Small Vertical Arrays

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Never
Pick Gain, F/B, or
Take-off Angle as a
Parameter for Weak
Signal Receiving!

Arrays
of small verticals
provide excellent
receiving
performance when
systems are designed
and installed
properly. As pointed
out in other
articles, there are
key differences
between receiving
and transmitting
systems. Parameters
considered important
in large
transmitting systems
are sometimes far
from optimum in
receiving
applications,
especially systems
using small
antennas. One key
item is gain.
Contrary to common
opinion, more gain
does not translate
to better receiving,
once the receiver is
limited by external
noise reaching the
receiver. The key
parameter is
directivity, which
may or may not have
a parallel
relationship with
gain. Gain includes
efficiency,
directivity excludes
efficiency. This
important
consideration
applies to systems
discussed
below.  

Noise
rarely comes from
one direction, or a
narrow range of
directions. With
that in mind,
antennas will be
compared by a
receiving
directivity factor
calculated with
Eznec’s (version 3)
average gain.
Removing efficiency
from the equation
allows direct
comparison of
receiving systems
since directivity,
not gain, is the
determining factor
in selecting an HF
or LF receiving
array.  S/N
ratio is very
dependent on nulling
or rejecting
unwanted signals or
noise.

It
is important to
locate receiving
antennas as far as
possible from
radiators or
re-radiators of
unwanted signals and
noise. Always
remember noise has
exactly the same
characteristics, so
far as an antenna is
concerned, as
signals from intentional
transmitters.
There is no way to
sort “good
signals” from
“bad
noise” except
through the
directional
characteristics of
your receiving
antenna. Noise is
not electric field
dominant. Desired
signals are not
magnetic field
dominant. The field
impedances are all
the same except near
the antenna or
source, and near the
antenna or source
coupling to multiple
unknown sources is
largely
unpredictable. The
truth is, it is
anyone’s guess what
field impedance is
actually best!

A
small loop antenna,
at a distance of a
few meters, is
magnetic field
dominant. Here is an
important fact few
people, outside of
those who work with
nearfield systems
know. At a distance
of an eighth wave
and larger a small
magnetic loop
becomes electric
field
dominant! 
Conversely, a small
voltage probe
becomes magnetic
field dominant at
about the same
distance! The fields
reverse dominance
because of phase
shift between the
fields as the
radiation fields
start to overtake
the induction
fields.

Susceptibility
to unwanted
near-field and
induction field
coupling between
receiving antennas
and large
transmitting
antennas or noise
sources is obviously
largely
unpredictable,
although  many
problems can be
corrected through
changes in antenna
placement or
detuning structures
and/or canceling the
radiation from
surrounding
structures. When
dealing with nulls,
a modest amount of
re-radiation from
surrounding
conductors can make
a large difference
in system
performance, but the
key is to watch
overall directivity.
Anything that
reduces directivity
will reduce the S/N
ratio of a receiving
system. The
reduction is
directly by the
amount of null
reduction ONLY
when noise comes
very predominantly
from within the area
encompassed by the
deeper areas of
null. We
always, unless we
have noise from a
specific direction
and angle all of the
time, want a wider
more modest depth
null in favor of
having a sharp point
with a deep null and
a wide-nosed
response.

One
of my best arrays on
Europe is only a few
hundred feet from a
transmitting
four-square, clearly
in the near field of
the four-square.
There is no
detectable influence
on this array when
it “looks
away” from the
four-square,
although there is a
quite noticeable
reduction of F/B
ratio when beaming
back into my
transmitting
antennas. The null
to the SW is very
deep, in excess of
35dB, regardless of
four-square tuning.
The converse is not
true, the null NE
when looking SW is
only 10-15dB deep
unless I detune the
antennas. Yet the
lower null depth
barely causes a
detectable noise
increase, because
the directivity does
not change much. It
is only when a very
dominant noise ( or
QRM) arrives from
the NE that this
array becomes almost
useless (compared to
other arrays with
deeper nulls in the
NE direction). Of
course detuning the
transmitting
antennas completely
restores southwest
performance, even
though spacing is
close.  
       

Building
Blocks For Arrays
Using Verticals

There
are four key areas
overlooked in most
published receiving
arrays using small
elements. Common
oversights in
element and phasing
system design cause
the antenna to be
more critical to
adjust, less stable,
and provide a poorer
pattern even if
array elements
happen to be
working.

The
most aspects are:

1.)
Elements must be
very low Q (wide
bandwidth). They
should have little
reactance change
with frequency or
weather.

2.)
Elements must be
heavily swamped with
loss. Mutual
coupling effects
must not change
element impedance.

3.)
The phasing system
must be designed for
the impedances that
actually appear at
the phasing system.
Transmitting-type
boxes, because the
systems have low
loss, must have high
mutual coupling
effects and
radically different
element impedances.
By definition, any
given system must
be seriously flawed
in one application
or the other!

4.)
The phasing system
must be stable and
have very broad
bandwidth
characteristics.

Before
building an array,
we must select an
element style that
we can live with.
Eznec and other
programs have made
this process simple.

The
Basic Element

Length

A
receiving array
element should be as
short as possible
but still maintain
sufficient
sensitivity (gain)
to ensure external
noise exceeds
receiver noise. My
160 Meter Band
elements are about
20-foot vertical
height. I’ve found
all of my arrays
with elements that
height have overall
sensitivities (gain)
on  par with my
Beverage antennas,
and that the signal
levels are very easy
to deal with. 

Keep
in mind that our
systems require more
gain as receiver
selectivity is
decreased. The noise
floor drops in
direct proportion to
selectivity
increase, and a
change from 2.5 kHz
selectivity to 250
Hz selectivity
reduces noise
voltage or power by
10dB. Signal level,
however, remains the
same for the same
transmitted power
within our
receiver’s
bandwidth.  

20-foot
tall elements with
reasonable element
spacing always 
provide more than
enough signal to
operate through
nearly 1/2 mile of
F-11 CATV cable
(similar to RG-11 or
RG-8 cables in size)
at my very quiet
rural location before
amplification. 3-5dB
noise figure
amplifiers are
adequate to
establish S/N ratio
by arriving noise
even when placed
after the signals
travel through those
long cable lengths.
To gauge my noise
floor, a standard
FT1000 with
preamplifier
“on” on my
200-foot vertical
has less than S3
noise in the SSB
position at mid-day.
Unless you have less
noon-time noise than
that, you will
certainly not
require an amplifier
at the antenna! The
only exception is if
you have very close
element spacing,
because close
spacing decreases
antenna sensitivity
(gain).     

Mechanical
and Electrical
Concerns

I
use two basic
mechanical
configurations of
elements. One system
uses steel
electrical conduit
on 1/2-inch
fiberglass rods
(rods driven
directly into the
ground) with four
“loading”
wires, while my
other system uses
stronger chain link
fence top-rail
mounted on wooden
posts. Both systems
handled weather from
ice storms (where
the antennas were
coated with almost a
radial inch of ice)
to high winds
without problems. I
have had no
electrical
connection problems,
and no weather
detuning problems. It
is not necessary to
“insulate”
the antennas mounted
on wooden posts
because system Q is
very low and
impedances are
modest. 
Wet posts will have
no deleterious
effect on
performance,
although I would
always place the
loading system (and
the base of the
verticals) above
snow depth. It
is not necessary to
use high-Q loading
inductors. The
only requirement is
that inductors
remain relatively
stable in
characteristics with
climatic
changes. 

My
systems are
normalized at 75-
ohms for several
reasons: 

1.)
75-ohm feed produces
a wider VSWR
bandwidth than
50-ohm feed systems,
the swamping
resistive losses are
about 50%
higher. 

2.)
CATV cable suitable
for direct burial is
inexpensive and
connectors are
inexpensive,
reliable, and easy
to install. 

I
use F6 flooded CATV
cable for local
cables in arrays,
and F11 (RG-11 size)
or 5/8″ flooded
CATV cable for trunk
leads. It is
NOT necessary to use
double or triple
shielded cable, you
gain nothing. But
you do want to use
good quality cable
that will last years
without weather
changes. 

Practical
Verticals

My
rectangular arrays
use elements with
four 20 foot long
#16 loading wires,
insulated by
fishing line used to
support the wires. I
terminate the guying
20 feet out from the
base of the antenna.
The entire structure
is self-resonant on
80 meters. The large
“hat”
makes current
essentially uniform
throughout the
vertical element
while minimizing
unwanted sensitivity
to high angle
radiation, and also
supports the
elements.

Eznec
Download 

Eznec RXvrhat

This
structure is base
loaded with a series
L/R

combination
of approximately 30

uH
and contains a
total loss
resistance of 75
ohms. This
resistance includes
resistive losses
related to inductor
Q, as well as ground
system loss
resistance. I used
small molded choke
inductors, although
other components
will work. My system
requires only 56
ohms of lumped
resistance to bring
base resistance to
75 ohms. 

Each element
requires a stable
ground system.
Ground loss is not
important, but long
and short term loss
stability with
climatic changes is
very important. I
use a minimum of
four buried radials,
each 1/8- to 1/4-wl
long, on each
element. Always
place radials
directly under each
hat wire. Do NOT
use small elevated
radial systems or
grossly
non-symmetrical
radial systems!
Elevated radials
will reduce VSWR
bandwidth of the
array, introducing
unwanted phase
shift. They also
make the system
susceptible to high
angle signals, and
are more susceptible
to common-mode noise
on feed lines and
other conductors
around the antenna
than buried or
earthed radials. It
is not necessary to
bury radials, but if
the radials aren’t
buried multiple
ground rods are a
good idea. The
feed line should also
be buried or if laid
on the ground
“choked”
with high
permeability ferrite
beads near each
element.

feed line
Length

Unlike
transmitting arrays,
it is not necessary
to use odd-quarter
wave lines. It might
be a tiny bit better
if you use exact
multiples of 1/4 wl,
but even 1/2 wl
lines work perfectly
fine. Multiples of
1/4 wl work better
in cases where you
might fail to match
antenna impedances
to the transmission
lines correctly. The
reason of this is
that phase shift in
a transmission line
is independent of
line SWR when the
feed line is ANY
multiple of
90-degrees. With ANY
phasing system
having standing
waves on the
feed line, you can
properly feed the
system by supplying
equal currents to
any feed line an even
multiple of 1/2 wl.
Any feed line having
odd multiples of 1/4
wl requires equal
voltages feeding the
line. The phasing
systems I use,
unlike transmitting
systems, are
designed to supply
either equal
voltages or equal
currents! The proper
ratio adjustments
are easily made. You
will, however, have
slightly less phase
error if you use any
multiple of 1/4 wl
when the lines are
mismatched.

Beware that foam
cables are NOT .82
or any other
standard velocity
factor. They range
from the .70 range
up to around 
.92 in velocity
factor, depending on
the ratio of gas to
material in the
dielectric. Only
solid dielectric
cable are
predictable without
measuring the
cable.       

Tuning the
Elements

After careful
planning and
selecting the type
of array, you should
install the elements
and the array’s
internal feed lines.
Each element must be
evaluated with an
antenna analyzer
that measures
resistance and
reactance. Connect
the analyzer at the
element’s feedpoint,
and follow these
steps:

1.) Using a two
foot long or less
jumper cable,
measure the antenna
without a loading
coil. Check the
resonance for
predicted values. It
should be within
several percent of
the modeled
self-resonant
frequency. 

2.) Touch the
shield of the
feed line in the
array to the case of
the analyzer or the
vertical’s ground.
If impedance changes
more than five
percent, you need a
better ground
system. 

3.) Install the
loading inductance
predicted, and sweep
the desired
frequency range for
lowest SWR and zero
reactance. If your
analyzer is working
correctly the lowest
SWR will be at zero
reactance, or very
close to that
frequency. 

4.) Fine tune the
inductance to make
the antenna resonant
at the desired
frequency. You do
this by adding or
removing small
inductors in series
with a main
inductor, or by
adding or removing
turns. Fine
adjustments can be
made by squeezing or
spreading turns on
the main loading
coil’s form, if it
is a toroid or
non-potted
construction.    

5.) Add enough
series resistance to
bring antenna
impedance to 75 ohms
at resonance
(assuming you use 75
ohm cables).

6.) Check the
feedpoint again for
stability by
connecting and
disconnecting the
shield of the unused
array transmission
line from the case
of the analyzer or
the ground system
connection point.
Again it should
remain within 5%, or
you need to improve
the ground. If you
can not improve the
ground, you will
have to isolate the
ground by using a
choke balun on the
feed line.   

Once one element
is tested and
proven, you should
be able to duplicate
that element with
near-perfect
results. I remove
the matching system
and take it to the
test bench, and find
a series C/R
combination that
produces the same
resonant frequency.
Multiple networks
can be constructed
on the test bench,
and then moved to
each element. My
elements normally
fall within 20 kHz
of each other, any
large difference in
impedance or
resonant frequency
is a sign of
potential
performance
problems.

Here is the 75
ohm SWR plot of this
element: 

Bandwidth
is excellent, and
sensitivity (gain)
including all losses
is -13 dBi. This
element is almost
perfect for use in
small receiving
arrays, since signal
level and bandwidth
are very good.
Because of the large
amount of
capacitance and the
resistance loading,
the element will not
significantly change
phase or sensitivity
with frequency over
the entire 160-meter
band!
In
addition, it has
very little response
to high angle
signals (and
noise). 

In
circular arrays, hat
wires can be
extended to ~35 feet
with only three
loading wires used.
My 350- foot
diameter arrays
position two loading
wires (using tarred
nylon fishing net
string for
insulation) in line
with the perimeter
of the antennas,
while the third wire
on each element is
used to “pull
out” from the
array center. This
tensions the
perimeter wires and
guys the entire
structure. Even
though hat wires are
not spaced exactly
120 degrees apart,
the effects on
sensitivity to high
angle signals are
insignificant.

Arrays
of Elements 

Arrays
of short verticals
have both advantages
and disadvantages
compared to other
antennas, such as
Beverages and
elongated loops. The
comparison is:

The
primary advantage of
arrays of short
vertical arrays are
excellent pattern
and reasonable
signal levels.
Unlike balanced
elongated loop
systems, they are
non-critical for
feed line routing
(other than keeping
the feed line on the
ground), matching
transformers
(transformers are
not even required!),
and earth conditions
around the antenna.
They have the same
approximate output
as simple Beverage
arrays, and can
still be made to
work over very wide
frequency ranges.
The disadvantage is
they are more
complex, and require
bandswitching to
work on two bands.

Note:
I am finishing the
layout of an FET
amplified array that
will allow ten-foot
non-hat verticals
that work from VLF
to 80-meters and
above, with the same
basic directional
characteristics over
that range without
switching.

Broadside-Endfire
Array

The
most directive
four-element antenna
possible is a
broadside-endfire
array. I use 70 feet
endfire spacing and
330-feet broadside
spacing between endfire cells, and
get a pattern like
this:

This
array has an RDF of
13dB (two dB more
than the large
4-square), and a
HPBW of 47 degrees!
The performance is
similar to a
circular array with
eight elements in a
350-foot diameter
circle, that allows
directional
selection every 45
degrees. The
circular array uses
techniques similar
to the
broadside-endfire,
except the relay
system is modified
to switch eight
antennas.

I’ve found that 1000
feet is about the
maximum physical
separation allowing
signals to be
reliably combined on
160-meters. Arrays
occupying areas
larger than 2
wavelengths have far
too much random
phase and amplitude
shift, preventing
reliable combining
of signals.