of this is covered in basic electromagnetics ...
73,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Al Williams
> Sent: Thursday, October 30, 2008 1:16 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Lossy traps?
>
>
> If I am reading this response correctly, then any metal
> anywhere decreases the radiated field from the driven element?!
>
> I am not discussing directivity loss or gain--I am just
> curious of whether the field in the vicinity of metal absorbs
> some of the field and somehow that causes the total field of
> the driven element to be reduced.
>
> k7puc
>
> ----- Original Message -----
> From: "Joe Subich, W4TV" <w4tv@subich.com>
> To: "'Al Williams'" <alwilliams@olywa.net>; "'Tower Talk'"
> <towertalk@contesting.com>
> Sent: Wednesday, October 29, 2008 10:07 PM
> Subject: Re: [TowerTalk] Lossy traps?
>
>
> >
> >> Do the "other elements" actually absorb some of the power
> creating an
> >> "IR???" loss of the source power?
> >
> > The current in the trapped, parasitic element creates an
> > I^2R loss in the traps of that element. The decrease in current
> > available for re-radiation represents an effective I^2R loss to the
> > driven element because there is less total power available for
> > radiation.
> >
> > ANY loss in the antenna system represents an overall decrease in
> > efficiency. The effective gain of an antenna is equal to the
> > directivity times the antenna efficiency.
> >
> > 73,
> >
> > ... Joe, W4TV
> >
> >
> >
> >> -----Original Message-----
> >> From: towertalk-bounces@contesting.com
> >> [mailto:towertalk-bounces@contesting.com] On Behalf Of Al Williams
> >> Sent: Wednesday, October 29, 2008 8:20 PM
> >> To: towertalk@contesting.com
> >> Subject: Re: [TowerTalk] Lossy traps?
> >>
> >>
> >>
> >> ----- Original Message -----
> >> From: "jimlux" <jimlux@earthlink.net>
> >> To: "Dan Hearn" <n5ar@air-pipe.com>
> >> Cc: "Tower Talk" <towertalk@contesting.com>; "Jerry K"
> >> <w5kp@hughes.net>
> >> Sent: Wednesday, October 29, 2008 3:55 PM
> >> Subject: Re: [ comparable: both are essentially arrays of
> >> coupled low Q
> >> resonators.
> >> >
> >> The F12 style beam
> >> > couples some nonzero amount of power into the other elements, so
> >> > there's some IR loss there.
> >> >
> >> >
> >> Do the "other elements" actually absorb some of the power
> creating an
> >> "IR???" loss of the source power? Or do the "other
> elements" become
> >> energized and then radiate in possibly a detrimental change of the
> >> pattern? k7puc
> >>
> >>
> >> _______________________________________________
> >>
> >>
> >>
> >> _______________________________________________
> >> TowerTalk mailing list
> >> TowerTalk@contesting.com
> >> http://lists.contesting.com/mailman/listinfo/towertalk
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
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> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
> >
> > --
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> Date: 10/28/2008
> > 10:04 AM
> >
> >
>
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