Snip

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!- Hide quoted text -

- Show quoted text -

OK this is getting interesting now !

Thanks for all your suggestions, I respect your judgement as I realise
what a wealth of experience I'm tapping into.

I had discounted random wires hanging from trees, as I wanted the
whole thing to be attached to the van. The G5RV is relatively easy to
deploy when I have the space, but I'm trying to operate from a
standard car park bay, without drawing too much attention to myself by
having lots of wires running across other bays which people are trying
to park in. Likewise I didn't want to use critical matching components
or high-Q tuned circuits which may require adjustments every time I
moved location or it started to rain (actually I should have said
stopped raining, as that is less frequent).

As I already have a Z-11 I thought I could use it as an autocoupler at
the base of the antenna. It's capable of doing this but it doesn't
have quite as wide a matching range as say the SGC range. LDG 6 to
1000 ohms vs SGC 0.3 to 3000 ohms (from memory) So providing I don't
use a coax feed to the antenna base and the impedance stays within the
6 -1000 ohm boundaries, so I don't need an additional 4:1 balun, then
the losses should be down to acceptable levels.

I realise that I could simply create a muliband trapped vertical, say
using coax based traps to reduce the size and weight. However I wish
to maximise the full length of the 10m pole as much as possible.
Bearing in mind that the lowest 2m or so of the pole would be below
the van roof, so I can only really use the remaining 8m or so, which I
need to optimise as much as possible.

The existing 7.2m wire works OK but it would benefit from a bit more
inductive loading to reduce the capacitive reactance at 3.5MHz to be
within the tuning limits of the Z-11. The other problem as indcated by
Cecil is the length being greater than 5/8 wave at 10m and 6m
resulting in a poor angle of radiation.

I thought that by adding a 3m phasing section in the form of a coil at
1.5m down from the top of the mast with approx 2.7m below, the phasing
section this would approximate to a 1/4 over 1/2 wave collinear for
6m, and add some loading at 3.5MHz. My thinking is that this would use
the length of the pole to it's best advantage on the 6m band and
reduce the radiation angle to acceptable limits on 10m.

I could play with overall lengths of wire (say 8.8m or 11.8m and the
length and size of the phasing coil to optimise the 6m gain and
impedance range on each of the bands if this was worth doing. But as I
said before I'm not agonising over the last dB of performance as I
would on the microwave bands. I believe it's a law of diminishing
returns, you need a lot of optimisation to gain a few dB but in most
cases an S point difference goes practically unnoticed. Especially
when the environment surrounding the vehicle will have a significant
effect on the overall radiation efficiency.

I had thought about the ATAS-120 but having 80m is useful for inter-UK
contacts and I would think that I could at least match the performance
of an ATAS on the higher bands and beat it on the lower ones. Has
anyone seen efficency figures for the ATAS ?

Another thought which struck me was that I could wind a counterpoise
coil on the bottom 2m of the pole which would be below the roof of the
van. This would be close coupled into the van body as well as other
nearby objects such as fences ect. but it may improve things slightly
by adding an additional counterpoise element. Something like the W2IK
IK-STIC design.

Keep the suggestions coming !

UKM