1. LOG PERIODIC
DuHamel – University of Illinois  altered the bi-fin antenna
linear polarization,
satisfied ANGLE PRICIPLE,
but not TRUNCATION PRINCIPLE.
needed to cause currents to fall off more rapidly
introduced discontinuities – “teeth” in fins –
to increase radiation and increase decay of “I “

2. LOG PERIODIC (CONT) To satisfy ANGLE PRINCIPLE concept of Rumsey,
teeth were made along circular arcs so their
length was determined by an angle
BUT HOW TO DETERMINE SPACING ?

3. LOG PERIODIC (CONT) Looking at equiangular spiral on a
radial line from center the spacing
of the arms are in a constant ratio
So radii of successive
teeth were chosen such
that

4. LOG PERIODIC (CONT) Resulting structure is not exactly frequency
independent, but it should have identical
characteristics at a discrete number of frequencies
Example
If the p-th tooth is at then the
(p-1)th tooth is at

5. LOG PERIODIC (CONT)  so identical performance at
as long as the structure is accurate at the feed
and there are negligible currents at the
truncation point.

6. LOG PERIODIC
(CONT)
Fig Planar and wire logarithmically periodic antennas

7. LOG PERIODIC (CONT) Since
performance is periodic function of the log of the frequency
LOG-PERIODIC ANTENNAS
Experimentally, they found that the variation in pattern
between the periods were relatively minor for designs
where  was not too far from 1.0

8. LOG PERIODIC
(CONT)
Still this periodic antenna was bi-directional and had a broad pattern z
BI-FIN had fields
polarized in dir.
Some models of
log-periodic were
found to have linear
polarization in the
dir.
(along dir. of teeth
instead of perpendicular
to them) z x x
BI-FIN
LOG PERIODIC

9. LOG PERIODIC
(CONT)
Isbell – ignored restrictions on planar structures by Babinet.
folded planar L-P structure into a wedge
got unidirectional pattern off the tip
same bandwidth
-dir polarization
could be made straight (trapizodal)
most currents were in teeth whose length was near
[see model]
Also found they could only just outline the periphery
of the sheet structure with a wire and it would still work

10. LOG PERIODIC
(CONT)
Fig Planar and wire trapezoidal toothed log - periodic antennas

11. LOG-PERIODIC (CONT) Fig. 11.8 Linearly polarized
flush mounted cavity-backed
log-periodic slot antenna