Here's a challenge !

Having recently acquired a Yaesu FT-879 HF transceiver I'd like to use
it for portable operation from my camper van. Ideally to take
advantage of the improving HF conditions during my lunch hour at work.

So I originally thought about buying a Yaesu ATAS-120A, but then I
realised that I wasn't that interested in mobile operation. So I could
consider something a bit more efficient.

See the Outbacker antennas http://www.outbackerantennas.com/products.html .
Some appear to meet your requirments.

One of our club's hams used one at Field Day last year and this. (Outbacker
8, I think) He did as well as the guys who had much bigger antennas. He
merely clamped it to a chain-link fence at head-level and he was on the air.
The mass of your camper van might be OK if you can't park adjacent to a
suitable fence or other structure.

Downsides: Not cheap; band changes require your attendance at the antenna
unless you switch among many of them. (You are quite wealthy, right? :-)

Based on the tests I've seen, the Outbacker is a bit more efficient
than Hamsticks (but not by much), and is probably less efficient than
all but the smallest of screwdriver-type antennas (including the ATAS).

If you're interested in a fairly traditional mobile-type antenna
(an inductively-loaded short radiator) with higher efficiency,
consider a bugcatcher - a lower mast, a large air-core center-loading
inductor, and a long whip on top (possibly with a capacity hat).
By winding the center coil properly (large-diameter wire or tubing,
turns spaced apart, with the right length/diameter ratio) you can get
a significantly higher Q in the coil (and thus lower losses) than
you'll get with a tightly-wound screwdriver coil or a helically-
wound Hamstick or Outbacker.

You may want one coil per band... although it's possible to work
multiple bands, with some additional loss, by using a jumper to short
out some of the coil turns on the higher-frequency bands. I've seen
some "dual-band" bugcatcher designs, in which the full coil is used to
load the antenna on 75 meters, and a series-resonant circuit shorts
out some of the turns on 40 meters.

Mount the antenna as high as possible on the van... ideally, so that
all of the antenna is up above the vehicle body.

Since you don't plan to actually operate while driving, you can use a
taller antenna than would be possible while mobile, and may not need
additional guying.

Another possibility is to simply throw up a random wire (perhaps an
inverted-L, suspended via thin rope from nearby trees... use the
vehicle body as a ground (perhaps with a counterpoise wire) and use a
wide-range antenna tuner. Any reasonably thin, flexible stranded wire
could be used... some specialist dealers such as The Wireman sell
antenna wires which are reportedly easy to coil without kinking.

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