----- Original Message -----
From: <K3BU@aol.com>
To: <Towertalk@contesting.com>
Sent: Thursday, September 04, 2003 8:40 PM
Subject: Re: [TowerTalk] measuring power, reference planes, and all that
> In a message dated 9/4/03 7:44:28 PM Eastern Daylight Time,
> jimlux@earthlink.net writes:
> My questions about measuring power have a real basis in the context
> of fairly sophisticated phased array designs where the amplifiers are used
> to supply reactive power instead of using passive L or C. It's not that I
> want to skirt the rules and run megaQRO with specsmanship and creative
rule
> reading... but neither do I want to artifically restrict the power
available.
> I had a look at your page describing the "problem". Here is practical
> (contester's) perspective.
>
> Commercial example of such transmitting array is at HAARP in Alaska. Nice
but
> EXPENSIVE!
Well, HAARP is a bit bigger installation than I would
contemplate..Furthermore, it's built to standards well beyond what any ham
would be willing to pay for and has capability well in excess (in terms of
each radiator and amp) of what a typical ham would want or need.
>
> Lets just take 4-square, which would be typical, efficient phased array. I
> can build phasing unit (3 relays, two toroid inductors, bunch of fixed
> capacitors) for about $95, or you can buy commercial unit for about $300.
Works on
> transmit and receive, simple, low losses, reliable, instantaneous. No need
for
> "$300 vacuum variables".
A 4 square only has good performance at one frequency (or narrow range of
frequencies) and only has 4 or 8 directions it can be pointed. Nor can you
adjust the elevation angle of the beam. A fully adjustable phased array
which would be comparable in performance to the SteppIR or a conventional
beam would require adjustable Ls and Cs. The 4square is an OK starting
point, but has a lot of limitations compared to the beam (or an active
phased array).
> 1500 W amp, used can found for around $500. Array you have to build
anyway.
Once you start rolling in used or surplus parts, any parts/construction cost
comparison goes out the window, unless the used/surplus part is truly a
commodity widely available at a standard price. Sure, as a prototype, I'd be
out scrounging like the rest of us, but if you're comparing designs, you've
got to use catalog pricing.
>
> "Distributed" amplifiers? You need to bring the AC power to them, weather
> (thief) proof them, you need LC components and tuning or switching systems
with
> tracking (4x). What about receive? Receive antenna has to be most
> discriminating. Another bunch of matching components and switching? How
much would that
> cost and then "brains" in running it?
I never said that it would be trivial... however, technology is pushing the
cost down on all the parts you identify. More specifically:
a) you'd run DC to the PAs
b) weatherproofing is an issue, but no more so than weatherproofing any
other part of an antenna. The high buck solution is NEMA 4X boxes. The
lower cost solution is stuff like Tupperware. Look at all the posts about
weatherproofing something as simple as a UHF connector.
c) LC and tuning... the whole virtue of supplying reactive power with an
amplifier is that you don't need any high power L or C. On the low power
side, Ls and Cs might be the way to do the phasing, but then, so might NCOs
and D/As... The latter solution is the one that is rapidly dropping in
price, thanks to the wireless industry.
d) The brains running it? While it takes a fair amount of computation to
figure all the phasing out, etc., it's something that can be done offline
and stored in a table, or even if done in real time, requires very few
compute cycles. While a PIC or 8085 might not hack it, a single board PC
certainly could.
>
> For contesters it don't look like attractive chick. Too komplikated :-)
No question there... And that is much like 20m SSB operations a few decades
ago. "What, you mean I can't use my rhombics?".. "Where's the plate
modulation transformer?" "If you can't run Class C, it's too inefficient,
and it's too hard to make a linear with low distortion."
20 years from now, I think the story will be different. (Although, there
will still be hams who long for the days of King Spark and the whine of the
rotary spark gap and Poulsen arc).
I find it fascinating to go back through old issues of the ARRL
handbook...Look at the first articles about using transistors, etc.
> For those who want to play around and "explore"? Maybe, why not.
precisely.
>
> Then there is a problem of reradiation and distortion from such a setup. I
> heard one HG station in the contest with raspy fuzzy signal just from such
setup
> - separate antennas fed with separate amplifiers.
I'd like to know more about that. Indeed, phase and amplitude matching
(particularly with respect to nonlinearities and differing distortion
products from each amp) is a challenge. It's a challenge today with any
amplifier that uses multiple amplifying components, which is why the usual
commercial design rule is use the biggest single device you can find that
can put out the power you need. I don't know about reradiation being an
issue. You're already reradiating in any multielement array, passive or
active.
>
> Not to flame or pore cold water on the project, but it would be
interesting
> to see if such setup was built and practical advantage shown.
Indeed, and that is precisely what I am going to do... Viewgraph (or
mailing list) engineering only goes so far.
\
>
> Yuri, K3BU
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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