Clay W7CE wrote:
> It sounds like we could debate the definition of resonance for days without
> reaching a consensus. So let's get back to the original topic: is resonance
> an important attribute of a good antenna?
Is resonance (in the single second order resonator sense) actually a
fundamental property of ANY antenna. No. It is not. The change in
feedpoint impedance IS a property of most antennas, some more, some
less, but the "single RLC" model is only a model of the feedpoint
impedance over a small range, useful for designing matching networks,
but not really having much to do with the antenna's properties as a
radiator. (consider a equiangular spiral or a discone or a helix or a
LPDA.. all provide a "good match" over a wide band, have good
efficiency, and don't really have any "resonance" to speak of)
A simple dipole actually has many "resonances" defined as places where
the reactive component of the feed point impedance is zero. ALready,
you're deviating from the "L C tuned circuit" model.
Then, there's the whole "Q" of the antenna thing. Q, for antennas, has
nothing to do with "resonance" in the sense of spring and weight or LC
resonator (i.e. a second order system). It's the ratio of the energy
stored in the antenna and it's surrounding fields to the energy radiated
away.
There's a fundamental relationship between the antenna's physical size
and the amount of stored energy and the directivity of the antenna.
That is, small antennas tend to have high stored energy (high Q), but
that is NOT the same as saying they have narrow bandwidth. (for SOME
antennas, this is true, but not all).
A resonant LC tank has a ratio between stored energy circulating between
L and C and that which is passing through, also defined as Q (in the
same way). And for that LC tank, it happens that the resonant frequency
divided by the 3dB bandwidth is equal to the Q. But that's just because
it's a second order system.
The fact that a dipole antenna is fairly well modeled by a simple single
R, L, C near the frequency where it's a half wavelength long does lead
to a lot of confusion.
However, one could have a simple wire antenna, with a very complex LC
matching network that presents a nice 50 ohms resistive impedance over a
decade bandwidth (the whole HF band), and it could be just as good a
radiator as any other of the same physical size. What autotuners (at the
feedpoint) do is basically create that "complex network", except by only
doing a small piece of it at a time. If you needed wide *instantaneous*
bandwidth, you'd need more sophistication.
There's a fair number of clever LC network designs out there in the
literature that do things like match a dual vertical (one that has two
"resonances") for a whole raft of bands (but not continuous) with no
"moving parts". There are also expedient solutions like the resistively
loaded folded dipoles that just accept the lossy matching solution, and
increase Tx power to compensate.
As W7CE points out, you need to look at the whole system design. And
also recognize that technology has changed a lot in the last 50 years,
so techniques that were "state of the art" or even "a good engineering
solution" in 1940 might not be either today.
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