Director/Driven Element 2-Element Yagis
Some Ideas for 12 and 17
Meters
L. B. Cebik, W4RNL
I have in other notes made mention of the
Director/Driven Element configuration of the 2-element Yagi. When pressed to
maximum gain, these antennas are capable of over 7 dBi free space gain, about a
dB higher than the conventional Reflector/Driven Element configuration tuned for
maximum front-to-back ratio. However, these antennas have a low feedpoint
impedance and fairly narrow band widths.
Jerry Haigwood, W5JH, reminded me that with a beta match, the low impedance
can be overcome, and on the WARC bands, narrow bandwidth is not a real concern.
Jerry is exactly correct, and his comments and other good ideas raised some
interesting design possibilities.
Jerry prefers spacing his director about 0.08 wl from the driven element. The
range from 0.07 to 0.09 wl is a good choice. Even though one might get a bit
more gain from the antenna, the 1/12th wl spacing holds the feedpoint impedance
of a D/DE Yagi set for a good front-to-back ratio at about 20 ohms, which is
quite workable.
Figure 1 shows the construction of a reasonable D/DE Yagi. The 1"
elements of this 10-meter model are feasible hardware store material, but the
model is designed less to build than to set the properties of the genre of beam.
With wider spacing, the antenna gain drops off, although the feedpoint impedance
goes up. Closer spacing raises gain for a while, but continuously drops the
feedpoint impedance. The model antenna, set for 28.5 MHz, has a feedpoint
impedance of 20.5 - j23.5 ohms, just about right for a hairpin or inductor beta
match of very conventional design.
Incidentally, HAMCALC has a very nice program for calculating beta matches,
including the hairpin. The equivalent inductance is also given, which permits
you to use another program on HAMCALC to create a coil instead. See the basic
"Radio" page for the address of VE3ERP, the master of this suite of GW BASIC
utility programs.
With only slight readjustments, the basic 2-element D/DE Yagi can be swept
through a variety of radiation patterns. Three of them are illustrated in
Figure 2. The Maximum Front-to-Back settings will yield a modest gain
(about 6.5 dBi free space) and a very good (greater than 20 dB) front-to-back
ratio with a feedpoint impedance of about 20 ohms. At the other extreme, maximum
gain provides over 7 dBi gain but under 10 dB front-to-back and a feedpoint
impedance of about 10 ohms. There is a midpoint setting shown in the patterns,
where the gain is intermediate, the front-to-back is respectable, and the
feedpoint Z is about 15 ohms.
These setting are not so very far apart, and a frequency difference of about
1.5% will sweep you through them. A little further, and you will experience
pattern reversal. On 10 meters, this is under 400 kHz. So unless your needs are
very frequency specific, the D/DE Yagi may not be a good design choice.
However, on the WARC bands, no such problems occur. On 12 and 17 meters, the
available bandwidth is about 1/2 of 1% of the frequency, and the D/DE design can
easily cover this spread with stable characteristics.
Back-to-Back 12-17-Meter Yagis
One of the chief attractions of the D/DE design is the short boom length
required for the antenna. For example, 0.07 wl is only 3.8' at 17 meters (with
elements between 26 and 28') and 2.75' at 12 meters (with elements between 18
and 20'). Modeling suggests that individual antennas for these two bands show
very little interaction when placed back-to-back and separated by about a foot
or more. Hence, an 8' boom would hold back-to- back individual antennas.
Individual antennas, however, require separate feedlines or a switching
system. We can make life even simpler and cut the feedline needs down to 1
feedline. By placing the driven elements close together, we can use open-sleeve
coupling. This involves connecting the feedline permanently to the 17 meter
driven element and letting it excite the 12 meter element when fed with 12 meter
energy. The required spacing is just about 4" which requires that we retune the
12 meter elements for this configuration. Once done, however, the two beams do
their jobs with few signs of other interactions. Moreover, the two antennas
require only a 7' boom.
To test this idea, I created models of a double D/DE Yagi using open sleeve
coupling and placed back-to-back. I used an aggressive stepped diameter tubing
schedule to keep the antenna array as light as possible. Figure 3 shows a sketch
of the overall dimensions of the antenna.
Since the tubing schedule would not show well in the sketch, here is the
antenna description file with a detailed list of lengths and diameters of
aluminum tubing.
--------------- WIRES ---------------
Wire Conn.--- End 1 (x,y,z : ft) Conn. -- End 2 (x,y,z : ft) Dia(in)
12-meter director
1 -9.570, 3.080, 0.000 W2E1 -6.500, 3.080, 0.000 3.75E-01
2 W1E2 -6.500, 3.080, 0.000 W3E1 -2.500, 3.080, 0.000 5.00E-01
3 W2E2 -2.500, 3.080, 0.000 W4E1 -1.000, 3.080, 0.000 6.25E-01
4 W3E2 -1.000, 3.080, 0.000 W5E1 1.000, 3.080, 0.000 7.50E-01
5 W4E2 1.000, 3.080, 0.000 W6E1 2.500, 3.080, 0.000 6.25E-01
6 W5E2 2.500, 3.080, 0.000 W7E1 6.500, 3.080, 0.000 5.00E-01
7 W6E2 6.500, 3.080, 0.000 9.570, 3.080, 0.000 3.75E-01
12-meter driven element
8 -10.120, 0.330, 0.000 W9E1 -6.500, 0.330, 0.000 3.75E-01
9 W8E2 -6.500, 0.330, 0.000 W10E1 -2.500, 0.330, 0.000 5.00E-01
10 W9E2 -2.500, 0.330, 0.000 W11E1 -1.000, 0.330, 0.000 6.25E-01
11 W10E2 -1.000, 0.330, 0.000 W12E1 1.000, 0.330, 0.000 7.50E-01
12 W11E2 1.000, 0.330, 0.000 W13E1 2.500, 0.330, 0.000 6.25E-01
13 W12E2 2.500, 0.330, 0.000 W14E1 6.500, 0.330, 0.000 5.00E-01
14 W13E2 6.500, 0.330, 0.000 10.120, 0.330, 0.000 3.75E-01
17-meter director
15 -13.000, -3.785, 0.000 W16E1 -9.000, -3.785, 0.000 5.00E-01
16 W15E2 -9.000, -3.785, 0.000 W17E1 -3.500, -3.785, 0.000 6.25E-01
17 W16E2 -3.500, -3.785, 0.000 W18E1 -1.500, -3.785, 0.000 7.50E-01
18 W17E2 -1.500, -3.785, 0.000 W19E1 1.500, -3.785, 0.000 8.75E-01
19 W18E2 1.500, -3.785, 0.000 W20E1 3.500, -3.785, 0.000 7.50E-01
20 W19E2 3.500, -3.785, 0.000 W21E1 9.000, -3.785, 0.000 6.25E-01
21 W20E2 9.000, -3.785, 0.000 13.000, -3.785, 0.000 5.00E-01
17-meter driven element
22 -13.850, 0.000, 0.000 W23E1 -9.000, 0.000, 0.000 5.00E-01
23 W22E2 -9.000, 0.000, 0.000 W24E1 -3.500, 0.000, 0.000 6.25E-01
24 W23E2 -3.500, 0.000, 0.000 W25E1 -1.500, 0.000, 0.000 7.50E-01
25 W24E2 -1.500, 0.000, 0.000 W26E1 1.500, 0.000, 0.000 8.75E-01
26 W25E2 1.500, 0.000, 0.000 W27E1 3.500, 0.000, 0.000 7.50E-01
27 W26E2 3.500, 0.000, 0.000 W28E1 9.000, 0.000, 0.000 6.25E-01
28 W27E2 9.000, 0.000, 0.000 13.850, 0.000, 0.000 5.00E-01
The feedpoint is on wire #25, the center of the 17-meter driven element. Do
not expect the model to be exact on the spacing for the open sleeve coupling
distance. Rather, experiment and measure the impedance on both bands as you
work.
The center of gravity should be just on the 17-meter side of the 17-meter
driven element, minimizing the need for coax to run along the boom. Almost any
boom in the 1.25" range should do, even a length of the lighter TV masting
(which is too light for mast use), as long as it is weather protected so that it
does not rust out in a year. Even hardware store aluminum with a wall thickness
of 0.055" should handle the job, although inserts at the element clamping points
should be used to prevent tube crush. Alternatively, you can use short sections
of the next tubing size up as strengtheners at the element clamping points. The
model is designed for elements insulated from the boom, so if you use direct
clamping to the boom, expect to adjust element lengths a bit.
The feedpoint impedance is just about 21 ohms resonant for both frequency
ranges. This permits the use of a broad-band balun to feed the antenna and
effect an impedance transformation to the 50-ohm coax line. alternatively, the
antenna lengths can be reset to show capacitive reactance. A Beta match usually
is effective not only at the frequency for which it is designed, but as well at
higher frequencies, and with a little juggling of dimensions, a 2-band match
should be obtained.
Figure 4 shows the patterns of the two antennas in one back-to-back
pattern at free space. At 70' up, the gain is in the neighborhood of 12 dBi, a
little under 5 dB better than a dipole, and with a strong front-to-back ratio.
The back-to-back open-sleeve D/DE Yagis are not world beaters, but they are
a. inexpensive to build, and b. lots better than some of the antennas being
pressed into service for these bands. The entire antenna, boom and all, should
weight in at less than 20 pounds, making it a good candidate for stacking on top
of an existing antenna.
If you separate the two antennas, you will have to adjust the
dimensions--especially of the 12-meter model--for independent use. Likewise, if
you use different materials or a different schedule of diameter steps, you will
also have to adjust the dimensions to restore the pattern and the feedpoint
impedance. But that comes with the territory of antenna experimenting and
home-brewing.
12-17-Meter 2-Element Yagis Facing the Same Way
Is it feasible to place the two antennas in the same plane using the
open-sleeve coupling system for a single feed line? The answer is yes and no.
Yes, it is possible to develop a set of dimensions that will produce good
performance at a desirable feed impedance for one frequency within the upper
band. (The lower band is not affected.) However, both elements for the upper
band are inside the elements for the lower band, producing a "cage" effect. The
chief problem created is an extreme narrow bandwidth for desireable
characteristics. In terms of this design, when the beams face in opposite
directions, they show an operating bandwidth for under 2:1 SWR in excess of the
100 kHz width of the bands. When caged (or facing in the same direction), the
operating bandwidth of the upper band beam is far less than the width of the
band. For building and tuning, this also means that hitting the precisely needed
dimensions is very tricky--and the settings are very susceptible to the need for
change with changes of antenna height below about 1.5 wl. Hence, my
recommendation is for opposing directions or for combinations of beam types,
where the upper band antenna is "uncaged." (Yes, I know, I watch too many
wildlife documentaries and it is affecting my choice of terminology.)
There is an alternative that achieves the uncaging. It has the disadvantages,
relative to the Janus-faced design above, of requiring a longer boom (10') and
sacrificing a good bit of 17-meter front-to-back ratio. On the other hand, the
beams face the same direction and require a single directly-matched 50-Ohm feed
to cover both bands.
The design is a DE-Reflector for 17 and a DE-Director for 12. By keeping the
elements for each band together, they remain uncaged. In addition, there is a
slight forward stagger effect so that the beams have marginally higher values
than when they are independent. With the spacing used, the feedpoint impedance
on both bands is close to 50 Ohms. The outline dimensions are shown in Figure
5.
The same caution about spacing of the open-sleeve coupled driven elements
applies to both designs: the builder will have to adjust both length and spacing
of the 12-meter slaved driver, since the modeling program is close to its limits
for handling closely spaced elements of different lengths. However, the model
should be quite close.
In addition, the antenna was designed with a tapering schedule in place. Here
is the detailed wire table.
--------------- WIRES ---------------
Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in) Segs
12-meter Director
1 -9.500, 3.130, 0.000 W2E1 -6.500, 3.130, 0.000 3.75E-01 5
2 W1E2 -6.500, 3.130, 0.000 W3E1 -2.500, 3.130, 0.000 5.00E-01 5
3 W2E2 -2.500, 3.130, 0.000 W4E1 -1.000, 3.130, 0.000 6.25E-01 2
4 W3E2 -1.000, 3.130, 0.000 W5E1 1.000, 3.130, 0.000 7.50E-01 3
5 W4E2 1.000, 3.130, 0.000 W6E1 2.500, 3.130, 0.000 6.25E-01 2
6 W5E2 2.500, 3.130, 0.000 W7E1 6.500, 3.130, 0.000 5.00E-01 5
7 W6E2 6.500, 3.130, 0.000 9.500, 3.130, 0.000 3.75E-01 5
12-meter Driver
8 -10.070, 0.380, 0.000 W9E1 -6.500, 0.380, 0.000 3.75E-01 5
9 W8E2 -6.500, 0.380, 0.000 W10E1 -2.500, 0.380, 0.000 5.00E-01 5
10 W9E2 -2.500, 0.380, 0.000 W11E1 -1.000, 0.380, 0.000 6.25E-01 2
11 W10E2 -1.000, 0.380, 0.000 W12E1 1.000, 0.380, 0.000 7.50E-01 3
12 W11E2 1.000, 0.380, 0.000 W13E1 2.500, 0.380, 0.000 6.25E-01 2
13 W12E2 2.500, 0.380, 0.000 W14E1 6.500, 0.380, 0.000 5.00E-01 5
14 W13E2 6.500, 0.380, 0.000 10.070, 0.380, 0.000 3.75E-01 5
17-meter Driver
15 -13.100, 0.000, 0.000 W16E1 -9.500, 0.000, 0.000 3.75E-01 4
16 W15E2 -9.500, 0.000, 0.000 W17E1 -6.750, 0.000, 0.000 5.00E-01 4
17 W16E2 -6.750, 0.000, 0.000 W18E1 -4.000, 0.000, 0.000 6.25E-01 4
18 W17E2 -4.000, 0.000, 0.000 W19E1 4.000, 0.000, 0.000 7.50E-01 11
19 W18E2 4.000, 0.000, 0.000 W20E1 6.750, 0.000, 0.000 6.25E-01 4
20 W19E2 6.750, 0.000, 0.000 W21E1 9.500, 0.000, 0.000 5.00E-01 4
21 W20E2 9.500, 0.000, 0.000 13.100, 0.000, 0.000 3.75E-01 4
17-meter Reflector
22 -14.100, -6.800, 0.000 W23E1 -9.500, -6.800, 0.000 3.75E-01 4
23 W22E2 -9.500, -6.800, 0.000 W24E1 -6.750, -6.800, 0.000 5.00E-01 4
24 W23E2 -6.750, -6.800, 0.000 W25E1 -4.000, -6.800, 0.000 6.25E-01 4
25 W24E2 -4.000, -6.800, 0.000 W26E1 4.000, -6.800, 0.000 7.50E-01 11
26 W25E2 4.000, -6.800, 0.000 W27E1 6.750, -6.800, 0.000 6.25E-01 4
27 W26E2 6.750, -6.800, 0.000 W28E1 9.500, -6.800, 0.000 5.00E-01 4
28 W27E2 9.500, -6.800, 0.000 14.100, -6.800, 0.000 3.75E-01 4
As always, since the taper schedule affects performance, you will have to
adjust dimensions to suit the materials you have on hand. Four light-weight
elements on a 10' boom should not stress an extended mast used to stack this
simple beam above a heavy-weight tribander.
As Figure 6 shows, the gain of the two beams is about the
same--between 6.3 and 6.4 dBi in free space. However, the 17-meter front-to-back
ratio is reduced considerably. However, until the WARC bands become seriously
over-crowded, this may not be a major problem.
Back-to-Back with a 30-Meter Dipole Between
A couple of requests came in
for an odd combo: a 30-m dipole wit the back to back 17-12 Yagi combo. This
combo would fit on an 8' boom, with the big dipole (48.6') centered at the
mast--but perhaps due to the greater weight of the 17-meter Yagi and the longer
boom to that side, offset so that the dipole did not sit right on the mast. Such
an antenna is possible, and the sketch shows its outline (Figure 7)
The dipole is now the fed element, with both Yagi drivers open-sleeve
coupled. The frequency ratio for the 17-meter driver (10.125:18.118) is low
enough to make it easy to preserve the full performance of the 2-element DE-Dir
beam (6.5 dBi/28 dB F-B) with a 53-Ohm feedpoint impedance. The frequency ratio
for the 12-meter driver (10.125:24.95) is large enough to adversely affect
performance to a small degree (6.3 dBi/20 dB F-B) with a 47-Ohm feedpoint
impedance. The dipole pattern is slightly flattened as the Yagis stretch it in
the planar directions--the amount is about 0.2 dB. The dipole impedance in free
space showed about 78 Ohms. In all cases, the impedance figures are good enough
for standard coax feed systems.
Here is the modeled wire table for the 3-band antenna. Unfortunately, I lack
ftp capabilities and cannot provide a downloadable .NEC or .EZ file.
--------------- WIRES ---------------
Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in) Segs
12-m director
1 -9.600, 3.340, 0.000 W2E1 -6.500, 3.340, 0.000 3.75E-01 5
2 W1E2 -6.500, 3.340, 0.000 W3E1 -2.500, 3.340, 0.000 5.00E-01 5
3 W2E2 -2.500, 3.340, 0.000 W4E1 -1.000, 3.340, 0.000 6.25E-01 2
4 W3E2 -1.000, 3.340, 0.000 W5E1 1.000, 3.340, 0.000 7.50E-01 3
5 W4E2 1.000, 3.340, 0.000 W6E1 2.500, 3.340, 0.000 6.25E-01 2
6 W5E2 2.500, 3.340, 0.000 W7E1 6.500, 3.340, 0.000 5.00E-01 5
7 W6E2 6.500, 3.340, 0.000 9.600, 3.340, 0.000 3.75E-01 5
12-m driver
8 -10.050, 0.600, 0.000 W9E1 -6.500, 0.600, 0.000 3.75E-01 5
9 W8E2 -6.500, 0.600, 0.000 W10E1 -2.500, 0.600, 0.000 5.00E-01 5
10 W9E2 -2.500, 0.600, 0.000 W11E1 -1.000, 0.600, 0.000 6.25E-01 2
11 W10E2 -1.000, 0.600, 0.000 W12E1 1.000, 0.600, 0.000 7.50E-01 3
12 W11E2 1.000, 0.600, 0.000 W13E1 2.500, 0.600, 0.000 6.25E-01 2
13 W12E2 2.500, 0.600, 0.000 W14E1 6.500, 0.600, 0.000 5.00E-01 5
14 W13E2 6.500, 0.600, 0.000 10.050, 0.600, 0.000 3.75E-01 5
17-m driver
15 -13.780, -0.620, 0.000 W16E1 -9.000, -0.620, 0.000 5.00E-01 6
16 W15E2 -9.000, -0.620, 0.000 W17E1 -3.500, -0.620, 0.000 6.25E-01 6
17 W16E2 -3.500, -0.620, 0.000 W18E1 -1.500, -0.620, 0.000 7.50E-01 3
18 W17E2 -1.500, -0.620, 0.000 W19E1 1.500, -0.620, 0.000 8.75E-01 5
19 W18E2 1.500, -0.620, 0.000 W20E1 3.500, -0.620, 0.000 7.50E-01 3
20 W19E2 3.500, -0.620, 0.000 W21E1 9.000, -0.620, 0.000 6.25E-01 6
21 W20E2 9.000, -0.620, 0.000 13.780, -0.620, 0.000 5.00E-01 6
17-m director
22 -13.050, -4.480, 0.000 W23E1 -9.000, -4.480, 0.000 5.00E-01 6
23 W22E2 -9.000, -4.480, 0.000 W24E1 -3.500, -4.480, 0.000 6.25E-01 6
24 W23E2 -3.500, -4.480, 0.000 W25E1 -1.500, -4.480, 0.000 7.50E-01 3
25 W24E2 -1.500, -4.480, 0.000 W26E1 1.500, -4.480, 0.000 8.75E-01 5
26 W25E2 1.500, -4.480, 0.000 W27E1 3.500, -4.480, 0.000 7.50E-01 3
27 W26E2 3.500, -4.480, 0.000 W28E1 9.000, -4.480, 0.000 6.25E-01 6
28 W27E2 9.000, -4.480, 0.000 13.050, -4.480, 0.000 5.00E-01 6
30-m dipole
29 -24.300, 0.000, 0.000 W30E1 -17.500, 0.000, 0.000 5.00E-01 5
30 W29E2 -17.500, 0.000, 0.000 W31E1 -12.650, 0.000, 0.000 6.25E-01 5
31 W30E2 -12.650, 0.000, 0.000 W32E1 -7.150, 0.000, 0.000 7.50E-01 5
32 W31E2 -7.150, 0.000, 0.000 W33E1 -5.150, 0.000, 0.000 8.75E-01 2
33 W32E2 -5.150, 0.000, 0.000 W34E1 -2.000, 0.000, 0.000 1.00E+00 3
34 W33E2 -2.000, 0.000, 0.000 W35E1 2.000, 0.000, 0.000 1.12E+00 3
35 W34E2 2.000, 0.000, 0.000 W36E1 5.150, 0.000, 0.000 1.00E+00 3
36 W35E2 5.150, 0.000, 0.000 W37E1 7.150, 0.000, 0.000 8.75E-01 2
37 W36E2 7.150, 0.000, 0.000 W38E1 12.650, 0.000, 0.000 7.50E-01 5
38 W37E2 12.650, 0.000, 0.000 W39E1 17.500, 0.000, 0.000 6.25E-01 5
39 W38E2 17.500, 0.000, 0.000 24.300, 0.000, 0.000 5.00E-01 5
As with the other designs, you will have to experiment for other taper
schedules to the elements. Likewise, the exact lengths and spacing of the
drivers from the dipole will also be am tter of experimentation.
If you shorten the dipole by any other means than a capacity hat on each end,
the performance of the Yagis will be thrown off due to the altered current
levels along parts of the dipole providing the drive for the coupled Yagi
drivers. Even mid-element loads throw everything off. Capacity hats do preserve
the current levels along the shortened dipole at the same magnitude as with a
full size dipole; however, the hats will necessarily by large enough to possible
couple to the Yagi driver ends.
No doubt further modeling time could yield improved performance from the 12-m
Yagi, with many iterations of element length, spacing, and spacing from the
dipole changes. But the figures shown here sould suffice as a starting point for
individual experimentation.
A Forward-Facing 17-12 Combo with Higher Gain on 12
It is possible to
obtain greater gain on 12 meters with a forward-facing combination of a
Driver-Reflector Yagi for 17 and a driver-director Yagi for 12, using the same
forward-stagger and open-sleeve principles of the earlier 4-element (2 per band)
we looked at earlier. The way to more gain is to add another director for 12
meters. The general outline looks like Figure 8.
If we compare this 5-element Yagi to the forward-facing 4-element version
earlier, we shall see several differences. First, the reflector spacing has been
increased. The chief result of this move is to increase the resistive component
of the feedpoint impedance. The essential performance on 17 is not changed. You
can feel free to use any spacing from the close spacing of the 4-element version
to the wide spacing of this version.
Second, adding a second director changes the director dimensions and
lengthens the boom considerable--a total of 17' for the entire array. However,
besides obtaining better gain on 12, the array is less sensitive to minor
changes of length of the 2 directors. What you might change slightly by a small
alteration of one element's length can be restored by slightly altering the
length of the other director.
The NEC-4 modeled performance of this array is given in the following table:
Freq Gain FS F-B R +/- jX SWR
18.068 6.30 11.89 62.1 - j 4.7 1.26
18.118 6.25 11.92 64.2 - j 3.1 1.29
18.168 6.20 11.91 66.3 - j 1.6 1.32
24.89 7.16 13.75 57.1 + j 6.9 1.20
24.94 7.21 13.82 54.9 + j 8.9 1.22
24.95 7.26 13.87 52.8 + j11.1 1.25
It is possible to optimize this design further. The 17-meter section can be
set closer to 50 Ohms, although significant increases in gain and front-to-back
ratio are not possible with only 2 elements. The gain and front-to-back ratio on
12 can also be altered, but improving one results in decreases to the other.
This version has opted for gain and only a modest front-to-back ratio.
Comparative patterns for the 2 bands appear in Figure 9.
As always, models of open sleeve coupling are approximate only. Therefore,
expect to play a bit with the length of the 12-meter slaved driver and its
spacing from the 17-meter fed driver to achieve the desired 12-meter feedpoint
impedance. Such adjustments have little or no effect on the 17-meter feedpoint
impedance or other operating parameters on that band. Also, if you select a
different tapering schedule for your elements, do two things. 1. Check out the
potential durability of the element, using a program like YagiStress or use a
model of element taper that has a proven record. 2. Adjust the element lengths
to suit the Leeson corrected substitute uniform diameter elements that are
equivalent to the ones shown here. This latter task can be handled with one of
the antenna modeling programs. The details of the model on which this note is
based appear in the following EZNEC antenna description.
--------------- WIRES ---------------
Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in) Segs
17-Meter Reflector
1 -14.100, 0.000, 0.000 W2E1 -12.250, 0.000, 0.000 5.00E-01 2
2 W1E2 -12.250, 0.000, 0.000 W3E1 -9.500, 0.000, 0.000 6.25E-01 3
3 W2E2 -9.500, 0.000, 0.000 W4E1 -6.750, 0.000, 0.000 7.50E-01 3
4 W3E2 -6.750, 0.000, 0.000 W5E1 -4.000, 0.000, 0.000 8.75E-01 3
5 W4E2 -4.000, 0.000, 0.000 W6E1 4.000, 0.000, 0.000 1.00E+00 9
6 W5E2 4.000, 0.000, 0.000 W7E1 6.750, 0.000, 0.000 8.75E-01 3
7 W6E2 6.750, 0.000, 0.000 W8E1 9.500, 0.000, 0.000 7.50E-01 3
8 W7E2 9.500, 0.000, 0.000 W9E1 12.250, 0.000, 0.000 6.25E-01 3
9 W8E2 12.250, 0.000, 0.000 14.100, 0.000, 0.000 5.00E-01 2
17-Meter Fed Driver
10 -13.167, 9.158, 0.000 W11E1 -11.250, 9.158, 0.000 5.00E-01 2
11 W10E2 -11.250, 9.158, 0.000 W12E1 -8.500, 9.158, 0.000 6.25E-01 3
12 W11E2 -8.500, 9.158, 0.000 W13E1 -5.750, 9.158, 0.000 7.50E-01 3
13 W12E2 -5.750, 9.158, 0.000 W14E1 -4.000, 9.158, 0.000 8.75E-01 3
14 W13E2 -4.000, 9.158, 0.000 W15E1 4.000, 9.158, 0.000 1.00E+00 9
15 W14E2 4.000, 9.158, 0.000 W16E1 5.750, 9.158, 0.000 8.75E-01 3
16 W15E2 5.750, 9.158, 0.000 W17E1 8.500, 9.158, 0.000 7.50E-01 3
17 W16E2 8.500, 9.158, 0.000 W18E1 11.250, 9.158, 0.000 6.25E-01 3
18 W17E2 11.250, 9.158, 0.000 13.167, 9.158, 0.000 5.00E-01 2
12-meter "Slave" Driver
19 -9.833, 9.575, 0.000 W20E1 -6.250, 9.575, 0.000 3.75E-01 4
20 W19E2 -6.250, 9.575, 0.000 W21E1 -2.500, 9.575, 0.000 5.00E-01 4
21 W20E2 -2.500, 9.575, 0.000 W22E1 -0.500, 9.575, 0.000 6.25E-01 3
22 W21E2 -0.500, 9.575, 0.000 W23E1 0.500, 9.575, 0.000 7.50E-01 1
23 W22E2 0.500, 9.575, 0.000 W24E1 2.500, 9.575, 0.000 6.25E-01 3
24 W23E2 2.500, 9.575, 0.000 W25E1 6.250, 9.575, 0.000 5.00E-01 4
25 W24E2 6.250, 9.575, 0.000 9.833, 9.575, 0.000 3.75E-01 4
12-Meter Director 1
26 -9.500, 10.683, 0.000 W27E1 -6.250, 10.683, 0.000 3.75E-01 4
27 W26E2 -6.250, 10.683, 0.000 W28E1 -2.500, 10.683, 0.000 5.00E-01 4
28 W27E2 -2.500, 10.683, 0.000 W29E1 -0.500, 10.683, 0.000 6.25E-01 3
29 W28E2 -0.500, 10.683, 0.000 W30E1 0.500, 10.683, 0.000 7.50E-01 1
30 W29E2 0.500, 10.683, 0.000 W31E1 2.500, 10.683, 0.000 6.25E-01 3
31 W30E2 2.500, 10.683, 0.000 W32E1 6.250, 10.683, 0.000 5.00E-01 4
32 W31E2 6.250, 10.683, 0.000 9.500, 10.683, 0.000 3.75E-01 4
12-Meter Director 2
33 -8.833, 16.967, 0.000 W34E1 -6.250, 16.967, 0.000 3.75E-01 4
34 W33E2 -6.250, 16.967, 0.000 W35E1 -2.500, 16.967, 0.000 5.00E-01 4
35 W34E2 -2.500, 16.967, 0.000 W36E1 -0.500, 16.967, 0.000 6.25E-01 3
36 W35E2 -0.500, 16.967, 0.000 W37E1 0.500, 16.967, 0.000 7.50E-01 1
37 W36E2 0.500, 16.967, 0.000 W38E1 2.500, 16.967, 0.000 6.25E-01 3
38 W37E2 2.500, 16.967, 0.000 W39E1 6.250, 16.967, 0.000 5.00E-01 4
39 W38E2 6.250, 16.967, 0.000 8.833, 16.967, 0.000 3.75E-01 4
The element taper used here is an adaptation of a commercial design for other
bands. 6061-T6 tubing or its equivalent is available from numerous sources. This
beam is far from optimized to its maximum possible performance. However, it may
provide a further alternative for experimentation. Since numerous commercial
beams use booms in the vicinity of 18', a design such as this makes a possible
conversion project once you spring the the very long boom tri-bander to use on
20-15-10 meters
The End Result is This . . .
As W5JH reminded me, the D/DE 2-element Yagi has a natural home on the WARC
bands, especially 12 and 17. Matching is straight forward, and performance will
be stable across these narrow bands. 0.07 to 0.09 wl makes a good element
spacing for a 20-ohm feedpoint impedance, an easy beta match. Combining DE-Dir
and DE-Ref designs is certainly feasible and can effect simplicities of matching
and use. Combining back-to-back Yagis with a 30-meter dipole is also possible.
Whether you go 2-faced or long-boomed, these designs and innumerable possible
variations on them should serve many needs on 12 and 17. Moreover, as this
little exercise has shown, the possibilities for experimentation are endless.
Updated 7-25-99. © L. B. Cebik, W4RNL. Data may be used for personal
purposes, but may not be reproduced for publication in print or any other medium
without permission of the author.
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