Topband: DX-Enginneering Active Antenna System

[David Raymond]

>
> > P.S.  K7TJR's controller is only $150 but it does not have the
> > capability of switching over coax.  If I had known about it before I
> > got a used DXE controller, I would probably have gone with it instead.
>
>
> Assuming the technology of the K7JTR 4 sq controller is the same
> as his 8 antenna design it is a single band design where the DXE
> (W8JI) controller uses crossfire phasing which works on both 160
> and 80 meters.
>
> The "phasing" part of the W8JI design was on Tom's web site for
> a very long time.  The design is not too difficult for a good
> home builder.  The most complex parts are the transformers and
> anyone who has built their own Beverage units will have no
> problems.
>
> 73,
>
>    ... Joe, W4TV

It's been interesting to see the reviival of interest in the four square
veritcal RX arrays.  I built, I believe, the first short vertical RX array
based on the original W8JI design back about a decade ago.  It turned out
the original design had an error in it which Tom caught immediately when I
first called him to discuss why the array was not wroking properly.   Tom
was very gracious to work with (and rather modest I might say, considering
his technical prowess) and quickly faxed me back a revised design.  The
corrected design subsequently got published.  In short, my array worked
well.

My array used 20' pieces of 3/4" electrical conduit with three 20' sloping
wire tophats that also guyed the elements.  Each element was mounted on a 3'
piece of treated 2x2 driven into the ground.  I used some WD1 wire I had on
hand for the tophat wires.  The elements had natural resonance at about 3.8
MHz.  I'm recalling it took about 34 uH of L to bring the elements to
resonance at 1.825 MHz..  I fed my array with 50 ohm coax (I would recommend
75 ohm for increased bandwidth) and used about 30 ohms of  R (can't remember
the exact amount) on each element to achieve 50 ohm impedance.

I  learned some key lessons building that first array which might help
builders of this array:

1.  For optimum F/B, it is critically important that the current be
balanced/equalized in the elements.  Small variations (more than a couple of
ohms) in impedance very quickly degraded the F/B performance.

2.  Proper quarter wave coax feedlines for each element are important to
help balance the currents.  Don't rely on velocity factors. . . take
measurements when feedlines are being cut.  The same goes for the phasing
line.

3.  Due to #1, the series resistance values for each element should be
carefully selected for each element so that elements impedances all agree
with each other within a couple of ohms (to agree with the impedance value
of your coax. .. i.e., 50 ohm or 75 ohm).  Five and six ohm variations
between element impedances substantially lowered the F/B (which is the whole
point of the array).  The measurement must be taken at the phasing box ends
of the element feedlines.  Remember, due to the properties of quarter wave
feedlines, increasing the R value at the antenna will decrease your
impedance measurement reading taken at the phasing box end of the feedlines.

4.  Also due to #1, equality of resonance in the elements is vitally
important.  When taking resonance readings (which is better done using a
short coax jumper rather than the quarter wave feedlines), only one element
at a time should be connected to the L, R, and coax due to mutual coupling
of the elements.  In my case, the required L varied by a uH or two from
element to element (not exactly sure why).

5.  Element impedance stability is also important as impedance variations
between elements create current imbalance (and F/B goes to heck in a hand
cart).  You only need enough radials to provide impedance stability between
wet and dry conditions.  In my case, three 65' radials on each element was
sufficient.  I used Radio Shack 4' ground rods at each element.  I also had
a 4' ground rod at the phasing/switching box.

I set the phasing/switching box  on top of a couple bricks to get it off the
ground and away from moisture.  I coiled up the phasing line and put an old
five gallon paint bucket over it and the phasing/switching box. . . then put
a couple of bricks on the top to keep it from getting blown away!

The original W8JI design is pretty simple to build.  My expenses were pretty
small. . .some ferrite cores, #30 enameled wire (from Radio Shack), some
relays, some 3/4" conduit, coax for element feedlines, phasing line, coax to
the shack, control cable, ground rods, and a box to put it all in.  The
entire array itself was passive. .. no preamps are required at the elements
with this design.  I did use a preamp (an old ARR) out at the phasing box
(the shack side) to provide some needed gain (15 db of gain was sufficient).
I used control cable to do the switching and feed DC to the preamp.  My coax
and control cables runs were about 400'.  My experience over several seasons
showed the array had performance similar to my 600' two wire reversible
Beverages.

You don't have to be an expert homebrewer to build the phasing control box
and make the array work.  And, it doesn't take a lot of money nor a lot of
space.

73 to all. . .Dave
W0FLS