Hi All,
There certainly isn't any free lunch with antennas, and we always
have limitations or tradeoffs on what we can do compared to ideal
situations.
The problem with comparisons is we often compare antennas using
very poor test methods or protocol, poor test situations (like
comparing antennas placed too close together), or compare
against poor reference antennas.
What we actually observe, even if we do it carefully, is just how
much better one less-than-perfect antenna works compared to
another less-than-perfect and unknown performance antenna.
> For some reason the loop does not reradiate noise as much as grounded
> verticals do.
That can only be two things. It can be from low efficiency, since
efficiency directly affects the "capture area" and the amount of
signals (signals behave no differently than noise) re-radiated. It can
also be that the delta was by luck or by design placed where the
receiving antenna was in a "null".
> >Seems to me that the vertically polarized delta loop would benefit
> >greatly from a conducting ground screen underneath each of the
> >vertical sections
>
> That's true as long as the ground screen extends many wavelengths away
> from the antenna, where the reinforcing reflection occurs. If the
> screen is right below the antenna it might reflect some of the
> radiation straight up.
It is also true because the electric and magnetic induction fields
below the lower wire of a delta is very strong. The electric field
actually is most concentrated at the voltage maxima, and so it
would be important to have a ground screen all along the lower
wire, not just at the "vertical" ends.
Often overlooked is the fact loss problems are due to concentration
of fields in the earth. There does not have to be a "path" to the
earth with a wire or connection for there to be significant induced
ground loss.
The voltage on the lower wire of the delta would have a strong
electric field impinging on earth below the wire, as well as some
magnetic field.
Walk below one sometime with a fluorescent tube while
transmitting with modest power, and you will see what I mean. It is
exactly the same problem as occurs with a few elevated radials.
The horizontal low wire of the Delta is acting
> rather like a pair of end-to-end radials I think, and when George
> Brown invented the groundplane antenna it had just two ground plane
> elements, and he added two more only because the customer thought they
> must be necessary.
Even a groundplane with four radials has significant current on the
shield of the coax because the groundplane is inadequate, and
even a four radial wire groundplane has pattern distortion caused by
radiation from the radials. You can clearly see this on modelling
programs, and I've measured significant coax shield current on four-
radial groundplanes. I always add a choke balun on four-radial
groundplanes, because the terminal voltage of the groundplane
common connection is never zero, it is always significant, causing
current to flow on the coax shield.
> >If it were up to me, I would consider putting some radials underneath
> >the delta loop to improve efficiency.
>
> But the efficiency of a dipole in free space is close to 100%, and
> this probably applies to a wire loop. I think you have to be careful
> about the relative definitions of efficiency and effectiveness.
In free space the efficiency is near 100%, not near earth. .05wl
from very good earth a dipole is less than 20% efficient.
> The point Bill made that you can open the apex to separate the
> vertical elements is worth remembering. This may also help keep the
> apex away from a conducting support, which is essential. But don't
> open the current-reversing point of the baseline and leave the
> baseline wires "in line". I think the ideal "squash" is .4 base line,
> .3 each slanting wire.
It would depend greatly on antenna base height above earth, and
ground characteristics. The larger the baseline, the more gain from
increased directivity. The larger the baseline, the more ground
losses from increased current in earth below the horizontal wire.
With either a perfect conductor for earth or a perfect dielectric for
earth, loss would be minimum. Someplace between all of this in
the real world is an optimum, but you can be sure that optimum is
far less than what a monopole over a perfect ground screen would
provide.
I'd wager the vertical delta is not much different in efficiency than a
two or four wire elevated radial vertical at the same base height.
73, Tom W8JI
W8JI@contesting.com
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