On Thu, 2006-07-13 at 18:44 -0700, Lee A Crocker wrote:
> Bill I will answer in line, it is a good discussion
> thank you for the questions.
>
> <Lee, I wasn't going to mention this
> but since you did, I've got to ask some dumb
> questions.>
>
> <1. Why would you ever choose 4 separate phase-
> controlled TXs instead of simply using one
> Comtek 4-square controller costing $350?>
>
> W8JI has introduced a 4 element receive antenna that
> covers an octave of bandwidth. His focus was on
> cleanliness of pattern and low noise. I would like my
> 4 element transmitting antenna to cover an octave of
> bandwidth. To accomplish this using comteck boxes
> would require about 5 or 6 boxes/antennas ground
> fields. How much work is that?
>
> <2. Why would you want to run 4 separate
> feedlines instead of one to the controller box?>
>
> <3. Why would you want to have 4 separate amps at
> 375 Watts output each instead of one at 1.5 kW?>
>
> 2&3 375 watts is easily do-able with transistors and
> automatic antenna tuners (like the LDG series), and it
> would not be too hard to actually mount the amp/tuner
> at the antenna, so you could use 4 cheap feedlines to
> drive them. Also on transmit you really only care
> about maximum forward gain in the given direction you
> are interested in reaching, and not "pattern", so you
> could choose the "phasing" and even the array geometry
> that maximizes gain in the particular direction you
> are interested in. If you mount the amp at the
> antenna you don't care if it has a lot of blower noise
> for example. Also there is no feedline loss. Given
> the class of amplifier they are considering it is not
> too far fetched to choose transistors that will give
> you 375 watts out of the flex radio box. If you are
> 80% efficient 100 watts of dissipation will cover you
> to 400 watts output power. If that's the case all you
> would need is a tuner at the base of the antenna for
> multibanding and some way to measure phase/gain at the
> base of the antenna.
You might want to control the antenna tuners as their tuning process
always introduces phase shift. That's how broadcast array drive networks
are tuned, with T, pi or L networks to introduce the the controllable
phase shift beyond that of the feed lines.
>
> <4. What would happen to phase coherency
> when passed through 4 separate amplifiers
> and tank circuits?>
>
> 4 Phase coherency could probably be dynamically
> controlled by measuring the phase/gain at the base of
> each element. Something like a souped up LP-100 meter
> could probably do this. Larry uses this idea in his
> beam antenna.
>
> <5. Has anyone actually computed cost
> comparisons for 4 TX, 4 amps and 4 feedlines
> versus 1 TX, 1 amp, 1 feedline and 1 Comtek
> controller?>
>
> 5. What is the cost of multiple 4 squares with
> multiple ground fields, multiple comtek boxes and an
> alpha 87?
>
> Note this particular array could easily fit on a lot
> an acre or probably a little less. Combine this with
> a W8JI receive array on another corner of the lot and
> you could put a very nice high performance station on
> the air. What is the cost of 30 acres in the country?
In Story County Iowa, that 30 acres runs about $141,000. Just beyond
giving a decent return from cash rent to a farmer.
>
> 73 W9OY
>
>
>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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