From: "Jim Lux" <jimlux@earthlink.net>
> At 05:08 PM 3/4/2005, Tom Rauch wrote:
> > > 4 SDR-1000 digital radios (w/100 Watt output) - 4@ $1300
> > > 4 SGC500 500W amplifiers - 4@$1200
> > > 4 high power computer controlled tuners 4 @ $500
> > > 4 radiating elements (one nifty approach would be a pair
> >of switch
> > > selectable horizontal dipoles at the top of each of 4
> >middling high posts)
> > > PC with necessary multichannel sound card interfaces $1000
> >
> >Sounds like a great system if you want less significantly
> >than 1500 watts output, pee-poor IMD, and have time and
> >money to burn.
This is a pretty interesting discussion. I don't quite have my head
wrapped around a few of the issues, but I would like to make the
following comments:
1). The concept Jim describes is used at higher frequencies
routinely in all sorts of applications.
2). Scanned phased arrays do suffer from problems related
to mutual coupling (scan blindness). This I think is an area that
has received a lot of research attention.
3). At very high frequencies ferrite isolators can be used to
solve the IMD the problem Tom describes. At HF frequencies
I could see how this might be a big problem. Thinking out
of the box a little, however, if one sampled the output of each
amplifier it might be possible to use the DSPs to predistort
the drive signals in order to minimize IMD in a manner similar
to what is done in cellular base station amplifiers. Even in
single amplifier situations, this is an area where software
defined radios using DSPs in the TX chain might be able to
really push the state of the art (as Tom loves to point out - the
state of the art in amateur amplifier IMD performance for
the most part sucks big time).
4) It isn't very constructive to compare ideal installations (100ft
tower with a steppIR) to the kinds of installations Jim is aiming
his concept at (e.g. CC&RVille). What would be useful to the
discussion is to focus in on a non-ideal installation and then try to
find out if distributed amplifiers would have any advantage in that
scenario. Say for instance, I can put three 20' whips along a chain
link fence (no additional ground beyond that provided by the fence).
A typical urban CC&RVille lot might be 50 x 100' with a chain link
fence running around the backyard perimeter (say a 50' x 30'
rectangle with the back of the house comprising one of the 4 sides
of the rectangle). Looking at that kind of environment (and assuming
the IMD problem could be solved with some kind of DSP based
linearizer), what, if any, advantage would there be to using the
distributed solid state amps versus those same amps combined
into a single output and then split back with passive couplers to
drive the 3 array elements?
5) I am a little confused about the advantages of the distributed
array. I can see how it provides you with fine amplitude and
phase control vis-a-vis the individual software defined radio
exciters for each element, but at the same time doesn't the
requirement to provide equal power to each element diminish
the relative advantage of that amplitude and phase control?
In other words does having to be mindful of mutual coupling
put a straight jacket on you that offsets the benefit of having
the flexibility of DSP controlled amplitude and phase?
6) Can you really ignore the mutual coupling problem? If I
am driving two mutually coupled elements with 500 watts
each from two independent amplifiers each one connected
to one of the two elements respectively, I would have to
provide a matching network so that the driving point impedance
of each element is transformed to 50 ohms. In that case,
each amplifier would see 50 ohms resistive and it would
therefore deliver 500 watts into each element. However,
wouldn't some of that power going into each element mutually
couple into the adjacent element and then get dissipated in
the ouput of that amplifier?
Okay my head hurts sufficiently now. I look forward to hearing
from each camp.
73 de Mike, W4EF............................
_______________________________________________
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