>
> Another overlooked aspect of both gamma and tee matches is that the gamma/tee
> rods hanging from the element makes it look like a big tapered element.
Gerald has put his finger on a key aspect of the gamma/Tee situation. I
had posted some notes on Tee matches before discovering some problems of
unknown size in modeling dissimilar diameter parallel wires on NEC-4 and
subsequently withdrew them. But one key proposition was clear from all
the models: the physical structure of a Tee or gamma becomes part of the
radiating element. Its diameter relative to the main driven element could
add or subtract from the antenna gain (although the amounts derived from
the exercise are no longer reliable). The gamma-match/driven-element,
with an effective element of dissimilar diamters from one side to the
other) in effect becomes an off-center-fed driven element with respect to
a change in the electrical element center, which can no longer be assumed
to be the boom position.
For non-critical situations, the differential may be small and of no
operational significance. For those wishing the last ounce of performance
from their systems, a balanced feed system like the Tee may be preferable.
For systems in the 35-40 ohms range, a beta match using a transmission
line stub (also called a hairpin) would likely be just as efficient (if
made beefy enough, that is, with something bigger than house wiring),
since the virtual L-circuit has a delta of well under 2 in this
transformation ratio for very high efficiency and fewer mechanical
connections to create maintenance concerns. The beta of course, calls for
a slightly short driven element to create the required series capacitance.
This applies, to monoband antennas.
Between the Tee and the transmission-line stub beta for antennas with
native feed Zs in the 35-40 ohm range there is likely little to choose, IF
each is precisely rather than casually designed. For systems with
impedances below 25 ohms, the beta begins to increase its delta above 2
for increasing losses above 2%, but has been used effectively. However,
well-designed Tees may begin to show some advantage in this region. But I
also suspect in this calendar era we call AL (after Lawson), 9-10 ohm
Yagis will grow more uncommon.
This will not resolve preferences folks have, but does suggest that
balance is better than imbalance in most critical cases.
-73-
LB, W4RNL
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