[HCDX]: compact cardioid-pattern antenna
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[HCDX]: compact cardioid-pattern antenna
Mark Connelly - WA1ION
30 William Road
Billerica, MA 01821-6079, USA
21 JUL 1998
Co-Located Passive Loop and Whip: another approach to a
compact cardioid array antenna system.
The terminated loop has been receiving quite a bit of attention
lately. I read (and enjoyed) the fine article "The K9AY
Terminated Loop - A Compact, Directional Receiving Antenna" by
Gary Breed (QST September, 1997). An important piece of
information is the fact that the electric and magnetic fields
are being combined to produce the cardioid null. I figured
that a co-located loop and whip would do the same and that
it could be done without having to use a conventional phasing
unit. If both the loop and whip are untuned and passive (not
amplified) prior to combining their contributions and if they
are located very close to each other, only amplitude balancing
is necessary to produce a null. This, in essence, is the same
thing being done in the K9AY loop. When you use a short vertical
and a loop, you can always get a cardioid pattern without
needing the ground rod.
In both the terminated loop and loop/whip cases, the best
cardioid null is obtained with the plane of the loop oriented
within a few degrees of the station to be nulled. If your loop
is east-west, good nulls of stations east or, with phase
reversal, the west can be expected. If a station to be nulled
is southwest, only a few dB of null might be realized. In that
case you would have to use a real phasing unit.
The set-up I tested consists of a square loop 6 feet (1.8 m)
on a side and a 6 foot telescoping whip.
The loop (magnetic field antenna) consists of two 6 foot whips
bolted to a 6 foot wooden board. These are connected top-to-top
with a wire having "alligator clips" on each end. Each whip has
a 3 foot wire at the base ends. As the two vertical sections
can be collapsed, this assembly is easily transported in a
vehicle and set-up on the car roof at a seaside DXpedition
site. The loop is fed at the midpoint of the lower horizontal
side. The two loop leads go to the primary of a 1:1 transformer
(Mini-Circuits T1-6-X65). The transformer's secondary goes to
the center conductor and to the shield of a 50-ohm coaxial
cable (Cable #1) which goes to the resistive combiner box at
the operating position.
The vertical whip (electric field antenna) is located about a
foot away from the center axis of the loop. The base of the
whip is connected to one side of the high-impedance winding
of a 16:1 transformer (Mini-Circuits T16-6T-X65). The other
high-impedance transformer winding is connected to a switch
that can connect it to a field-site earth ground rod or to
the shield of Cable #2 or to both the field-site and cable
grounds. The secondary (low-impedance side) of the transformer
goes to the center conductor and to the shield of a 50-ohm
coaxial cable (Cable #2) which goes to the resistive combiner
box at the operating position.
Instead of a controller box that operates a Byan Vactrol
remote termination, you have an in-shack combiner box consisting
of a 1K level pot for each of the two channels (loop, whip).
The center of each coaxial cable goes to the wiper arm of the
respective pot; one end of each pot (e.g. clockwise) is the
adjusted signal and the other end goes to chassis ground.
One of the two adjusted signals goes to the one of the primary
leads of a 1:1 transformer and that transformer's other
primary lead goes to ground. The two secondary leads go to
arms of a 3-pole 4-position rotary switch. The other adjusted
signal (the one that doesn't go to the combiner box's internal
transformer) goes to the third arm of that switch. The rotary
switch is wired such that its four positions provide output
consisting of the loop's contribution only, the whip's contribution
only, the signals combined with a 0-degree phase shift, and the
signals combined with a 180-degree phase shift (through reversal
of the secondary leads of the combiner box's transformer).
Note that this looks a bit like the switching in some phasing
units, but the combiner box is not a real phasing unit because
there is no continuous phase shifting (as done in the MFJ-1026
with a bridge circuit, in the DL-2 with a delay-line, and in
MWDX-5 etc. with L-C-R tuning). All continuous phase shifting
circuits add adjustment complexity and they cause attenuation
that degrades the signal-to-noise ratio.
Luckily, the resistive combiner that is used to obtain a
cardioid with the co-located broadband passive loop / whip
system does not have the adjustment complexity and S/N
degradation associated with true phasing. It is no more
complicated to operate than the controller to be used with
the terminated loop. The hit-or-miss nature of the field
site ground is also eliminated.
The "sigma" (summed) output signal of the combiner can go
directly to the receiver or to the input of a phasing unit.
However, with antennas of the diminutive dimensions of the
system described above, low-noise amplification is necessary.
I use an amplifier (VN10KM VMOS FET based) with about 23 dB
of gain and a good noise figure. Because small antennas
yield small signals, it's more important that the amplifier
chosen has a good noise figure and ample gain. Strong
signal parameters such as intercept points are of lesser
concern except, perhaps, at urban sites.
As with the terminated loops, two of these co-located loop/
whip systems can be presented to the inputs of a phasing unit
to get very deep nulls and a narrower, higher-gain forward lobe.
You can use two systems of similar orientation (e.g. peak-
east null-west cardioids) spaced about 1/4 to 1/16 wavelength
along the desired peak-null axis. You can use two systems
closer to each other and oriented for cardioids +45 degrees
and -45 degrees of the desired peak-null line. You can
phase a loop/whip cardioid system against a different style
antenna such as a loop (figure-of-8 pattern), a Beverage
(lopsided figure-of-8), or a vertical (circular pattern).
If the co-located loop/vertical system is configured with
physically larger elements, amplification may not be necessary.
One "caveat" is that the loop will behave less like a purely
"magnetic" antenna if it is made too large. In that case,
nulling may become more of a hit-or-miss proposition if you're
using a combiner not having variable phase shift.
I hope this discussion stimulates some further experimentation
and, as DXpedition season approaches, rare DX loggings.
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