On Saturday 26 September 2009 8:18:17 pm Lee K7TJR wrote:
> And lastly Victor K1LT has changed from the loaded elements to
> Hi-Z amplifiers in his arrays. I was hoping he would chime in here
> with his opinions as well.
Chiming some random thoughts:
Yes, the W8JI design works great. His web page says 20 feet, but his
downloadable eznec file says 25 feet. I compromised at 23 feet because
thats the length of 2 12-foot 2x4s overlapped by a foot (as a support).
The exact length doesn't matter so long as you resonate it. With 23
foot wires (vertical and top hat wires), about 31 uH resonates at 1.83
MHz.
I found 8 25 foot radials was barely enough, but that 16 is more than
enough. The nature of the ground seems to make a difference of how
many radials you need. Like Tom's web page suggests, you have enough
radials when adding one more doesn't change the SWR. Note that these
radials don't have to be long or numerous, because we don't mind some
extra ground loss.
I built my first phased array in amongst the area ("Beverage Central")
where all my 2 wire Beverages cross (which makes feeding them in the
center easier). I found that there was coupling between the ends of
the top hat wires and nearby Beverage wires. However, the Beverage
wire passing within a couple of feet of the main vertical wire made no
difference. Probably the ends of the top hat wires are high impedance,
so a couple of pF of capacitance makes a big difference, whereas the
main vertical wire is low impedance and capacitive coupling is much,
much less. I moved the offending Beverages (much work).
On the other hand, the entire high-impedance active antenna is high
impedance, so you must be very careful of nearby wires and metal
stuff. This is coupling as opposed to nearby other antennas that
affect the pattern by re-radiating. The re-radiating phenomena is
a resonance thing.
I don't have an opinion about which type of vertical is "better". There
are trade-offs both ways. I can't really form an opinion until I build
a linear array with active antennas, and a circle-array with short
verticals.
Victor, K1LT
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