At 01:02 PM 4/4/01 -0400, Tom Rauch wrote:
>
>> Just make sure that you isolate the grounds so you
>> have no spurious resonances due to common shields
>> that will detune the "reflectors" to somewhere that
>> limits their usefulness. Just means that you need
>> double-pole relays to switch both the center and shield
>> for each dipole.
>
>
>The reason you have to use DPDT relays is the system has no
>balun at each element, which is a VERY bad move in a directional
>array.
>
>If you install choke baluns (aka current baluns) at some point
>between the element and the common switching system, there is
>no need to switch the shields.
>
>If the feedlines are 1/4 wl long electrically on the outside of the
>cable, you also would not need a balun or a DPDT relay. A balun
>would do very little good because the common mode impedance
>would already be high on the shields.
>
>In the case of 1/4 wl or near 1/4 wl long feedlines **without a
>balun** switching the shields will make things worse, and should
>be avoided . In this case the shield and 1/2 of the dipole would be
>RESONANT or near-resonant when the shield was opened!!!
>Bad news.
Several points --
The most recent pubished source on this antenna is K3LR's article in the
August 94 QST. Authors also include Jim Breakall, WA3FET, who knows a
little about antennas. They switch both shield and center conductor and
use a choke balun at the feedpoint. Since I don't know any better, that is
what I did, too.
The design has been in use by WA7LT, ex-W9LT for at least 25 years, and he
told me personally that the antenna did not show directivity until he
switched both.
The feedline length from the switchbox to each feedpoint is somewhere in
the vicinity of 150 degrees. In my modeling, that dimension is relatively
non-critical, while the length of the dipoles themselves is quite
important. Above their resonant frequency, they demonstrate good gain and
directivity for ~10 percent of the resonant frequency, while below their
resonant frequency the pattern goes bidirectional within 200 kHz. I have
empirically verified the first of these statements in the 80 meter DX
window, with an array cut for the bottom of the CW band.
My modeling also indicates that the parasitic array should produce about 2
dB less gain than a perfect 4-square using elements of the same
configuration, with equivalent or better F/B. Contrary to the folklore
about vertical dipoles over ground at HF, the modeling results are VERY
sensitive to ground parameters. Simply substituting EZNEC "Good" for "Very
Good" ground raises the modeled takeoff angle from 17 to 22 degrees and
reduces the gain by 2.5 dB.
For whatever it's worth.
73, Pete N4ZR
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