...I am thinking about a PVC sleeve-type arrangement where the fixed, lowest height would be 8' and I could raise the smaller PVC to 15' or so - maybe a third PVC to raise another 6' when in use. I'm still working this through.
I have used PVC for antenna supports, but I generally
don’t think it is a good solution. It is relatively heavy
and bendy. Black ABS pipe is lighter and stiffer - stop
by your local store and try holding each type of pipe
horizontal from one end, and you will see what I mean.
ABS has a minimum size of 1 1/4 or 1 1/2 inch. It will
bow, however, if you try to tighten the guy ropes too
tight.
Fiberglass is a better solution. There are any number
of fiberglass fishing poles sold for this purpose, with
popular heights of 20’ to 33’. (SpiderPole sells heavy
duty versions up to 80’.) The top section is often
too wimpy to support an antenna, so don’t plan for
the full advertised height.
They are, however, useful for wire verticals, in which
case the full height can be used.
Antenna choice will depend on what bands are of
particular interest to you, and whether you can
bend the ends of the wires around the corners
of your property. With a bit of creativity, you might
fit a 33’ dipole for 20m into that space, or even
a coil-loaded dipole for 40m (which could also
work 10m). But, unless you feed tha antenna
with twinlead or ladderline instead of coax, the
performance won’t be very good on other bands.
(A 15m dipole probably wouldn’t be too bad on
17m or 12m, but will degrade much beyond that.)
At some point, you just have to put up something
and experiment.
If your budget will stretch that far, consider the
telescoping masts from Max-Gain Systems
(www,mgs4u.com). These ate much stronger,
and have thumb latches to lock the sections
rather than friction fit. I have one that I take
out and strap to the back fence, that goes up
26’, and you have the options of section
lengths and diameters to go up to 50’.
MFJ sells similar masts for a similar price,
and if you have a local ham radio store you
might be able to get it there and save shipping.
One of the common options is a painter’s pole,
but make sure you can raise the top section
with the bottom section vertical. The taller ones
use 8’ sections, which may require a ladder.
My favorite is in the window washing section of
Home Depot - the Unger 24’ telescopic pole,
UPC 61475-97298. It is aluminum with a
triangular cross section, and has thumb locks
on the sections instead of twist locks. There
are numerous other telescoping handles of
varying sizes - I also have a 15’ one for
changing light bulbs. Probably the best
deal was a red and green 13’ pole designed
for stringing Christmas lights, that was on
super sale on 27 December.
Some of these may not be ideal for a
permanent installation, but probably will
work better than a home-made telescoping
PVC mast.
Now, there are some other considerations
to increasing the height. It means that the
angle in the center of the antenna gets
smaller, unless you can also elevate the
ends. With 20’ of width, the feedpoint can
only be 10’ higher than the end anchors
for a 90 degree angle. Yes, inverted vees
can work with sharper angles, but you are
approaching the area of diminishing returns.
...I thought it best to stay away from conductive material for the center pole...
A lot of hams with steel towers hang inverted vees
from them.
There can be some interaction right at the feedpoint
that can shift the tuning a bit. You can always add
a foot or two of PVC between the top of the mast and
the feedpoint if you want. (It is fine for short lengths.)
I sometimes use a wood spacer at the top to move
the feedpoint away from a metal mast a little bit,
but I don’t think it really makes much difference.
Some of the telescoping poles don’t make good
electrical contact between the sections, so they
don’t act like a long piece of metal.