Just about any inductor will withstand the power level, so your
tradeoffs are among size, weight, expense, weather resistance, and
loss. The higher the inductor Q, the lower the loss. But to get the
maximum Q you need a large, air wound inductor that stays dry -- any
water between turns of any kind of inductor will spoil the Q. I've
made an antenna similar to the one you describe (but half the size,
for 40 meters) with reasonably low loss using two large powdered iron
cores for loading at the feedpoint. It was conveniently matched by
link coupling to the cores which of course you won't be able to do if
the loads are away from the feedpoint. If you can tolerate the size
and/or weight, the use of multiple conductors -- fanned or parallel
--, or (not as good) a single large diameter conductor will reduce the
loss two ways. First, it reduces the conductor's I^2*R loss which can
become noticeable in a short antenna. Second, it reduces the amount of
inductance you need and therefore for a given Q reduces the amount of
inductor loss. I think you'll have trouble getting reasonable Q and
therefore reasonably low loss from a small, multilayer air wound coil.
You're best off with a single layer air core solenoid or, not quite as
good but decent if done with care, a single layer winding on a large
type 2 or 6 powdered iron core.

Just remember to protect the inductors to keep water from between the
turns. Also, remember that just beyond the inductors the impedance is
pretty high, so keep the part of the antenna beyond the inductors, in
particular, away from leaves, branches, and the like, especially if
they're wet.



Thanks, Roy. Since my antenna will be very portable, with a
collapsable fibreglass mast I can't take much weight, ergo, the need for
small wire, and light weight inductors. However, it appears I would be
better off with feedpoint inductors instead of putting them halfway out
the legs of the dipole. I will plan on doing that, and will look into
using iron core inductors instead of air wound.... if I can keep it light
enough. (the mast sags under any appreciable weight). Thanks again.

Ed K7AAT

However, it appears I would be
better off with feedpoint inductors instead of putting them halfway out
the legs of the dipole.

Myself, I would prefer them out at 1/2 or even 2/3 from the center.
Better current distribution. What will the antenna be made from?
Wire, or metal tubing? Regular solenoid coils can be made from
any light plastic tube, using light wire. I use insulated wire to act
as a spacer. I then cover them with shrink wrap, or tape.
If the antenna is hard to match to coax, you can use a third coil
at the feedpoint, and tap it for the best match.
If the antenna was tubing, I would use wooden dowel that would
fit in between a section of metal tubing, and wind the coils on those.
I'd weatherproof those some way also.
When I make small loaded dipoles, I think exactly the same way as
I would designing a mobile whip. I'd make the most efficient design
of a 24 ft whip, IE: coil placement, etc, and just double it for a
dipole.
Whats best for a short whip, will also be best for a short dipole
of the same size per leg. The very best location is probably at
the 2/3 to 3/4 mark, but it's not that critical. Even 1/2 out is good.
Reg's vertload can model all that, and tell you the best place, coil
info, etc, etc... In comparing to the real world, it's close nuff for
gov
work. If the apex was high, even a center load could be ok, but if
you only have 5 watts, I would go for every last scrap of efficiency
in the design. Actually, I'd also carry a spare 120 ft of wire to use
as
a full size dipole in the off chance the location could support it. :/
Many would, if they have trees to hang wires from, etc.. Even if the
full dipole is perverted, and making turns, a "Z",etc, it will usually
outdo
the loaded antenna for 75m.
MK