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 beteeen there and the
base feedpoint. A length of 3/4 wavelength of wire is suggested there.
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.
Any tips & help appreciated.
Thanks,
Adrian
VK5AW

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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Duno wot happened to cause the repeats.....Sorry.


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 beteeen there and the
base feedpoint. A length of 3/4 wavelength of wire is suggested there.
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.
Any tips & help appreciated.
Thanks,
Adrian
VK5AW

Well that is indeed food for thought thanks Cecil! I have never heard of
that before, but it certainly figures. I just wound another for 20m,
with abt 46 feet of wire on it, and it GDO's on approx 48 MHz ONLY!
Although the GDO coupled into the z bridge says it is resonant at 14250,
20 ohms. But the SWR is hopeless!
Now it figures....I was going to re-wind the same wire onto another
thicker whip and see what happens to the resonant freq.

Do you think it might help if I keep the close wound section to a half
wave or less, and make the remainder above and below the coil well
spaced? I know I'll need to try I guess as each antenna I try is different.
I am going to make a dummy test rig of mounting a mobile base on a piece
of scrap iron, with a simple 2-3 turn loop on the base. No coax. Then I
can GDO any antenna I screw onto it! I'm gunna solve this!
All observations appreciated!
Adrian.

Oops, almost missed this......

Cecil Moore wrote:
A length of 3/4 wavelength of wire is suggested
there.


By who?

Someone quoted ARRL handbook chapter on mobile ants.

mustang wrote:
Do you think it might help if I keep the close wound section to a half
wave or less, and make the remainder above and below the coil well
spaced?

Here's what I would try as a wild hair experiment. Wind the electrical 1/2WL
helix and find the frequency at which the impedance is purely resistive and
maximum. A GDO should accomplish that. That's the resonant frequency for an
end-fed 1/2WL monopole. Use some spacing and wind the remainder of the coil
opposite to the original, i.e. CW Vs CCW. This would theoretically put the
fields in phase on each side of that current node. It sounds very tricky and
I have never heard of it being done before, but it might work.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

mustang wrote:
Do you think it might help if I keep the close wound section to a half
wave or less, and make the remainder above and below the coil well
spaced?

Here's what I would try as a wild hair experiment. Wind the electrical
1/2WL helix and find the frequency at which the impedance is purely resistive and
maximum. A GDO should accomplish that. That's the resonant frequency for an
end-fed 1/2WL monopole. Use some spacing and wind the remainder of the coil
opposite to the original, i.e. CW Vs CCW. This would theoretically put the
fields in phase on each side of that current node. It sounds very tricky
and I have never heard of it being done before, but it might work.

And of course, this would work perfectly on only a single frequency. Off
to each side of that frequency, the problem would again manifest itself.
--
73, Cecil http://www.qsl.net/w5dxp



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Hmm, an interesting approach!
But its all about trying things!
I'll give that one a go too. It doesn't take long to wind & try!
Adrian.




Here's what I would try as a wild hair experiment. Wind the electrical
1/2WL helix and find the frequency at which the impedance is purely
resistive and
maximum. A GDO should accomplish that. That's the resonant frequency
for an
end-fed 1/2WL monopole. Use some spacing and wind the remainder of the
coil
opposite to the original, i.e. CW Vs CCW. This would theoretically put
the
fields in phase on each side of that current node. It sounds very
tricky and I have never heard of it being done before, but it might work.


And of course, this would work perfectly on only a single frequency. Off
to each side of that frequency, the problem would again manifest itself.