[TowerTalk] Re: linear loading

[w8ji.tom]

Hi Guy,

This is about all I can add.

>.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.

>This is the one point I'm having problems with. A loading coil does
>not radiate significantly, and it's properties will not vary with
>distance from the ground (say 65 feet vs. 100 feet on 40m). 

I think the confusion stems from thinking it is proximity of the coil to
ground that causes the detuning.

That is not true at all, of all the parts of the antenna the coil has the
LEAST coupling to surrounding objects, and is the least affected by what
happens a few coil diameters away from the coil. That's why a coil makes a
lousy antenna, and why the books indicate a coil diameter or two away is
sufficient spacing to avoid affecting inductance or Q. Proof the coil is
very immune to surroundings comes when current at each terminal of the coil
is almost perfectly equal, something that is easily achieved.  

The elements, on the other hand, couple very well to the surroundings.
That's why they radiate so well and why current tapers in the elements!

A stub falls in between. The wider the stub is spaced, the more it
radiates, the more it acts like an antenna,  the more it is affected by its
surroundings. Same thing if current in the two conductors is not exactly
180 degrees out-of-phase and exactly equal in both conductors of the stub.
That's why open wire lines need to be close spaced and transposed at
fractional wl intervals, and also carefully balance in voltages and
currents!

It is the capacitive and inductive coupling (induction fields) and
radiation fields that cause the antenna to detune or change impedance, and
an antenna with higher Q (more energy storage in the induction fields) will
detune quicker than a lower Q antenna. Two things lower Q, reducing field
intensity at the boundaries of the antenna (like making the outer ends
where the highest voltage occurs very thick or fat) and adding loss
resistance. That's why bowties, fat dipoles, and mobile antennas with big
hats at the tip  are broader, and yet have higher efficiency.

If you didn't make the outer ends very thick, and bandwidth or tuning
sensitivity dropped, you almost certainly added loss resistance. There
ain't no bandwidth magic in the loading system for a given current
distribution, except the magic of loss resistance.

>This is not a design flaw, or something wrong. It is not lossier than
>a lumped solution, unless it has physical implementation problems.

I wouldn't know about that (specifically one manufacturer's system), since
without a direct A-B comparison of a simple single element of that type to
the other optimized lumped component it would just be a wild guess. I don't
win the lottery every time I play, I try not to guess about answers.

But let me toss this out for thought on loading systems in general.... 

A coil with air dielectric primarily has losses confined to I^2 R losses in
the wire. For all practical purposes, air is lossless. Only the small
dielectric supports would add dielectric losses (unless someone stupidly
close wound enameled wire on a lossy form like PVC). The turns spacing and
form factor controls circulating currents in the coil, and those currents
must be minimized without reducing mutual coupling from turn to turn (which
causes the desirable effect of increasing inductance by the square of the
wire length).

The same is true in a stub.

To get the same inductance for a given surface area conductor, it takes
less wire length in a proper form factor coil than in a stub. The exception
is if the stub has shunt C that increases inductive reactance per unit
length of wire by increasing energy storage in the stub (in the form of
internal circulating currents). Keep in mind circulating currents increase
loss without showing up as external currents leaving the stub or coil.

So why would a stub have less loss than a coil, if the loss is primarily
due to I^2 R losses in the conductor and the coil has a shorter conductor
with about the same internal circulating currents?

If I stretch a coil out, does the Q increase? Very few things go through a
dip in Q only to peak again at a wildly different form factor. A stub is a
stretched out coil, with poor form factor. It can be better than some
coils, and worse than others. Depends on how poorly each is made.

>I will have more runs with the unlimited mininec in NEC4WINVM before
>long, and may be able to quantify this a lot better.

When you find a program that models near fields accurately, let me know.
The best I can do is cut and try and measure.

Understand I'm not saying linear loading is bad. 

I'm saying:

1.) In a large antenna you'd never know the difference between the two
unless one is "done wrong".

2.) Claims of 2 dB or 5 dB difference are nonsense, just like that
feedpoint dual element 5 dB gibberish, unless something is seriously wrong
with the reference system.

In a mobile antenna or any system requiring lots of reactance, lumped
loading is better than linear loading no matter how the linear loading is
implemented. The less conductor in the path to the termination (without
unduly increasing circulating currents) the more efficient the system for a
given current distribution.

In my opinion, either system would work so close to the other that we are
debating "true north". The exception would be if one is not optimized. I
care less which one I use, I just like lumped components because they are
easier to optimize and a mechanically challenged person like me can handle
the mechanics better.  

Lossless loading is in the pot of Gold at the end of the rainbow, next to
all the packets of 5 dB free gain and half-size radials with 150%
efficiency.

73 Tom


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