More musing: Does the entire "J" have to be upright? Can't I put an elbow
where the quarter-wave and half-wave sections meet, so the radiating element
is vertical, but the quarter-wave section is horizontal?

No, it doesn't, and yes, you can.

The quarter-wave matching section may radiate somewhat (it's not a
perfectly-balanced transmission line) but the effect should be
minimal.

You'll still want to mount the antenna so that the quarter-wave
section remains some distance from the mast.

You can extend this approach a bit further, and make a fully- balanced
version of a bent J-pole - a.k.a. a "double zepp". This would be two
half-wave radiating sections, fed through a quarter-wave matching
section.

The coax is connected along the matching section, using a 4:1 coaxial
balun.

A further variant of this is an "extended double Zepp" (or "double
extended Zepp"), which is two 5/8-wave radiators with a somewhat
shorter matching section.

For both the standard and extended double Zepp antennas, the use of a
balanced feed allows the "inner" end of the matching section to be
grounded to the mast - it can even be part of a metal mast, or can
have extensions addd to allow it to be clamped to the mast.

See http://www.tcarc.ca/public/2mdez.php for complete plans for the
extended double Zepp. The simple "double Zepp" can be made in a
similar fashion, with shorter radiators and a longer matching section.

I built a 440 version of the EDZ for use as a linking-radio antenna
for a local repeater system. It works nicely. One thing I'd do a bit
differently - I'd actually do the matching (selection of feedline
attachment point and shorting bar) after mounting it on the tower,
rather than in free space - the tower mast does have enough of an
effect on the impedance to change the SWR a bit.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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