CM 17-10m Log Per - ARRL Ant Book, converted with 4nec2 on 29-Mar-06 22:08
CM 	LogPer (with warnings)
CM This is an example of a three-band 17m-10m log periodic
CM antenna from the ARRL ANTENNA BOOK, 16th Edition, p. 10-6.
CM Look at this antenna with the View Antenna display, using
CM the zoom and offset features to see the details. This
CM method of construction, with one transmission line
CM conductor above the other, frequently is used for VHF and
CM UHF log periodic antennas. Note the feedpoint. The source
CM is necessarily on a wire which is much shorter than the
CM 0.02 wavelength minimum recommended for wires containing a
CM source. In addition, the source is close to multiple wire
CM junctions and to connections to wires of different
CM diameters, both of which are difficult for NEC-2 in the
CM vicinity of sources. Run a second 3D far-field request,
CM select the average-gain option and notice the Average Gain
CM reported on the 'Main (F2)' window. The Average Gain of
CM -1.7 dB (-0.74 dB when using the NEC-4 engine)
CM shows that source placement is a problem. Experimentation
CM shows that lengthening wires 23 and 24, which moves the
CM source farther from the multiple wire element junction,
CM improves the average gain. However, the transmission line
CM formed by these two wires has an impedance of more than 400
CM ohms, so the impedance match will be degraded as the wires
CM are lengthened.
CM This illustrates the difficulty of accurately modeling this
CM type of antenna, but also the usefulness of the Average
CM Gain feature. Notice that the maximum antenna gain is 3.05
CM dBi and Average Gain is -1.69 dB. (With NEC-4, 3.96 dBi and
CM -0.74 dB respectively.) The corrected gain is then 3.05 
CM - (-1.7) = 4.75 dBi. This compares very favorably with
CM LogPerTL's reported gain of 4.69 dBi (with Average Gain of 
CM -0.01 dB).
CM The design feedpoint impedance of this antenna is 200 ohms,
CM so it would normally be fed from a 50 ohm system through a
CM 4:1 transformer. To see what the SWR would be on the 50 ohm
CM line when fed this way, select the alternate Z0 of 200
CM ohms. An SWR sweep shows that the SWR of this antenna is
CM high at 18 MHz, although the ANTENNA BOOK design indicates
CM that it should work at this frequency. Not having built and
CM measured an actual antenna, the cause of the discrepancy
CM remains unknown.
CM See the LogPerTL.nec Antenna file for additional information.
CE
GW 1 8 0 -163.46 -1.2 0 0 -1.2 0.625
GW 2 8 0 0 1.2 0 163.46 1.2 0.625
GW 3 8 39.23 -130.76 1.2 39.23 0 1.2 0.5
GW 4 8 39.23 0 -1.2 39.23 130.76 -1.2 0.5
GW 5 8 70.62 -104.62 -1.2 70.62 0 -1.2 0.375
GW 6 8 70.62 0 1.2 70.62 104.62 1.2 0.375
GW 7 8 95.72 -83.69 1.2 95.72 0 1.2 0.3125
GW 8 8 95.72 0 -1.2 95.72 83.69 -1.2 0.3125
GW 9 8 115.81 -66.95 -1.2 115.81 0 -1.2 0.25
GW 10 8 115.81 0 1.2 115.81 66.95 1.2 0.25
GW 11 3 0 0 -1.2 39.23 0 -1.2 0.04040404
GW 12 3 39.23 0 -1.2 70.62 0 -1.2 0.04040404
GW 13 3 70.62 0 -1.2 95.72 0 -1.2 0.04040404
GW 14 3 95.72 0 -1.2 115.81 0 -1.2 0.04040404
GW 15 3 0 0 1.2 39.23 0 1.2 0.04040404
GW 16 3 39.23 0 1.2 70.62 0 1.2 0.04040404
GW 17 3 70.62 0 1.2 95.72 0 1.2 0.04040404
GW 18 3 95.72 0 1.2 115.81 0 1.2 0.04040404
GW 19 1 120 0 -1.2 120 0 1.2 0.04040404
GW 20 1 0 0 -1.2 -6 0 -1.2 0.04040404
GW 21 1 0 0 1.2 -6 0 1.2 0.04040404
GW 22 1 -6 0 -1.2 -6 0 1.2 0.04040404
GW 23 1 115.81 0 -1.2 120 0 -1.2 0.04040404
GW 24 1 115.81 0 1.2 120 0 1.2 0.04040404
GS 0 0 in		' All in inch.
GE 0
EX 6 19 1 0 1 0
GN -1			' Free space
FR 0 1 0 0 21 0
EN