Just a note that I've updated my OWA writeup with curves for a 3-element
Yagi with bent driven element and hairpin match. This design has a good
pattern on the low end of the band and relaxes it higher up where it may
not be needed. It has more gain than a wideband 4-element OWA and less
gain than a narrowband one. It might be an interesting alternative if
you'd like to eliminate an element.
I think of the close-spaced OWA director as a distributed matching
network. Why not use a lumped network and eliminate the element? I show
curves for a 3-element Yagi that trades the hairpin for a pi-network.
The response is really nice, but the design is impractical due to the
very high capacitor current. So far I haven't found a lumped matching
network that can fully replace the extra OWA director and survive
high-power operation. High-current capacitors exist for broadcast
applications, but they seem to cost hundreds of dollars.
If you limit the capacitor values to try to minimize current, a lumped
compensating network still can be useful. An automatic optimizer can
adjust the other variables to yield the best design possible within the
constraint. I've simulated both pi-network and parallel-resonant tank
circuits. I've seen benefit with capacitor values as low as 1000 pF on
40 meters. Values like this may be practical, at least at lower transmit
levels. For example, CDE CDV16 mica capacitors can take amps of RF and
are available for a few dollars each at Mouser. You might be able to
parallel some to create a useful matching network. Each capacitor has
its own RF current spec:
http://www.cde.com/resources/catalogs/CD-CDV16.pdf
For example, if you parallel ten CDV16FF101J03 100-pF capacitors, you
get 1000 pF that will handle 9.2 amps. For 1500 watts, it needs to
handle 12.6 amps. It also needs to survive temporary overvoltage due to
a nearby lightning strike. I'm not at all sure it's wise to put a
critical component out of reach atop a tower, but the idea of replacing
an element remains attractive.
http://ham-radio.com/k6sti/owa.htm
Brian
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