Hi Guy,
in a multiband array using a single boom it's not an easy task to find
out where to place planar and different bands elements without
affecting negatively the antenna performances on one or more bands.
Sometimes exist a place were interaction and detrimentel effects are
minimal, some other times a critical coupling developing a sort of open
sleeve element (parasitic open sleeve elements may exist) is better
than a bigger but insufficient spacing and smartly converts two
separate elements in a single one on two bands (that element finally
behaves like a trapped one).
This concept is used to overtake the problem of a lower band element
that may disturb an higher band antenna, althoug the open sleeve
parasitic element for the higher band has to be broad banded using more
than a single sleeve element.
When designing a multiband antenna on a single boom the above processes
must be checked several times, and it's not sure that an optimal
solution always exists and some compromises have to be taken in account.
The same considerations can't apply between two antennas one above the
other and a bigger whole antenna over another smaller one is a further
more complex structure than single isolated elements.
We can't be sure to model everything so perfectly (or that modeling
software can do it) and a bigger whole antenna over a smaller one is
far more complex structure than single isolated elements.
We like to beleive in cheap miracles but reality is often different
althoug some gain reduction or a worse F/B are not so perceivable
without the chance to compare with or without the close stack and
doesn't prohibit that one works DXes or held pile ups with a
compromised antenna.
Neverthless, if one decides to benefit the full performances of a well
designed multi element antenna should carefully evaluate if to place a
bigger metal structure in close proximity and expecially above it.
In my experience this is something that doesn't work as I pretend and
as I consider worth to be realized.
73,
Mauri I4JMY
> ---------- Initial message -----------
>
> From : owner-towertalk@contesting.com
> To : n4kg@juno.com, towertalk@contesting.com
> Cc :
> Date : Wed, 31 Jan 2001 17:23:24 -0500
> Subject : Re: Re: [TowerTalk] Shadowing of small antennas by larger
ones?
>
> Read the description carefully. On the C39, the 40 DE is BETWEEN the
15 DE and the 15 reflector, NOT behind the 15 reflector.
>
> They HAVE had to deal with removing the 21 mhz resonance from the
40 "N" elements. The diagram is at home, but the distance from the 40
DE to the 15 DE on the same boom is a LOT less than the vertical mast
separation the original poster proposed between the separate antennas.
>
> >
> > From: n4kg@juno.com
To: <towertalk@contesting.com>
> > Date: 2001/01/30 Tue PM 11:51:25 EST
> > To: TOWERTALK@contesting.com
> > Subject: Re: [TowerTalk] Shadowing of small antennas by larger ones?
> >
> > You can put lower frequency elements BEHIND higher frequency
> > elements because the high frequency reflectors isolate other
> > elements to the rear.
> >
> > That is NOT the case for vertical separation where the lower
> > frequency elements are symmetrical about the mast as are the
> > higher frequency elements on the triband antenna.
> >
> > de Tom N4KG
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