Standard Delay Line Systems
Many phasing systems use standard 180-Spacing=Delay line
phasing. In other words, the end-fire distance in degrees is subtracted from
180-degrees, and the result is the delay line length. This is an acceptable
method in single frequency or narrow-band systems.
Conventional delay line systems have the following problems when used in
endfire unidirectional arrays:
- Phase shift almost always generally changes the wrong direction as
frequency is varied
- Phase shift only equals line length when the line is perfectly terminated
or when the line is lossless and an exact multiple of 1/4wl
- Voltages or currents at both ends of the line are equal only when the line
is perfectly terminated or an exact multiple of 1/2 wl and has negligible
loss
- Currents at the load end of an odd-quarter wave (or multiple) line are
only equal when the lines are fed from the same voltage and have negligible
loss
Hybrids
Some systems use 90-degree Hybrids or L/C phasing systems. Hybrids offer ideal distribution of power and
provide the expected phase-shift only when perfectly constructed and terminated.
While there is some tendency to self-compensate phase, they still suffer
bandwidth limitations. Hybrids are very useful in electronic systems,
amplifiers are one example. In an amplifier system, we might want a constant
90-degree phase shift despite slight frequency shifts.
Unidirectional antenna systems are never optimized when phase shift is
fixed at some arbitrary value that remains constant as frequency is varied.
ALL unidirectional endfire arrays require phase to track with
element spacing change in degrees, as frequency is varied. Lumped component systems and Hybrids might save space,
but they do not enhance array performance over the proper choice and
design of transmission line phasing systems. This is true in both transmitting
and receiving antenna systems!
If hybrids are so poor, why do we see so many of them in antennas? There are
a few reasons authors and manufacturers use hybrids. They often think:
- Hybrids assist in maintaining proper currents in elements
- The dump resistor only absorbs a portion of reflected power that would be
wasted anyway
- Power is wasted only when the load SWR is high
- The association with Collins engineering must mean the system works well
in any application
- Well-tuned Hi-Q components must work better than transmission lines
In reality, none of the above are true in a broad sense. Hybrids have their
place, but it certainly is not in wide-bandwidth phased arrays.
Transmission-line phasing systems are a bit more tolerant than Hybrids. For
example, a 90-degree long transmission line has zero degree phase error,
even when grossly misterminated. A 90-degree phase delay transmission line has
less phase error than a quadrature
(90-degree) lumped component Hybrid when mismatched. Phase
error peaks in misterminated transmission lines when the line is any odd-multiple
of 1/8 wavelength, and is minimum with lines any multiple (even or odd) of
1/4-wl. For example, a 45-degree long transmission line provides 27
degrees phase lag when terminated in 25 ohms…not the 45 degrees people often
expect! (Remember this when you see phasing designs that just throw a
certain length of cable in series with a mismatched impedance!)
With all systems, amplitude errors are a problem. There isn’t any
passive system that provides correct phase and amplitude when load impedance
changes, especially over a wide frequency range.
Crossfire Phasing
I prefer cross-fire
phasing, rather than the conventional narrow-band phasing methods
discussed above. Cross-fire phasing, when designed
properly, ensures phasing
is always correct regardless of frequency. When elements (in this article Beverages) offer a near-constant impedance that is almost entirely resistive
over a wide frequency range, cross-fire phasing can function
perfectly from VLF to LF all in one antenna. Phase and amplitude can be designed
for a back-fire null, with the upper limit in frequency set by element spacing
and the lower limit set by array sensitivity. It is possible to design
cross-fire receiving arrays that maintain the same basic directional response
over several octaves of bandwidth.
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