> the elements. For instance, a log periodic (which has all
elements driven)
> doesn't really care which end it's driven at (aside from
some matching issues).
The log periodic uses crossfire phasing, and that means it
must always fire towards the feedpoint.
The delay as you move back through the transmission line
feeding the elements is nearly D=S for the open wire lines
used for feeds.
The crossing of the feed gives 180+S for phase, meaning the
spatial delay from element to element is also s. Everything
away from the feedpoint is automatically in a null. Moving
from the feedpoint S (transmission line delay) equals s
(element spacing delay) and thus cancels, so the remainder
is the 180 transposition creating a null away from the
feedpoint. Moving towards the feedpoint the delay is S+s
with an added 180 degree flip. Assume the elements are 45
degrees apart then phase would approximately be to the -45
to the back. The 180 flip rotates the phase to a 135 degree
lead for the second element. Thus the array satisfies the
requirement that the feedpoint has lagging phase.
There is no way around that, so the statement a "log doesn't
care which end is fed" is absolutely not correct in an
aperiodic design.
In order to make an array fire away from the feedpoint, you
must VERY lightly couple the elements and use techniques
that speed the velocity through the transmission line. The
antenna in NOT in a log mode, but rather in a traveling
wave mode. This can be done through series capacitive
reactances, but the gain and F/B is always less than a log
of similar length and bandwidth. The Fishbone antenna, if I
remember correctly, is a example of a traveling wave array
that may from some distance away or at casual glance appear
to be a long yagi or log.
As for the Yagi, optimum pattern and gain require elements
be excited in a binomial current distribution. While you
could feed the front and use two rearward parasitics in a
three element antenna (the middle in NO way would be tuned
accurately as a reflector, or the furthest to the rear would
have almost no current), by far the optimum feedpoint is the
center which will produce a nearly perfect 1:2:1 current
distribution and a null of at least 40dB with proper tuning
and spacing.
The idea of adding a reflector behind a reflector is a waste
of time, because there is essentially no field there to work
with. This is why Yagi's have developed into multiple
directors with single reflectors. Even trigonal reflectors
(staggered above and below the boom) have largely proven to
be a waste of material.
Of course if you DON'T properly tune an element for null
then adding a second reflector can help.
I think the confusion in all of this comes from people
thinking reflector reflect and directors direct. That just
isn't what they do. What all of the elements do is cancel
radiation towards the undesired direction. The minute a
director starts "directing" as a primary gain mechanism it
barely affect pattern. Patterns are formed by squeezing
radiation by forming nulls, like squeezing a balloon.
Stretching the pattern forward is a very inefficient way to
obtain gain.
Traveling wave antennas, like the Fishbone, Beverage,
Inverted V, and V Beam, use in-phase addition more than
nulling to achieve directivity. That's why they require huge
amounts of linear area for minimal gain. I can't imaging
why anyone would want to mess up a Yagi by making it work as
a traveling wave antenna, unless they pound for pound had
more element and boom material than brain material. Hi hi.
Example of a traveling wave antenna? Some UHF TV antennas.
Two wires run along the a row of insulated full wave
elements without crossing, the antenna fires away from the
feedpoint. They have been largely superceded by conventional
Yagi construction with a corner reflector, however. Why?
Because directivity is poor in a traveling wave antenna for
a given area.
Example of a crossfire or log array? The VHF TV antenna that
has alternating elements cross fed and fires towards the
feedpoint.
73 Tom
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