User manual

Revised 14 June 2003
E-3
Tune-up Instructions for the Cal-Av Labs, Inc. Model 2D-40A Antenna
This manual contains tabular data on the tuning and matching stub shorting strap
positions and element lengths for an antenna mounted at the minimum recommended
height of 1/2 wavelength above local earth and additional data for antennas mounted at
one wavelength. However, the local environment (ground constants, vegetation, site
clutter, etc.) makes it impossible to predict exactly what you will find when you raise the
antenna at your site. So you may need to make some minor adjustments "in place".
NOTE: The shorting strap on the transmission line stub attached to the front
(shorter) element adjusts the array match to 50 ohms. This is the same
assembly to which the coax feedline from the transceiver is attached.
The operating frequency of the array is adjusted primarily by adjusting the lengths of the
elements. The element length is adjusted by changing the exposed length of the
outermost "tip" sections. When changing the array operating frequency, all four
element tips should be adjusted by exactly the same amount. A reasonable “sanity
check” is to verify that all four tips are exactly equal length before raising the antenna.
Wherever referred to in this appendix, "element" lengths in inches are actually half-
lengths. That is, the "element length" dimension refers to the distance from the end of
the outside "tip" section of the element to the center of the boom (or half way across the
2-inch gap at the element feedpoint). "Tip" lengths refer to the exposed portion of the
outermost 3/8-inch diameter element tube. “Tip” lengths will be given for settings and
adjustments.
The tuning stub on the rear element sets the array reactance so that the pattern is
optimized at the same frequency where the antenna feedpoint impedance is exactly 50
ohms. It should normally be set at the recommended length and not adjusted further.
However, if the array resonant frequency must be moved slightly, and the element tips
cannot be reached for adjustment, the rear stub may be adjusted to move the array
resonant frequency a small amount (approximately 70 kHz) without significantly
degrading the pattern F/R characteristics.
Operating the antenna at a height above ground that is significantly different than 1/2
wavelength may necessitate an adjustment to the rear tuning stub to restore the
antenna's excellent front to back ratio. Careful modeling of the proposed installation and
consultation with the factory is recommended before proceeding with such an installation.
A very accurate EZNEC (NEC2) model of the antenna is available for download from the
Cal-Av Labs, Inc. web site <www.cal-av.com>.