----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Jim Lux" <jimlux@earthlink.net>; "w7xu" <w7xu@iw.net>;
<towertalk@contesting.com>
Sent: Monday, June 14, 2004 4:17 AM
Subject: Re: [TowerTalk] 80 4-square inside a 160 4-square? Pros
andConsplease!
>
> > Complex yes, but probably less complex than two sets of
> elements and trying
> > to deal with the interactions. Essentially, you'd wind up
> with a big
> > multipole relay to switch between two sets of tuning
> networks. Does he
> > really need to run two bands simultaneously? If so, it's
> a bit trickier,
> > but then again, it would be no tougher (probably easier,
> in fact) than a
> > diplexer for the 600 kHz spacing on a repeater.
>
> A duplexer allows simultaneous reception and transmission
> with one receiver and one transmitter at the same time, a
> diplexer allows multiple receivers or multiple transmitters
> to be used on one antenna. Repeaters use duplexers.
>
> To use just four elements, we would need a controlled phase
> shift two-band diplexer at each element (with about 100dB
> isolation) and then AFTER that we would need a two band
> phasing system. Everything would be phase-critical.
The band splitter only has to be phase stable and have good isolation. I
agree that this is a significant challenge, but, it's a qualitatively
different challenge than erecting 4 additional towers and dealing with the
mutual interactions, which would still exist in the "two radios
simultaneously" concept. After all, nesting two antenna arrays is just
relying on the antennas different tuning and physical separation to do the
splitting and isolation, and I doubt you'd get 100 dB from that without
working at it.
Once you have the two band splitter, what ever its phase shift
characteristics are, they could be compensated by the steering phasing
networks. For that matter, there's nothing that says that the splitter has
to have 50 ohm resistive connections to the phasing network.
It's a complex, very non-trivial design problem, but one that is certainly
solvable.
If you don't need simultaneous two band operation, then you probably don't
need 100 dB isolation through the band splitter.
>
> It would be MUCH less complex to do what he originally
> thought of, and that would be to use two antennas. That
> would only require simple traps and pass systems that were
> not phase-critical. Two bands in four phased elements is
> workable one band at a time, but a complex nightmare
> (although it could be done with limited bandwidth) on two
> bands at the same time.
At this stage, it's all speculative. I think it goes to show that there's a
lot of potential for phased arrays, and that amateurs have only begun to
scratch the surface.
>
> 73 Tom
>
>
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