User manual

Revised 14 June 2003
E-1
Appendix E
OPERATIONAL NOTES
Feed System Description for the N7CL Array
The N7CL version of the two element driven array has been described incorrectly as
being essentially an HB9CV antenna. While the N7CL driven array antenna is definitely in
the same class of antenna as the HB9CV (also confused with the ZL-Special), the N7CL
Array is not technically very similar to either of the above versions of the two element
driven array. The primary differences are in the element lengths, element spacing, and
the method of feeding and controlling the magnitude and phase of the current in the
elements.
The HB9CV and the ZL-Special are typically fed and phased with high impedance open
wire transmission line with a velocity factor in the high ninety percent range. Matching for
the ZL-Special is by means of a shorted stub located some distance towards the
generator from the antenna’s feedpoint. “T” matches at each element are used in the
HB9CV design. Element spacing for the original HB9CV is 0.125 wavelengths.
The Cal-Av LABS 2D series antennas incorporating the N7CL feed system have been
issued U.S. Patent No. 6,411,264. This feed combines element stagger, shorted feedline
stubs, and coax fed inside one arm of shorted open-wire transmission line stub to
produce a feed system with the following characteristics:
1. Physical spacing of the elements significantly less than 0.125 wavelengths for
increased forward gain and, conveniently, a more compact boom. The actual spacing
used in the N7CL arrays produced by Cal-Av Labs, Inc. is 0.11 wavelengths. So the
array is more properly characterized as a “close-spaced” driven array.
2. The feed system uses phasing line with a velocity factor in the 0.66 range. This
permits the use of common coax to form the bulk of the phasing line. This allows the
phasing lines to be placed inside a metallic boom.
3. The feed system permits setting the accuracy of the match to 50 ohm feedline and
the array operating frequency independently using two separate adjustable “hairpin”
shunt inductors.
4. These hairpin inductors are adjusted by sliding shorts. The arrangement of the
hairpins on the boom brings the matching and tuning adjustments inboard towards the
mast. This makes the adjustments reachable from a position on the antenna support
tower. Thus final adjustments to the antenna can be easily made with the antenna in
place on the tower while the antenna is mounted at its intended operating height
above earth.
5. The array dimensions and feed system are configured so that the pattern optimization
is coincident with system resonance and match to 50 ohms. When adjustment is
made to the tuning hairpin to compensate the array for its mounted height, site clutter,
etc., the good pattern characteristics of the array are maintained.