Practical_Antenna_Handbook_0071639586

160 p a r t I I : F u n d a m e n t a l s
have a reactive component to their impedance, as well. Even in free space, the second
dipole in a loop may well cause the entire structure to have a feedpoint impedance
other than 73 W. (The exact change depends on the distance between the centers of the
two dipoles.) But the methods for canceling out the reactive part and transforming the
real part to 50 W are the same as for a simple dipole. As a general rule, open-wire line,
used in conjunction with an antenna tuning unit (ATU) at the transmitter end of the line
or at ground level directly below the loop, is an excellent choice and usually represents
less of a downward drag on the antenna. But 50-W coaxial cable may actually be a closer
match to the loop’s natural impedance.
Next in ease of implementation are the bidirectional line arrays with equal element
currents. Whether in phase or 180 degrees out of phase, the principle of symmetry assures
us that the element feedpoint impedances are identical, and transmitter power
can usually be equally divided among them simply by feeding multiple identical transmission
lines in parallel. Often it will be useful to incorporate one or more impedancetransforming
sections of appropriate transmission line to convert the typically low
junction impedance to something higher and more appropriate for the main run back to
the transmitter.
When we move to element phase angles other than 0 and 180 degrees, feeding the
all-driven array becomes much more difficult because the impedances of the individual
elements differ from each other and, hence, have a tendency to take differing amounts
of power from the source or passive component divider networks unless special techniques
are employed for ensuring equal power or equal current at all the element feedpoints.
As we saw in Chap. 4, all transmission lines exhibit a velocity of propagation that is
somewhat less than the speed of light in free space or air. As a result, a length of transmission
line used to create a 90-degree delay at some operating frequency will be
shorter than the physical distance between two array elements spaced l/4 apart, as
measured in air or free space. If the layout of the array is such that the physical distance
of each such delay line from a central combining point to each element is substantially
shorter than the distances between the elements themselves, it may be possible to phase
each element with a minimum transmission line length. However, frequently it is necessary
to make the phasing lines longer, such as using a 3l/4 line to provide a 90-degree
delay.
Some important points when constructing or procuring phasing lines for arrays
with separately fed elements:
• Make all phasing lines out of the same transmission line. If the lines are coaxial,
be sure to use cable from a single reputable vendor. Ideally, all phasing lines to
individual elements of a given array should be cut from the same spool.
• Be sure you know the velocity of propagation (as defined by the cable’s
propagation factor) for the cable you are using. Don’t guess, and don’t rely on
published specifications—measure it! Use a time domain reflectometer (TDR), a
vector network analyzer (VNA), or other suitable instrument capable of allowing
you to accurately calculate the propagation factor or equivalent line length of
your chosen cable material. (See Chap. 27 for some instruments and techniques.)
This is not quite as important for broadside arrays where all the elements are
fed in phase through identical lengths of cable originating at a common junction,