On 11/28/16 11:10 AM, Mike Smith VE9AA wrote:
Greeting Antenna Gurus,
Put the finishing touches on a 100% homebrew K1WA/(K8UR) K3LR type 5-dipole
sloping dipole (all parasitic) array for 15m only hours before the contest
this weekend and came away quite disappointed.
NE/NW/SE/SW dples are all spaced at 90* from one another but the 5th dipole
(south) is squeezed in between the normal SE and SW directions. Maybe it's
too close.
What's connected to the unused dipoles? Coax to a double pole switch,
like in the 1994 QST article, so that you basically have a open
circuited stub?
The article comments on the large change in F/B in the model as the
transmission line parameters were changed (which isn't surprising.. it
affects the phase on the "parasitic" elements.
. (not 100% of the time, but most of the time.)
This is built almost EXACTLY like Tim's 4-element 160m array at K3LR (or
like his array was in the 1990's anyways - not sure if he still uses it.)
except I used the 5th dipole due south for Caribbean/SA and my 5 radials are
on the ground, and not raised. Phasing lines, tops with ropes, etc. all the
same. Most of the mast is PVC (except the bottom 12'_)
Since the dipoles are vertical, I would expect that in K3LR's case there
was a fair amount of interaction with the conductive mast. That's
something you'd need to factor in.
and the bottoms of
the dipoles are off the ground maybe 6' (not overlapped at the tower bottom
like Tims). Box and coax grounded, radials attached. All per his article.
What I would like to know from anyone that has either built one for 10-15 or
20m (or any band, and I'll do the math) is what is the actual spacing used
dipole center to dipole center?
In K3LRs, assuming that the angles are all 45s, the distance from the
tower to the feedpoint is 93 ft, and 93 feet from the top of the tower.
That makes the "feedpoint to feedpoint" distance, pretty much the same
as the distance along the hypotenuse from top to feed, 135 feet (41.15
meters)
At 1.84MHz (K3LRs apparent design frequency) the wavelength is 163
meters, so the center to center distance is pretty close to a 1/4
wavelength.
Or anyone that knows antenna modeling programs, can you see if this has any
useful gain or F/B with only 6.66' spacing at the centers at 21.1MHz. (which
is roughly 0.142WL I believe)
That is "very close" and almost certainly interacting.
I went this close because all the hardware was taken from an old 6m array
that I used many moons ago.
I am thinking I should rebuild it with a much larger spacing, maybe around
.25WL(11.6') and ditch the south and extra, 5th dipole.
That would be a start, but this is one of those antennas where some
modeling will help a lot.
Bear in mind that for most arrays, gain is fairly tolerant of phasing
errors, but F/B (null depth) is VERY picky. If the phase in the
parasitic element is off, then the null goes away. That's illustrated in
Figures 3 & 4 in the original article.
Tnx
Mike VE9AA
Mike, Coreen & Corey
Keswick Ridge, NB
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