Thanks for your responses. I emphasize
that I may well be one of the first to get an SDR-X
(2 RX version) because of its potential for inband
interference rejection, but I simply question the
practicality of the 4 TX idea.
>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?
I believe it would only require two
phasing boxes (octave = 80/160), not 5 or 6.
Use the same 4 verticals, ground systems and
equal length phasing lines to the controller(s).
You simply do phasing at the controller instead
of in the transmitters.
>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 dissapation 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.
Count me as one who is paranoid about
having much electronic content at my antennas.
You should see the sooty mess inside the box
housing my series capacitor for my 180' vertical
for 160 (even though this was the first time
I've seen anything similar in the 7.5 years it's
been up). No damage after I cleaned it up but
imagine if it contained an amp and tuner. I'd be
concerned about having much of value at the base
of TX verticals. My feedline is 350' of LDF-550
which I bought for about $150 and has 0.2 dB of
loss on 160, so I'll spot you that loss in the
next pileup. :-)
>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
Anything is possible, but you need to
compare cost and survivability. Installing
or replacing a $350 phase box (or two) will be
much less than multiple amps, tuners and phase
> What is the cost of multiple 4 squares with
multiple ground fields, multiple comtek boxes
and an alpha 87?
One 4-square (like W8JI uses with 98'
spacing), one ground system (which must be much
more extensive than for the RX-only version)
2 phase boxes ($700 total) and one 1.5 kW amp.
$700 for 2 controllers, $5500 for an ACOM 2000A,
$150 for feedline (zero for CATV hardline), and
one transceiver ($3650) for a total of $10k
(excluding land, verticals and ground systems
which are identical for either case).
Versus a 4 TX 4 RX SDR-X (assume you want
phase control for RX also, otherwise you need
another RX system and more land), 4 each 400W
amps/tuners/phase detectors), assume zero for
4 feedlines. Since only 6 modules fit into the
first SDR-X mainframe, you will also need two
mainframes to house the 8 modules. How much
will this cost versus the simpler alternative?
>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?
It would be more than that because you
need a much better TX ground system. Think 98'
square for the 4 verticals and 60 X 120' radials
at the base of each. I think that's roughly a
square with 270' sides. That's closer to 1.67
acres without a separate RX system. BTW, why
would you need a separate RX system? Use the
same system for both TX and RX. 30 acres is not
necessary whether you use the SDR-X or not, so
I'm not sure of your point. If you are talking
phased Beverage arrays, they will also be much
higher performance than a W8JI 4-square. That's
actually what Tom uses himself (phased Beverages).
Lee, I'm not saying it's impossible to do
such a 4 TX system, I'm simply saying it's not
very practical. The SDR-X has some very exciting
potential, but I really don't feel this is a good
73, Bill W4ZV
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