In GENERAL, of course, I have NO argument with your statement of
PRINCIPLE, nor have I posted such.
But "shadowing" as used by various posters in this thread is an obsolete
term that does not intuitively point to the real mechanism. The other
antenna can cause just as much trouble being that close UNDERNEATH. That
IS easy to model, and the modeling HAS been done. The proof is thousands
or horror stories. (I've heard them too.) The proper term for it is
UNWANTED INTERACTION, not shadowing. And I do know someone locally who
tried to remedy a 40/15 interaction by reversing the vertical position.
No luck of course. Lot of sweat and some expense went into that
experiment.
SPECIFICALLY though, the original poster placed a question about two
SPECIFIC antennas (F12 C19 & 240N in case the reader has never seen or
forgotten). Those HAVE been modeled and tested together, even on the
same boom. All the worry-warting over a general principle will not
remove the fact that the requisite work has already been done for THOSE
TWO antennas, both vertically separated and horizontally separated. They
work together.
And since they have done it for other pairs, the suspicion grows (though
some may never be satisfied because Proof-Man has not showed up on
Planet Earth with the ultimate "prover" to check it out) that Tom et al
at F12 are actually on to something that allows them to dodge the 40/15
interaction conundrum. "We like to believe in cheap miracles" seems NOT
to apply at F12. He does the work. Refreshing, isn't it.
A note here that Cushcraft advertised an interlaced 10/15/20/40 in
advance but was unable to deliver, and truthfully retracted the
advertisement when they couldn't. Not like it's easy to do.
And no, I have ZERO connections with Force 12. Just don't want to drown
one of the good guys with Mauri's fire hose. When I was a fireman, I
had to take care that all the water went on the FIRE to minimize damage
to parts of the house NOT on fire. Only the rookies go in and spray
water on everything in sight.
73,
-----------------
Guy Olinger, K2AV
Apex, NC, USA
----- Original Message -----
From: <i4jmy@iol.it>
To: <towertalk@contesting.com>
Sent: Thursday, February 01, 2001 4:19 AM
Subject: Re: Re: [TowerTalk] Shadowing of small antennas by larger ones?
> 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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>
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