The 2 element hang from the top of the tower in opposite directions to an
anchor point 40 mts from the base but suspended at that point at 3 mts from
the ground and keep that hight returning to the tower as the final anchor
point.>>>>
There are a number of compromises with an array like this. You probably want
the current maximum someplace low, but not at or near ground level. I'm not
sure what the typical optimum place is.
The parasitic elements are not what we might think. We can pretend like it
is a half wave ground independent element, but it is really a sloped
vertical with a single counterpoise near earth. They are really sloped
verticals with a single, low, elevated radial.
1.) You want maximum parasitic element current as far away from the main
element as possible. This dictates having maximum current at the base of the
sloped wire, and as much ampere/feet in the sloped wire near the bottom as
possible.
2.) The problem with the above in number one is the sloped element, which
likes a low current maximum for best alignment and maximum effective element
spacing, has a single low radial. The loss in this radial increases with
lower position of sloped element current, and as the radial is closer to
ground. This is contrary to the optimum of number 1.
The system should work better, or with more repeatability, if the
counterpoise wires increase in number and area around the sloped parasitics.
It would have less loss and more bandwidth. Then you could move current to
the bottom more, and not have loss increase.
The trade off is unpredictability and loss of efficiency and
performance-bandwidth for simplicity in design. There is no free lunch.
<<<I followed (I think) the tunning procedure suggested by the authors which
consisted in keeping the driven element connected and using a vertical
polarized signal from the back (700 mts in my case) tune the reflector to
max attenuation, then the director for max attenuation and iterate both
until maximum attenuation from the back is reached. That procedure is
suppose to give a f/b centered in your chosen fcy (thats is what I got, more
than 20db across 25Kc) and max gain >3db from 1800 to almost 1900. >>>>
There are several compromising adjustments in this antenna. There are:
1.) Effective spacing. Maximum effective spacing is with the maximum
vertical current as far away as possible from other elements. This array is
**always** spaced closer than distance out because the wire slopes. The
slope angle is one compromise.
2.) Ground loss. This is a sloped vertical with single ground radial. This
makes it ground dependent, and there is no imagined theory that can rid us
of that. Radials have to radiate in the near field around the radial, or
they cannot be counterpoises. That means they always couple to earth, no
matter what the design. What makes the vertical "better" makes the radial
loss worse.
3.) You can get into poor tuning situations when you just optimize for back
null. The reason for this is you can have poor current ratios in the
parasitic elements. The ideal ratio would be 1:2:1 for gain and F/B. I've
seen arrays be far from that and have good back null, and gain like two
elements. I've always found it best to tune first for maximum forward gain
back and forth several times, and then do a minimal change for peak back
null (if that is the goal).
4.) All of this is worse because of the spacing and counterpoise compromise.
The slope and single radial add more variables. So does the thin long wire,
which can add element loss while decreasing bandwidth.
5.) Current positioning in the element will affect gain, because it affects
losses in multiple ways. It makes the effective spacing closer or further,
and it changes ground losses.
You could have any number of problems. I generally do not like unpredictable
systems with many things that interact, but unless you want to add radials
and make it less critical as a traditional sloped element array, you could
try a different tuning method. I would tune for maximum forward gain first,
going back and forth several times. Then, if F/B is important, I would tune
a minimal amount from both elements for maximum back null.
Maximum F/B in a two step tuning method does not guarantee equal or
near-equal parasitic currents, and the ideal 1:2:1 ratio. (Since the
elements are sloped and have compromised grounds, it will not really be
1:2:1 but it should be close.)
I also have doubt you can effectively tune the parasitic by resonance with
the driven element or other elements functional. That makes no sense at all
to me. Given the variability in effective spacing and driven element
characteristics, I think every system will be different for resonance
tuning. A real parasitic reflector, for example, has close to optimum gain
and F/B when close to exact self-resonance. But that tuning requires no
other elements be present. Mutual coupling changes resonance we see, so the
other element characteristics change what we perceive as the element tuning
through mutual coupling.
You have probably ruined the simplicity and beauty by actually measuring
gain. :-)
73 Tom
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Topband Reflector
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