Re: NEC-LIST: Electrically "short" antennas ...

Doug McKean wrote:

> One of the tests I performed involved a short 1 inch wire off the end
> the end of a 50 ohm coax (small monopole) and a 1 square inch loop
> antenna off the end of another 50 ohm coax. "Short" antennas being
> those where voltage and/or current is effectively constant over the
> length of structure.
>
> The span of the SA was from 0 to 1 GHz. I fed the antennas with the

Although it has nothing to do with the observations you mention
further on, I must mention that your loop antenna is not "short" at
the high end (1 GHz) of your frequency range. It is more than a
quarter wavelength in circumference, and therefore the voltage and
current (which vary more or less sinusoidally along the wire) can have
the full range of values between a minimum and a maximum of a standing
wave.

> ...
> Well, the short of it is, they aren't much different from one another.
> ...
> Is this to be expected? Am I looking a virtual point sources in the
> far field so it really doesn't matter if they're stubs or loops?

No, the main source of radiation is the unbalanced current (common-
mode current) in the feed cable. It exists because capacitive current
dissapears from the antenna to the surroundings. This is about the
same for both situations. The loop may have some more capacitance to
surroundings, but it presents a heavier load, so the voltage on it
will be a bit lower. Not much difference than.

The direct radiation (from the antennas, which may indeed be
closely resembling point sources) is usually an order of magnitude
lower than the radiation from the common-mode current.

>
> This flies in the face of advice that's been kicked around concerning
> printed circuit board design - that loops formed by traces and their

A lot of the advice is nonsense. Don't forget that lesson!

> respective return paths are more of a concern in circuit boards than
> traces.

Nevertheless, there is some thruth in that. The highly assymetrical
situation of a narrow trace above a wide ground layer, often with some
holes and slits in it, combined with the presence of a dielectric,
does make it more complicated than your very fundamental
experiments. It can indeed make the "loop" situation radiate more than
the "open trace" situation.

Cheers,
Jos
Received on Sat May 13 2000 - 11:05:05 EDT