[TowerTalk] Enhanced OWA

[Brian Beezley]

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