On Dec 4, 10:23 am, Richard Clark wrote:
On Mon, 3 Dec 2007 18:29:24 -0800 (PST), K7ITM wrote:
I can make the antenna conductor be the outside of a piece of
coaxial cable, and use the coaxial inside as a shorted stub which
reflects a pretty good (fairly high Q) inductive reactance back to a
particular point such as a quarter of the antenna length back from
each end, where the stub connects across a gap in the outer
conductor. Can I use such an inductive reactance to tune the
antenna? Will there then be a difference in current at each end of
the gap across which that reactance connects?

Hi Tom,

Interesting proposition. I like it.

73's
Richard Clark, KB7QHC

Hi Richard,

Note that it's also possible to make the stub in the form of a helical
resonator operated below resonance--that is, a loading coil that's
shielded by the tubular conductor whose outside surface is the
antenna. A problem with using plain coax is that the length is
prohibitive. For example, if you make an 80-foot long dipole from
RG-213-size coax, you find that you need about 550 ohms reactance at
points a quarter of the total length in from the ends, to get it to
resonate at 3.9MHz. But using a shorted stub of 50 ohm line requires
about 85 electrical degrees of line. Even with solid polyethylene
dielectric, that's 39 feet of line. Ooops. We only have 20 feet to
work with. Lengthen the antenna to, say, 120 feet, and the required
reactance drops to a low enough value to be practical to do with a
shorted stub co-axial with the antenna wire, but at that point, why
bother? You'd only have to add a few feet of wire to get the antenna
to resonate without inductive loading.

Mostly I find value in thinking about things like this because they
make more clear what's really important: it's primarily the inductive
reactance that tunes the antenna; the parasitic capacitance from a
loading coil to the outside world, which is what causes it to behave
like a helical delay line, is of much lower importance in determining
the antenna tuning. In a long antenna that's capacitively loaded, the
capacitors can have negligible parasitic series inductance and shunt
capacitance to the outside world, but they still strongly affect the
antenna loading.

Of course, the closer to the end of the antenna you put a large
loading coil, the more effect its capacitance will have. In the
limit, you can dispense with the coil and just add a capacitive hat
after all. Even modest size conductive balls on the ends of a thin-
wire dipole will have a significant effect on the resonance.

Cheers,
Tom