[TowerTalk] Lossy traps?

[Joe Subich, W4TV]

> 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 at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Al Williams
> Sent: Thursday, October 30, 2008 1:16 PM
> To: towertalk at 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 at subich.com>
> To: "'Al Williams'" <alwilliams at olywa.net>; "'Tower Talk'" 
> <towertalk at 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 at contesting.com 
> >> [mailto:towertalk-bounces at contesting.com] On Behalf Of Al Williams
> >> Sent: Wednesday, October 29, 2008 8:20 PM
> >> To: towertalk at contesting.com
> >> Subject: Re: [TowerTalk] Lossy traps?
> >>
> >>
> >>
> >> ----- Original Message -----
> >> From: "jimlux" <jimlux at earthlink.net>
> >> To: "Dan Hearn" <n5ar at air-pipe.com>
> >> Cc: "Tower Talk" <towertalk at contesting.com>; "Jerry K"
> >> <w5kp at 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
> >>
> >>
> >> _______________________________________________
> >>
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