At 08:57 AM 1/30/2006, Jim Miller wrote:
>Speaking of broadbanding and the possibility/probability of increased loss
>as the actual cause, does this apply to the dipole cut to a specific band
>and then running several wires in a box (for instance) configuration
>separated by a foot or so with spreaders at the ends and all driven and
>connected together on each end? Does this add loss to the single wire
>dipole or does it simply make it more broadbanded without adding the loss?
>(realizing it will be a slight bit shorter than the single wire dipole).
>
>Tnx es 73, de Jim KG0KP
Larger element diameter does make it more broadband, and without increasing
the loss. In fact, it might actually have less loss (more copper area,
after all). If you plot the feedpoint impedance vs frequency on a grid of
R and X, you'll see that it follows a roughly circular shape (crossing the
X=0 line at roughly the frequencies where it's a multiple of a half
wavelength). skinny dipoles have bigger circles (the maximum X is bigger,
and the R is bigger at the even multiples of half wavelength).
For all antennas, there's a tradeoff between physical size, efficiency, and
antenna Q. BUT, antenna Q is defined as the energy stored in the antenna
divided by the energy radiated away. It's NOT like Q for an inductor (X/R)
or a LC tuned circuit where there's an implication that low Q means more
loss. Low antenna Q just means that the power is radiated away, rather
than stored in the antenna, which is a good thing (since, inevitably, there
are losses in storing the energy).
It happens that the feedpoint impedance of a simple antenna can be fairly
accurately modeled by a tuned LC circuit over small frequency ranges, so if
you plot it, you get that familiar resonance curve, which seduces you into
thinking they're the same thing, but they're not. One can get wrapped
around the axle about antenna bandwidth and antenna Q, and trying to
directly compare that to tuned circuit bandwidth and Q. They're similar,
but different: in one case, the power is radiated away (good), and in the
other it's lost as heat(bad). And, then, there's the whole "match
bandwidth" issue, which is more of a transmitter compatibility thing.
But, to return to your original point... Big fat conductors: good. Skinny
conductors: bad. It's worth noting that a biconical antenna is quite
efficient AND has wide bandwidth. Fat conductors are just a transitional
form between infinitely thin wires and the bicone. A discone is basically
a specialized version of a biconical: just like a 1/4 wave ground plane
vertical is electrically much like a dipole. But these are physically
large, getting back to that tradeoff between bandwidth, efficiency and size.
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