Offset Crossed Yagis
Offset-Crossed Yagis analyzed using 4nec2, where a Vertically oriented Yagi is mounted
with a Quarter-Wavelength Offset forward of a conventional, Horizontally oriented Yagi.
Hi-VHF 8-El. Yagi design is from on-line K7MEM calculator:
http://imageevent.com/holl_ands/yagis/hivhf
Since the K7MEM designs are NOT optimized for wide-band use, usable Gain is only
realized on Ch7-10, with reduced F/B and F/R Ratios on Ch7.
Much better LHCP suppression was found in K7MEM 8-El. Offset Crossed Yagis than Crossed Yagis:
http://imageevent.com/holl_ands/circularpolarized/crossedyagis
When RHCP signals are reflected (e.g. multipath), it changes to a LHCP signal, which will be
SUPPRESSED by antenna's Cross-Polarization Loss. Hence importance of LHCP suppression,
also known as Cross Polarization Loss (CPL = RHCP - LHCP).
Note that Polarized Gain figures do NOT account for the DOUBLING of Transmitted Power (H+V),
which results in TWICE as high a receive signal power level when RHCP is effective. The 4nec2
program assumes a fixed amount of power input to a "standard" RHCP antenna, which splits
that power into Horiz. and Vert. components. At the receiver, the two components are summed
together via the interconnecting Transmission Line. But DTV uses TWO transmitters.
When/If circular polarization optimization parameters are added to nikiml's optimizations
scripts, re-optimizing the entire antenna might be able to improve on below performance.
Date(s): 3 Feb 2011. 1 - 7 of 7 Total. Shared |
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