[TowerTalk] Re: linear loading

[w8ji.tom]

Hi Guy,

I'm not sure what W4AN's policy is on postings, so this is only to you and
Pete. If you want to re-post it...feel free. I personally thought the
reflector was pretty good, but maybe it is not what W4AN wants.
Maybe questions like this belong on antenna qth.net reflector?? I'm
confused by the sudden policy change.

>How would you comment on the spread-out variant of LL used by Force
>12. That would seem to minimize capacitance, particulary in what
>remains as the "stub" which is a couple feet apart on the 40m design.

It is safe to say, as a general rule, anything that reduces capacitance
across a "loading inductance" reduces loss. That's why the certain
commercial mobile coils are so lossy, they use tight wound coils with huge
metal end caps. Maxwell covers these loading coils in his book
"Reflections". Stubs are not exempted from that rule.

>It has been my experience, based on direct FS measurements, that properly
>designed lumped loading is better than linear loading when the loading is
>at the same effective location on the elements and similar conductor sizes
>What is the "best" effective location for the loading, other than at
>the ends? 

Very complex problem. It depends on the distributed loss resistances and
capacitances throughout the system. It can be anywhere but most of the time
it is somewhere from the middle out. The lossier the antenna **outside**
the loading system, the less critical loading loss and the more critical
loading location becomes. 

>What is the effective location for the loading on the F12
>design?

Don't have any idea. Maybe someone will model the antenna and make results
available. What it would take is a overall view of the common mode current
distribution of the element. I don't know if any commonly available NEC
based programs do that, but all of the articles I've read ignore that
effect and it can be significant. It could be done as a close approximation
long hand, but it would take a lot of time.

Keep in mind this is about like worrying about true north.

What happens is current (differential current) in the linear loading
section is not equal and 180 degrees out of phase with current in the
opposing wire. That means the linear loading section radiates (has common
mode current). Since it is folded out along the element, radiation from the
linear loading, in effect, moves the loading further out on the element.

How much that helps depends on distributed losses in the system. 

>>IMO, seeing a large difference means something was wrong with the design
of
>>the poorer system rather than the fact the loading "method" was changed.

>Or the sensitivity of a standard setting on an LL antennas to
>environment, placement, etc, making setting up in the field
>problematic?

A linear loaded antenna should be no more or no less sensitive to
surroundings than any other loading method that has the loading at a
similar point in the element, unless something is wrong with one of the
designs.

73 Tom



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