Paul,
I've used one of these things when portable. Worked
better than the mobile antenna, but I did use a tuner
(wasn't all that worried about losses at the time, no
harm to the tuner).
To keep the weight/cost/bulk down I used RG-58 coax (it
was going to be wound up and thrown in the trunk). If I
were going to make it a 'permanent' (no such thing!) antenna
I might use something larger, but I doubt it.
The center 'feed point' was weather proofed by sealing it
with some kind of 'goo', can't remember what it was, probably
silicon. Also found that winding a 'choke' a few feet from
the end where it came out of the folded back braid cut down
on RFI problems. Not that they were 'bad', but it didn't
hurt anything either.
I wouldn't count on it ever being a 'DX antenna' (whatever
that means), but it is an alternative...
'Doc