[HCDX] article: Phasing Improves Kaz Antenna Nulls

[Mark Connelly <MarkWA1ION@xxxxxxxxxx>]

For HTML version of the following article, go to
"http://www.qsl.net/wa1ion/kaz/phased_kaz.htm";
It contains hyperlinks and some additional material.

Phasing Improves Kaz Antenna Nulls
(Mark Connelly, WA1ION - 10 JUL 2001)

The "Kaz" antenna was introduced by John Bryant's article "Testing Two 'Kaz'
Squashed Delta Antennas" (reference 1).  Further test results were presented
in my article "Pennant and Kaz Antenna Tests" (reference 2).  The antenna
has the form of a delta with the apex spaced above the center of the
horizontal base at a distance of 1/4 to 1/3 the length of the base.  Base
height above ground can be as little as 0.3 m / 1 ft., though I find
improved performance at 1.5 m or higher.  The top wire (forming the two
sloping sides) is one conductor and the base wire is a second conductor. 
Feeding and/or termination occurs at the pair of wires on each end of the
base at the point of approach of the upper conductor.

This article describes how the nulling abilities of the Kaz antenna can be
enhanced by having combination feed and termination boxes installed at each
end of the antenna.  The two coaxial feedlines are presented to inputs of a
phasing unit.

Homebrew phasers such as DXP-2, DXP-3, Superphaser-1, and Superphaser-2 work
well for this application.  Articles on these units can be obtained via
links on my RF Circuits page (reference 3).  Commercially-available
broadband phasing units such as the Quantum Phaser, modified MFJ-1026, and
JPS ANC-4 could also be used.

Each feedline supplies a pickup that is somewhat cardioid in shape.  If the
Kaz antenna is set up on an east-west axis, the feedline coming from the box
at the west end of the antenna has a pattern which, to some extent, nulls
signals from the east (+/- 30 degrees typical).  The feedline coming from
the box at the east end of the antenna has a pattern that tends to null
signals from the west.  Depending on antenna layout, height, surrounding
conductive objects, and termination resistance, the maximum null of each
cardioid can vary from as little as about 6 dB to as much as 40 dB.  In many
cases Vactrol control of termination can improve null depth over what can be
had with a fixed termination value (typically chosen to be about 1000 +/-
200 ohms).  Even if the terminations are fixed and cardioid front-to-backs
are only coming in around 10 dB each, very deep nulls can still be obtained
by phasing the two opposing direction cardioids against each other.

In the case of wanting to null a signal from the west with the above set-up,
the feedline from the west end of the antenna may be presented to phasing
unit Channel 1 and the feedline from the east end to Channel 2.  Channel 1
has a western station signal about 10 (and maybe more) dB stronger than that
same signal on Channel 2.  Also, desired eastern signals are on the order of
10 dB or more weaker on Channel 1 than on Channel 2.  When equalizing
Channel 1's western pickup to be about equal with that from Channel 2, the
level pot adjustment introduces about 10 dB of loss on Channel 1.  At this
point there is at least 20 dB of strength difference between the two
channels on eastern signals.  When the phase adjustment to null the western
signal is enacted, there is virtually no effect on eastern signals even if
they too are made to be 180 degrees out of phase between the two channels. 
The system's overall front-to-back ratio can be pushed to better than 50 dB
on groundwave and 25 dB on most skip with this arrangement: superior to a
single feedline approach even with Vactrol control.

The simplest way to do a combination feed and terminate at each end of the
antenna is to connect the antenna wires through a step-down transformer
matching 950 ohms to 50 ohms.  John Bryant's article "Fabricating Impedance
Transformers for Receiving Antennas" (reference 4) recommends an FT114-75
(FT114-J) Amidon ferrite toroidal core with a 20-turn high-impedance primary
winding (antenna) and a 5-turn low-impedance secondary winding (coaxial
feed).  He also mentions the alternative of an FT114-43 core with a 45-turn
high-impedance primary and a 10-turn low-impedance secondary.  A
Mini-Circuits T16-6T-X65 transformer has also been used successfully in some
Kaz and Pennant antenna installations.  If the "shack end" cable
terminations (e.g. at the two phasing unit inputs) are reasonably close to
50 ohms, the correct termination impedance for each side of the antenna will
be passed through the respective transformer (acting as a step-up in that
direction).

Because adjustment of a deep null is accomplished by phasing, there is less
need for Vactrol remote termination than with a single-feedline system.  Two
opposing cardioids of fairly mediocre null depth can be combined to produce
impressive front-to-back ratios.  This is quite the same as noted here with
opposing-pickup slopers even when each antenna is only good for about 8 - 10
dB of front-to-back on its own.  Also, as noted with slopers, a bit of
spatial separation helps too.  Phase shift on desired signals is less likely
to be in the 180 degree null range executed on opposite-direction signals
when feedpoints are separated by 1/15 to 1/3 wavelength (as compared to
being co-located or at some multiple of 1/2 wavelength).  As compared to a
small 12 m base version, the larger size Kaz antennas (base in the 20 - 40 m
/ 65 - 130 ft. range) will deliver heftier signals in the first place and
will also have the added benefit of a lower likelihood of collateral nulling
of desired-direction signals along with null-direction "pests" when inherent
cardioid null depths are below the ideal of at least 12 dB each.

The transformer feed / terminate scheme without additional amplification is
usually adequate for Kaz antennas having areas of 100 square meters or above
(e.g. base 20 m, apex 5 m above base).  Smaller antennas can benefit from
amplification.  Since coaxial cable has very little loss below 2 MHz, MW
DXers and 160-m hams will usually have a 50-ohm input / output amplifier on
each coaxial feedine at the "shack" end, out of the weather.  The W7IUV
design (reference 5) is a good choice.  The Kiwa broadband amplifier and the
Mini-Circuits model ZHL-6A are ready-made, though pricey, options.

There is one instance in which amplification located at the two antenna feed
points may be desired.  This is the circumstance when Vactrol control of
termination resistance is desired.  Because DC (typically 12 volts) to power
an amplifier and a separate Vactrol DC voltage both must be presented to
each of the two combination feed / terminate boxes, a single wire (carrying
Vactrol DC) must accompany the coaxial feed than carries DC to and RF from
the amplifier at a given end of the antenna.  This separate wire should be
broken up with several RF chokes to keep it from influencing the antenna. 
It can be physically attached to its accompanying coaxial line by means of
nylon cable ties.  The amplifier should be a high-impedance input to 50-ohm
output buffer type.  My BUF-E and BUF-F models (links via reference 3) work
well here.  The Vactrol's variable resistance can be placed from the input
of the buffer amplifier card on one side and, on the other side, to a 220
ohm resistor to circuit ground.  The two antenna leads go to the primary of
a custom 1:1 high-impedance transformer (to be described); the secondary of
this transformer goes to the buffer amplifier card input and to circuit
ground.  This arrangement gives about 15 dB of gain compared to no
amplifier; it also enables simultaneous Vactrol control of the null observed
when the phasing unit is set to the channel corresponding to the output of
the amplifier on the opposite side of the antenna.  The custom 1000 ohm 1:1
balun transformer consists either of 21 turns primary / 21 turns secondary
on an FT114-75 (FT114-J) core or, alternately, 45 turns primary / 45 turns
secondary on an FT114-43 core.

The Vactrol controller should be a dual version incorporating aspects of the
model presented in Figure 3 of my article "Pennant Antenna with Remote
Termination Control" (reference 6).  Chokes, dropping resistors, and diode
protection of the LED portion of the Vactrol (in each feed / terminate box)
should be configured similarly to Figure 5 of the Pennant article.

Having the independent Vactrol controls and "field site" buffer amplifiers
at each end of the antenna adds a good deal of complexity compared to the
simple transformer-feed method, but it is seen as a valuable approach to
take on smaller Kaz antennas like the 10 X 40 ft. / 3 X 12 m model that is
becoming popular for temporary installations.

You could use the buffer amplifiers without the Vactrol if desired.  In that
case you'd install a 1K fixed resistor or, better yet, a 2K pot across the
input of each buffer.  During installation each end's pot could be tweaked
to null a target station in the middle of the frequency range.  You'd
"listen" to the output of the amplifier opposite the one where you were
adjusting the pot.  An adjusted value of 800 ohms to 1.2K would be the
typical result.

Phasing is achieved by observing the normal operating procedure for the unit
being used.  This generally consists of, first, equalizing the amplitudes of
Channel 1 and Channel 2 on the signal to be nulled and, secondly, adjusting
the phase shift control to produce a null.  The procedure concludes with
small interactive adjustments of one or both amplitude pots and the phase
shifter.

If feedline pickup is a problem, one or more coaxial chokes (consisting of
17 turns of RG-174 on an FT140A-J core) may be inserted in series with the
coaxial line.

Anyone contemplating the use of a Kaz, Pennant, Flag, or Delta terminated
loop should look into two-feedlines-to-phaser schemes similar to those
outlined above.  The enhanced nulling performance makes the slightly greater
system complexity well worth the effort.

References:
(Note: Over time Web URL's may change.  If this occurs, it may still be
possible to retrieve the articles by going to known DXer Web sites or to
search engines for links.  Hard copies are likely to be available from the
National Radio Club and International Radio Club of America reprints
services.)

1. Testing Two 'Kaz' Squashed Delta Antennas, John Bryant, 2001
"http://members.aol.com/DXerCapeCod/kaztests.pdf";

2. Pennant and Kaz Antenna Tests, Mark Connelly, 2001
"http://members.aol.com/DXerCapeCod/pennant_v_kaz.htm";

3. RF Circuits page (links to construction articles)
"http://www.qsl.net/wa1ion/index.html";

4. Fabricating Impedance Transformers for Receiving Antennas,
John Bryant, 2001
"http://members.aol.com/DXerCapeCod/z_transformers.pdf";

5. W7IUV Amplifier
"http://www.qsl.net/wa1ion/amp/w7iuv_amp.htm";

6. Pennant Antenna with Remote Termination Control, Mark Connelly, 2000
"http://members.aol.com/DXerCapeCod/pennant.pdf"; and
"http://members.aol.com/DXerCapeCod/pennant.htm";






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