Ed wrote:
I plan on building a short dipole (inverted V) for traveling in my tiny
RV. It will only be designed for 75M. The mast is 20' tall, and I wish
the legs to be about 24 feet each.

I still haven't decided where to place the inductors, but in any case, I
have a question about their design. This antenna will only be used at the
5 watt level with a FT-817 Yaesu; an LDG Z-11 tuner is also available, but
probably not needed.

If I make my 75M dipole legs for about 24 feet long, then I figure I
will need about 70 uH inductors if they are center located.

My Question: What would be the requirement for a 70 uH inductor that
only needs to work with 5 watts? Do I need open air inductor with spacing
between windings, or would a smaller multi wound coil with insulated wire
suffice?

Thanks for any opinions on this.


Ed K7AAT

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.