Never Pick Gain, F/B, or Take-off Angle as a Parameter for Weak Signal
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
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
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
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
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:
Elements must be very low Q (wide bandwidth). They should have little reactance
change with frequency or weather.
Elements must be heavily swamped with loss. Mutual coupling effects must not
change element impedance.
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.
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
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
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
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.
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.
This structure is base
loaded with a series L/R
combination of approximately 30uH 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 feedlines 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 feedline should also be buried or if laid on the ground "choked"
with high permeability ferrite beads near each element.
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 feedline is ANY multiple of 90-degrees. With ANY phasing system having
standing waves on the feedline, you can properly feed the system by supplying
equal currents to any feedline an even multiple of 1/2 wl. Any feedline 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
After careful planning and
selecting the type of array, you should install the elements and the array's
internal feedlines. 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
feedline 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
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
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 feedline.
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 short verticals
have both advantages and disadvantages compared to other antennas, such as
Beverages and elongated loops. The comparison is:
Verticals and Wide-spaced
1/2- to 3-wl
modest to large
worse than 2-el vert,
about same as 4-sq, middle
middle to best
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 feedline
routing (other than keeping the feedline 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.
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 (soon to be an
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.
My next planned array is an
array of four four-squares modified to fire eight directions, with 500 foot center point
separation between each four-square array. 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.