ANTENNAS FROM THE GROUND UP

6. But My Yard's Too Small
or a 102' Center-Fed Multi-Band Dipole Data Compendium

L. B. Cebik, W4RNL





If you cannot fit a 135' dipole in your yard, perhaps an antenna about 102' long will fit. 102' is about 3/8 wl at 80 meters, which is about as short as you dare go for decent feedpoint impedances and minimal gain. (In dire circumstances, of course use what you have, even if shorter. It will work, but just not as well as something longer.)

We shall feed the 102' wire with parallel transmission line all the way to the ATU. With this feed system, the exact length is not critical 5' or so. The patterns will be mostly unaffected, but variations in length and height will change the feedpoint resistance and reactance more significantly.

100' flattops or doublets have been used as long as any old timer remembers. The precise 102' length became famous when G5RV developed a feed system that he hoped would allow hams to use the antenna on harmonically related bands with about 33' of 450-ohms line or 29' of 300- ohms line, and the rest 50-ohms coax. Two problems changed fame into controversy. First, the WARC bands opened, adding nonharmonically related frequencies to the multiband antenna wish list. Second, on the low bands, small changes of length and height alter the feedpoint impedance, thus disrupting the low SWR match effected by the parallel line lengths.

But the 102' doublet (= any center-fed wire) does work well with parallel line all the way to the ATU. However, its patterns and impedances along parallel transmission line differ from those of the 135' dipole. Therefore, the 102' antenna deserves a data compendium of its own.

See the preceding installment of this series for instructions on interpreting the patterns and the feedline impedance tables, along with the method for calculating the ballpark impedance presented to your own ATU.

The line impedances are calculated from the feedpoint impedance of the NEC- 2 model of the antenna at a 50' height. For intermediate values at 5 deg. intervals, see the transmission line performance program in HAMCALC. If you need values that account for line loss, ARRL's N6BV has written a fine program, but it calculates one value at a time.

The impedance values are intended only as indicators of the magnitude of resistance and reactance and the rising or falling direction of those magnitudes along the line. The accuracy of the values for any given ham installation, with its typical domestic "clutter," is no more than about 20%, considering variations in height and antenna length. At most, they can tell you that a longer or a shorter line might be better for a given band. In other words, they can suggest why your tuner may be having difficulties in matching the antenna on a given band. It is usually cheaper to add a little line length than to add a new tuner to the system.

80 meters: 3.6 MHz
AZ plots:   Elevation angles = 45 deg.
EL plots:   Azimuth angles = 90 deg.

Feedpoint Z (R  jX): 35 - 420 ohms

TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0        0         35 - 420
20   14.4      4.4       20 - 190
40   28.8      8.8       20 - 25
60   43.3      13.2      20 + 135
80   57.7      17.6      30 + 335
100  72.1      22.0      60 + 685
120  86.5      26.4      350 + 1870
140  101.0     30.8      1165 - 3330
160  115.4     35.2      90 - 880
180  129.8     39.6      35 - 420

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        35 - 420
20   12.1      3.7       15 - 205
40   24.3      7.4       15 - 80
60   36.4      11.1      10 + 30
80   48.6      14.8      15 + 140
100  60.7      18.5      25 + 305
120  72.9      22.2      70 + 650
140  85.0      25.9      1475 + 3010
160  97.1      29.6      165 - 1065
180  109.3     33.3      35 - 420

40 meters: 7.15 MHz
AZ plots: El. Angle = 45 deg. @ 35';
                      39 deg. @ 50'
EL plots: Az. Angles = 90 deg.
Feedpoint Z (R  jX): 450 + 1045 ohms

TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        450 + 1045
20   7.3       2.2       3260 + 150
40   14.5      4.4       480 - 1075
60   21.8      6.6       150 - 520
80   29.0      8.9       85 - 255
100  36.3      11.1      65 - 80
120  43.6      13.3      65 + 75
140  50.8      15.5      80 + 250
160  58.1      17.7      145 + 505
180  65.3      19.9      450 + 1045

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        450 + 1045
20   6.1       1.9       1380 - 1500
40   12.2      3.7       145 - 580
60   18.3      5.6       55 - 280
80   24.5      7.5       35 - 130
100  30.6      9.3       30 - 20
120  36.7      11.2      30 + 85
140  42.8      13.0      45 + 215
160  48.9      14.9      95 + 435
180  55.0      16.8      450 + 1045

30 meters: 10.125 MHz
AZ plots: El. Angle = 39 deg. @ 35';
                      27 deg. @ 50'
EL plots: Az. Angles = 90 deg.
Feedpoint Z (R  jX): 2220 - 3200 ohms

TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        2220 - 3200
20   5.1       1.6       155 - 925
40   10.3      3.1       60 - 440
60   15.4      4.7       35 - 205
80   20.5      6.3       30 - 35
100  25.6      7.8       30 + 125
120  30.8      9.4       45 + 320
140  35.9      10.9      90 + 645
160  41.0      12.5      435 + 1625
180  46.1      14.1      2220 - 3200

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        2220 - 3200
20   4.3       1.3       80 - 680
40   8.6       2.6       30 - 315
60   13.0      4.0       15 - 150
80   17.3      5.3       15 - 35
100  21.6      6.6       15 + 75
120  25.9      7.9       20 + 200
140  30.2      9.2       35 + 405
160  34.5      10.5      160 + 995
180  38.9      11.8      2220 - 3200

20 meters: 14.15 MHz
AZ plots: El. Angle = 26 deg. @ 35';
                      19 deg. @ 50'
EL plots: Az. Angles = 37 deg. @ 35'
                       40 deg. @ 50'
Feedpoint Z (R  jX): 100 - 50 ohms
TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        100 - 50
20   3.8       1.1       105 + 100
40   7.3       2.2       140 + 275
60   11.0      3.4       60 - 270
80   14.7      4.5       780 + 930
100  18.3      5.6       1920 - 485
120  22.0      6.7       500 - 790
140  25.7      7.8       200 - 430
160  29.4      9.0       120 - 215
180  33.0      10.1      100 - 50

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        100 - 50
20   3.1       0.9       100 + 45
40   6.2       1.9       125 + 145
60   9.3       2.8       200 + 275
80   12.4      3.8       450 + 410
100  15.5      4.7       925 + 25
120  18.5      5.7       475 - 40
140  21.6      6.6       210 - 285
160  24.7      7.5       125 - 155
180  27.8      8.5       100 - 50

17 meters: 18.1 MHz
AZ plots: El. Angle = 21 deg. @ 35';
                      15 deg. @ 50'
EL plots: Az. Angles = 54 deg. @ 35'
                       54 deg. @ 50'
Feedpoint Z (R  jX): 2040 + 1640 ohms
TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        2040 + 1640
20   2.9       0.9       815 - 1400
40   5.7       1.8       185 - 635
60   8.6       2.6       90 - 320
80   11.5      3.5       65 - 130
100  14.3      4.4       60 + 30
120  17.2      5.2       70 + 195
140  20.1      6.1       110 + 415
160  22.9      7.0       285 + 835
180  25.8      7.9       2040 + 1640

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        2040 + 1640
20   2.4       0.7       325 - 955
40   4.8       1.5       75 - 405
60   7.3       2.2       40 - 200
80   9.7       2.9       30 - 75
100  12.1      3.7       25 + 30
120  14.5      4.4       30 + 145
140  16.9      5.2       55 + 305
160  19.3      5.9       155 + 640
180  21.7      6.6       2040 + 1640

15 meters: 21.15 MHz
AZ plots: El. Angle = 18 deg. @ 35';
                      13 deg. @ 50'
EL plots: Az. Angles = 60 deg. @ 35'
                       61 deg. @ 50'
Feedpoint Z (R  jX): 375 - 1135 ohms
TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        375 - 1135
20   2.5       0.8       115 - 525
40   4.9       1.5       65 - 255
60   7.4       2.2       50 - 80
80   9.8       3.0       50 + 80
100  12.3      3.7       65 + 255
120  14.7      4.5       110 + 520
140  17.2      5.2       370 + 1125
160  19.6      6.0       4300 + 75
180  22.1      6.7       375 - 1135

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        375 - 1135
20   2.1       0.6       75 - 445
40   4.1       1.3       35 - 220
60   6.2       1.9       25 - 90
80   8.3       2.5       20 + 20
100  10.3      3.2       25 + 130
120  12.4      3.8       40 + 280
140  14.5      4.4       110 + 585
160  16.5      5.0       1215 + 1830
180  18.6      5.7       375 - 1135

12 meters: 24.95 MHz
AZ plots: El. Angle = 15 deg. @ 35';
                      11 deg. @ 50'
EL plots: Az. Angles = 34 deg. @ 35'
                       35 deg. @ 50'
Feedpoint Z (R  jX): 205 + 335 ohms
TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        205 + 335
20   2.1       0.6       415 + 590
40   4.2       1.3       1215 + 660
60   6.2       1.9       1160 - 685
80   8.3       2.5       400 - 575
100  10.4      3.2       200 - 325
120  12.5      3.8       140 - 145
140  14.6      4.4       125 + 5
160  16.6      5.1       140 + 155
180  18.7      5.7       205 + 335

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        205 + 335
20   1.8       0.5       560 + 515
40   3.5       1.1       1050 - 240
60   5.3       1.6       360 - 460
80   7.0       2.1       155 - 270
100  8.8       2.7       100 - 135
120  10.5      3.2       80 - 30
140  12.3      3.7       85 + 70
160  14.0      4.3       115 + 180
180  15.8      4.8       205 + 335

10 meters: 28.5 MHz
AZ plots: El. Angle = 14 deg. @ 35';
                      10 deg. @ 50'
EL plots: Az. Angles = 44 deg. @ 35'
                       44 deg. @ 50'
Feedpoint Z (R  jX): 3235 - 65 ohms
TL = 450 ohms; VF = .95
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        3235 - 65
20   1.8       0.6       460 - 1050
40   3.6       1.1       145 - 510
60   5.5       1.7       80 - 250
80   7.3       2.2       65 - 75
100  9.1       2.8       65 + 80
120  10.9      3.3       85 + 255
140  12.8      3.9       150 + 515
160  14.6      4.4       475 + 1065
180  16.4      5.0       3235 - 65

TL = 300 ohms; VF = .80
Deg  Feet      Meters    R  jX (ohms)
 0    0         0        3235 - 65
20   1.5       0.5       220 - 765
40   3.1       0.9       65 - 350
60   4.6       1.4       35 - 170
80   6.1       1.9       30 - 50
100  7.7       2.3       30 + 55
120  9.2       2.8       35 + 170
140  10.7      3.3       65 + 350
160  12.3      3.7       225 + 770
180  13.8      4.2       3235 - 65




Updated 9-1-97. 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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