K9AY Flag Pennant Ewe Terminated Loops

K9AY Flag Pennant Ewe Terminated Loops

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Terminated loops have only recently become popular in Amateur radio, even though
such systems were available commercially in the 1960’s and earlier.

Elongated
and Terminated Loops

In the early 1970’s,
the late W1BB, Stew Perry, forwarded
an advertisement brochure for commercial loop arrays. The brochure described
various arrays
of receiving loop antennas. The antennas were manufactured and marketed by
a Canadian manufacturer for commercial and military use. The brochure had pictures of diamond-shaped terminated
loops, individual small loops, and loop arrays that looked like a number of Flag
antennas strung end-to-end where each rectangular “loop cell” fed the
next loop. The
brochure gave array directional patterns over wide frequency ranges. Most patterns looked much
better than patterns of other popular receiving antennas, such as Beverage antennas.
The least expensive array’s price, as I recall, was
around ten thousand dollars! That early
brochure started me thinking in terms of antennas other than Beverages.

Other things influenced
me also, including a few antenna lectures by John (Jack) Kuecken and his book,
“Antennas and Transmission lines”.  

Although
detailed technical information was withheld, and even though this was before NEC
modeling programs, it wasn’t difficult to work out an acceptable electrical design
for each array. I
experimented with small loop antennas that would form each element or cell in a
phased array, and eventually installed an array of
eight in-line loop cells. That system allowed me to work Japan and other Asian
countries through interference from  multiple multi-KW
LORAN transmitters.
I was actually the first station in the eastern part of the USA to work Japan
after WWII.

As the system was refined, I could work JA’s on a regular basis. This was long before
the broad LORAN pulse transmitters were removed from 1900 kHz, when the JA’s
transmitted between 1907.5 and 1912.5 kHz. (ZL’s were also confined to LORAN
frequencies, as were west coast stations.) Working DX through LORAN pulses was
possible only because of the clean pattern and wide deep nulls of loop arrays.   

Are
they quiet because they are loops? Not at all!

One of
the most damaging rumors to good antenna science are claims loop antennas
have a mystical property that somehow rejects noise. Nothing is further from the
truth, with one exception. Corona.

Any
sharp point extended from the area of any structure, even from tall wooden ship’s masts
on old
sailing vessels, is subject to corona in inclement weather. Sailors
feared  St. Elmo’s Fire long before anyone knew what electricity was, and
amateurs are burdened with it today when trying to receive in harsh or
threatening weather. We often just don’t know what causes the noise, and we can’t see
the fire in brighter skies, so we acknowledge this phenomena as
“precipitation static”. 

The
sharp point in a dipole, where the wire or tubing ends, not only promotes corona
by virtue of the fact it is “out in space by itself”, the open ends almost always
have the disadvantage of being at a very high impedance point. The tiny
random charge movements, with little current and very high voltage (high
impedance source), inherent in conductors around the corona discharge, are driving
a high impedance part of the system. This is an ideal situation to maximize transfer of
tiny amounts of noise power into the receiving system. A dipole element is not
only more subject to corona from sharp ends, corona is also impedance matched to
the receiver system.

Loops do
not have sharp points sticking out into air at high impedance points, and that
can be an advantage in antennas exposed to high electric fields in storms or
inclement weather. This is
almost certainly where the rumor loops “reject” noise comes from. We overlook
the fact corona noise is a function of the high impedance in the area where the antenna is
sharply shaped. We fixate on the fact it is a “quiet
loop” because of electromagnetic
radiation voodoo, where the time-varying electric field is somehow
independent of the magnetic field in a radio wave. A second effect occurs in
full wave loops or very small loops, where the antenna has higher common mode
impedance than a dipole, which can reduce common mode currents.

Are terminated loops actually loops?

Terminated loops do not behave like conventional small loop
antennas. Terminated loops do not carry uniform in-phase currents over the
entire antenna perimeter. They don’t behave like directional couplers either.
Small terminated elongated loops are
antennas…not nearfield coupling devices coupled to other conductors (we hope).

Terminated loops are really just short verticals, with a phasing
system inherent in
the longer horizontal component of antenna wires. The loop’s vertical areas receive the desired signals,
while the horizontal conductors
merely serve to act as transmission/phasing lines for the vertical (or sloping
vertical) ends. A small elongated terminated loop acts like a simple two-element
vertical array.
Elongated terminated loops, and arrays of elongated terminated loops, are a
special form of short phased verticals where phasing and feed systems are an
integral part of the antenna. These
antennas “want to” be verticals because the earth below them and
somewhat closer spacing prevents the horizontal components from being
efficiently coupled to space while the earth simultaneously enhanced the vertical
radiation component
of the gradually sloped conductors making up the antenna. The
horizontal sections act as a wide-spaced air dielectric transmission line. This
integrated system of elements, feed system, and phasing solves construction
problems associated with arrays using more recognizable vertical elements.

This is true even in the K9AY Loop
and Pennant antennas, which have sloped conductors. The sloped conductor behaves as a vertical (think about this when people follow the mistaken advice
that sloping the last few feet of a Beverage “stops noise pickup of
vertically polarized signals”), ten feet of vertical drop is still ten feet
of vertical conductor exposed to vertically polarized signals. The fact sloped
wire antennas work so well as verticals is testimony to how sensitive a sloped wire is to vertical
polarization.  If sloped wires weren’t sensitive to vertical signals, the Pennant and K9AY Loop wouldn’t work! 

With any system there are tradeoffs, elongated loop antennas
only allow very limited control of phase and unwanted high-angle response. We can’t
obtain optimum patterns (although a properly placed series capacitance
will help) because the integrated phasing and feed system allows very limited control of current distribution and phase
shift. 

These
antennas work best when propagation time of signals along the horizontal wires
matches propagation time of the wave in space around the antenna, and when the earth or
a ground system below the antenna is good. Suppression of high-angle horizontally
polarized radiation, and maintaining velocity of propagation near unity, are
why EWE antennas (and other forms of this family of
antennas) thrive on good ground systems or good earth below the antenna. The
desired earth effects are opposite those desired with Beverage antennas, an
ideal situation for highly conductive soil!

 For best performance:

S1 is smaller than S2

S2 is as large as possible, but smaller than 1/4λ

Currents are equal at all points

Phase is approximately 180 – S2 between feed and load

The antenna is the equivalent of two small crossfire verticals,
with horizontal component of wires forming a transmission line (radiation
cancelled by earth) and vertical component of wires forming antennas (which are
supported by earth reflections). A loop like this fires toward the feedpoint,
away from termination.

Eznec file courtesy of Roy Lewallen.
K9AY_Lewallen

Looking at outer points highlighted above, we find a phase
difference of about 22 degrees. Since one wire is fed from the top, and the
other from the bottom (they run in a circle), there is also 180 degrees phase
inversion. This is cross-fire phasing. The
resulting phase is 180+ -22 = 158 degrees phase shift with the junction of 1 and
2 lagging phase from the junction of 3 and 4. This lagging phase causes the
unidirectional pattern, firing in the direction of 1 and 2.

This K9AY is about 30-feet long, and about 25-feet high to the
apex, but the effective height is a bit lower. This is because wires 2 and 3,
being folded back on themselves at the apex, carry 180-degree out-of-phase
currents.

160 meters is about 1.5 feet per electrical degree, so the array
is about 20 electrical degrees spacing between front and rear elements. Ideal
phasing would be about 22 degrees from 180, which closely matches what the
horizontal component of wires adds! On 80 meters, the array is around 40 degrees
long. In this case phase shift prior to counting the 180-degree inversion should
be 40-degrees, which it is!

Pattern on 160 is:

Two short sloped verticals, at the same effective spacing,
height, and phasing, show:

The K9AY behaves just like two short sloped verticals with the
same phase shift.

The EWE

While outwardly appearing different, the Ewe operating mechanism
is no different than a Flag, Pennant, or K9AY. It behaves as two small crossfire
verticals. Instead of a wire return to complete a transmission line path, the
EWE relies on earth. As such, it requires a good soil path, or help from
grounded conductors.

Again we have crossfire phasing. One vertical is fed from the
top end, and the other at the bottom. The phase difference is about 30 degrees
or so, making the feed end lag the termination end by 180+ -30 = 150 degrees.
Again, because of crossed feed and electrical distance of S2, we have proper
phasing for the vertical ends. The feedpoint, because of the phase flip
associated with the current path, lags the termination end. All close spaced
end-fire arrays fire toward lagging current elements. 

Flags, Pennants, and Ewes all behave in a similar fashion,
because they all work on the same principle. There is no magical to them, and
they are easily analyzed. They are all effectively phased verticals with
acceptable, although less than perfect, phasing systems. While we might stumble
on a shape that works by modeling or experimenting, we could also create a new
shape just by following the guidelines of creating properly connected hidden
feed lines. However they appear by eye, they do not function as loops. They do
not carry uniform in-phase currents over the perimeter.  

Why
use terminated elongated loops?

It is
easy to understand why “loop” systems, even very small loop systems, have become popular. Arrays of
terminated loops vertical elements produce effective low-angle receiving performance along
with a somewhat clean pattern. EWE’s, Flags, Pennants, and K9AY loops are
effective methods of building two-element broadband vertical arrays. They are easy, small,
and inexpensive, even if noticeably less directive than two optimally phased
driven verticals.

They cannot be fully optimized
because horizontal components are not totally cancelled by ground effects
and the opposing wire, and because we cannot vary phase and current level
independently (electromagnetic radiation
comes from current, not voltage). The inherent limitation of a “leaky” radiating
phasing section that cannot be adjusted for phase and level independently distorts pattern a bit from
ideal.

On the positive side, elongated loops are easier to construct than phased
verticals. Phased verticals require more complex loading systems, grounds, and
interconnecting coaxial cable feed systems. We have
the standard old antenna tradeoff we just can’t seem to get away from. We always must
balance complexity and cost against performance.

Many people are working with various arrays of
elongated loop
antennas, so there are few contributions I can make other than describing how or
why they work. I would like to
suggest it is possible to extend the arrays end-to-end for some distance without
external feed systems, and well-placed reactances can be used to modify
patterns. Very little amateur radio work, other than my systems in the 1970’s
and 80’s, has been done in that area. I’d suggest experimenting
with series capacitors, perhaps placed mid-way in phasing (the horizontal wires)
areas, can increase velocity of propagation through elongated loop arrays and
increase directivity. But it all remains a compromise of phase and current
distribution, and minimizing “leakage” from the required horizontally
distributed physical component in wires, when feed systems are integrated into
structure.

Are
small loops hyper-sensitive to vertical masts?

There is
no compelling evidence that any of these antennas are more sensitive to vertical
metal masts than any other antenna would be. As a matter of fact, the only basis
for such claims appears to come from models that fail to pass simple recommended
tests for model accuracy and stability. If we build a model that is flawed and
oversensitive to changes in things, like the number of segments used in the
model, we can expect it to be hypersensitive to nearly any wire change!

Other than keeping a short mast a few
feet away from vertical wires and NOT connecting that mast directly to the
antenna or feed line, I wouldn’t hesitate for a second using metal supports. My
large arrays of loops in the early 70’s used metal masts, my arrays in the 80’s
did, and as have commercial arrays.

Feedpoint
Matching 

Great care must be taken in decoupling the feed line from the
antenna in the balanced versions of these antennas, although the EWE (being
unbalanced) is
relatively immune to such problems. Keep in mind the antenna generally looks
capacitive as a common-mode structure, so inductive decoupling (i.e. a choke
coil of coax) can actually increase
system problems. The best common-mode isolation system would be an isolated winding transformer designed with
minimal capacitance between the antenna winding and the rest of the
system.  I use a small transformer with stacked 73 material binocular cores
in feeding some of my high impedance “log-Beverage” arrays, and
similar transformers should work with ~1000 ohm impedance elongated loop
antennas.

Because this transformer only has a single turn primary (two
turns with the balancing pass), I’m able to reduce stray capacitance to a dozen pF or
less. It has excellent balance, low SWR over a wide bandwidth, and very low loss. The reasonably low transformer capacitance, when used in concert
with proper feed line grounding and routing, should make the system relatively
immune to common mode problems. I’d route the feed line horizontally directly
away from the end of the antenna for a few dozen feet (but never a distance
approaching 1/10th wavelength or longer), and then drop the feed line to ground
earthing the feed line shield at that point. Decoupling beads or sleeves belong
on the receiver side of the shield grounding point, not between the ground and
the antenna! 

I wind this transformer on three Fair Rite Products 2873000202
cores (about 1/2 inch square and 1/3 inch thick 73 material). The high impedance
(secondary) winding is #26 enameled wire through Teflon tubing, while the
primary is Teflon coated wire-wrap wire wound outside the tubing. The small
extra pass that “dangles” on the low-Z primary winding helps balance
the system, even though it adds a few pF of capacitance.

By the way, a Faraday shield will only make things worse. It
will increase unwanted stray capacitance and might deteriorate the high
impedance winding’s balance if the shield is not properly grounded. The proper grounding
point for a Faraday shielded primary is opposite the exit point of the
primary winding, or on the secondary winding’s exit side of the transformer. Most
Faraday shields described for Beverage and other transformers are not only
useless, they are often incorrectly grounded and actually increase unwanted
coupling! 

©2003 W8JI