From: awpaeth@watpix.uwaterloo.ca (Alan Wm Paeth)
Subject: Wullenwebers, Phased Arrays, Radar and N-gons (Re: Big Antennas)
Organization: University of Waterloo
Date: Fri, 14 Aug 1992 02:09:35 GMT

In article <1992Aug12.233323.2612@intelhf.hf.intel.com> zardoz@rx-db (zardoz@rx-db.hf.intel.com) writes:
>In article <rwa.713655224@aupair.cs.athabascau.ca> rwa@aupair.cs.athabascau.ca (Ross Alexander) writes:
>>
>>I'm waiting for the antenna book to have a good discussion of Webenwullers :).
>>
>A Wullenweber is any arrangement of directional antennas designed to be
>selected electrically so as to provide a rotating beam sorta like radar.
>Picture this:
>A 200 foot mast with sloping veebeams running in all directions which are

Zardoz is correct. I came across a Wullenweber design recently and as these
are interesting, thought I'd add my own two bits (and a schematic):


                        NW      N      NE
                           \    |    /                /
                            \   |   /                /   = antenna
                             \  |  /
                                _                   _
                      W  ----- ( ) -----  E        ( )   = rotating commutator
                                                           5P5T (see text)
                              / | \
                             /  |  \
                            /   |   \
                         SW     S     SE

Since the antennas are in a radial pattern (verticies of a regular N-gon),
the entire structure is periodic every 360/N degrees. The trick lies in the
central commutator, which in the picture is making contact with only five
elements: W, NW, N, NE and E. Rather than being a single pole piece, it is
a 5-pole 5-throw switch, in which the five poles are connected to the common
source of RF using cables whose lengths have a distribution looking something
like this:

   |-                 edge pole
   |------
   |---------         center pole
   |------
   |--                edge pole
   |
   ^ point of common drive.

This means that with the commutator in the switch position illustrated, the W
and E elements are driven essentially from the common feed-point (with a bit of
connecting cable having length '-') and begin radiating outward. When the
wafefront passes the NW and NE elements, they likewise present an in-phase
wavecrest, as their RF energy arriving via the 5-pole commutator presented a
longer delay: '------'. The center feed receives current last and hence is the
last to radiate. Note that the distribution drawn is a lobe of a sine wave
for firing due north ("broadside" along a line through two opposting vertices),
as this presents the phase delays of a leaving (or arriving) wavefront.

Rotating the switch one position to the left (CCW) moves the shaped beam to the
NW direction, etc. A true system will provide many additional antennae and the
spread of the commutator will likely not be through 180 degrees, as shown for
illustration here. (I picked "5P5T" as an example).

When used at UHF frequencies the Wullenweber can drive into a central
reflecting hub (a coaxial cylinear suitable distorted along the Z axis but
showing radial symmetry) to beam (un)shape along the Z axis as the basic
Wullenweber only steers radially (in the XY plane).

A more modern design uses Butler arrays to provide an N-by-N port (eg, 8x8)
in which excitation is presented to one of N inputs and then all N outputs
(which drive the radiating elements) provide a properly phased beam at
eight discrete locations, arranged radially (ie, end fire left, through
broadside in one direction through broadside right in 22.5 degree steps).
Such arrays use "magic Tees" and other hybrid couplers (power dividers). A
basic element is a 2x2 port when driven at In1 provides 50% inphase output at
Out1 with 50% going to Out2 with a 90 (or 180) degree phase lag. Driving In2
likewise presents 50% inphase to Out2 with 50% going to Out1, AGAIN with a 90
degree lag. The Butler array builds these into larger structures much as the
phone company builds up NxN crossbars having less than N^2 elements by using
smaller crossbars and an interesting interconnection hierarchy. A much better
model is the FFT butterfly diagram (which also provides "phasing" along points
of a regular 2^N-gon).

At the high end, the modern top-flight (bad pun) phased arrays used in UHF
avionic radar use ferrite or step recovery diodes to provide phase shifting.
(These are called "phasers" in this application - no 'kiddin. ["Couldn't aquire
lock on the enemy, Cap'n, my phasers must be burned out..."]). Here, active
(circuit) elements -- RF amplification -- drive each antenna element. Then the
phased array becomes a flat array of small transmitters in which each might not
put out more than 5W. This keeps the solid-state RF drive simple, yet given a
few thousand elements, many thousands of watts can be concentrated and directed
along a select path. The return signal likewise takes advantage of the antenna
gain along the common path. Some ferrite systems show "remanence" (as in the
hysteresis of ferrite core memories), so that the array remains properly
directed for reception of the reflected wave even after the steering signals
have been removed.

My personal interest (and recent bedtime reading) in all this lies in designing
a six-element driven vertical array for use on 432MHz. (I recently cornered the
market on SPDT coax relays with silver-plated N connectors and want to build
something worthwhile). I've seen a number of designs for four element "square"
vertical systems with unidirectional radiation off the diagonals; if anyone has
seen designs based on a regular hexagon I would be delighted to receive e-mail.

  /Alan Paeth
  VE3AWP/KD3XG