At 08:31 PM 10/2/97 -0600, Matt Trott wrote:
>I am so impressed with the single 80m sloper I put up last year I have
>decided to put up 3 more and have a "steerable" array. The ARRL antenna
>book says to feed the antennas from the switchbox with "a little more than
>3/8 wavelength" of feedline to provide the appropriate inductance to
>electrically lenghthen the 3 non-fed antennas (which then act as
reflectors).
>
>I know K3LR wrote an article in Aug. 94? QST about this type of antenna
>system. I don't know if he, more precisely than the antenna book, explained
>the feedline lenghths. My problem is I cleaned house and must have tossed
>that issue out a few years back. I do have the issue where Bob, W7LR
>discusses this array, but the lenghth of feedline he mentions is 62' of
>RG213 (vf .66?) which equates to ~3/10 wl for the 40 meter antenna
>discussed in the article.
>
>Is the feedline lenghth from the switchbox REAL critical or just critical?
>I am going to use RG8X.
>
>If you are using this antenna what electrical lenghth feedline are you
using??
I'm using one -- I was introduced to it by W9LT, who has been using them
since the 1960's (but never wrote an article...). Before building it I
modeled it extensively with EZNEC. This antenna is a lot simpler to
construct than a 4-square, and I believe the performance is pretty close.
The model and the real antenna provide good F/B (on the order of 20 dB)
with relatively low side rejection and useful gain (the model says 3-4
dBi). Modeled take-off angle (with no radials) is around 23 degrees. I
obviously can't testify to the gain, but there has to be some, judging from
the directivity, and it sure "plays loud."
A few points:
1. The antenna works much better above reasonance than below, both in real
life and in the model, where both gain and front-to-back fall off quickly
below resonance. Oddly enough, this is true over a fairly large range of
feedline lengths, but performance is quite sensitive to antenna element
lengths. I used K3LR's feedline dimension (roughly 87 feet of RG8-X) with
elements about 66 feet a side, and got resonance at 3500. The antenna
still provides useful F/B and some gain at 3800, despite an SWR of over 3:1
there.
2. Be sure to bring the bottom ends of the antenna elements back to the
tower, about 5 feet or so above ground. This seems very helpful in
cancelling high-angle radiation and enhancing the relatively low angles.
You'll find that the lower halves of each antenna describe something more
like a quarter-circle than a straight line making it back to the tower.
This is not a problem, except for the fidelity of your model, but the
impedance of the elements does seem to be quite dependent on proximity to
the ground. Two of my elements are over upward sloping ground, and they
show better SWR at resonance(1.1:1) than the other two over flat ground
(1.4:1). I also brought the feedines back to the tower at an angle roughly
bisecting the included angle at the feedpoint, hoping to minimize coupling
to the shields. Finally, I used 50 type 73 ferrite beads over teflon coax
for feedpoint choke baluns on each element, also hoping to minimize stray
currents.
3. Don't overlook the need to float both the shield and the center
conductor of each unused feedline. This requires either using terminal
strips on the switchbox or an insulated switchbox with SO-239s. Also use
good center insulators (Budwig RF-1, for example), because the feeders tend
to move a lot in the wind, and connections to the elements are prone to
breakage.
I hope this is helpful.
73, Pete Smith N4ZR
n4zr@contesting.com
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