mustang wrote:
I'm trying to wind a set of helicals to use mobile on HF, and I'm
wondering what guides there are. I have made them by close winding a
half wavelength of wire on the rod, and have tried close winding on the
top section of the rod, then widening the turns between there and the
base feedpoint. A length of 3/4 wavelength of wire is suggested there.

By who?

But I am getting very weird effects. Can someone who has done this give
me some tips please? I have searched the net, but find very little on
this subject. I use a GDO, simple z bridge, and a vswr meter of course.
Briefly, the weird effects include the freq DROPPING as wire is pruned!
And an inability to get above 13.5 MHz if aiming at 14 MHz...it
fluctuates as wire is cut and spacing is altered.
Frustrating.

One of the effects that you are seeing is when you add a turn to a helical
longer than 1/2WL, the field generated by that turn subtracts from the field
generated by other turns because the cosine of the current's phase angle has
changed from plus to minus. Thus the *electrical* length of the antenna can
be decreased by the addition of turns under certain circumstances. Of course,
when you remove a troublemaking turn, the resonant frequency drops. Seems
to me, a helical shouldn't be made electrically longer than 1/2WL. Most are
1/4WL.

If you locate an SWR current node in the middle of a coil, it's inductance
will be drastically reduced. Instead of an n^2 term, there will be an
(n1-n2)^2 term. A current minimum node should never be located inside a coil
if an inductance is desired.

This is akin to putting two windings on a toroid, one CW and one CCW.
Fed in parallel, the result will be a very low magnitude of flux and
virtually no choking function.
--
73, Cecil http://www.qsl.net/w5dxp



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