This is useful information to know. I don't know why the section about small
loops in the Antenna Handbook doesn't mention this. BTW, how far does the
near-field extend?

"Reg Edwards" wrote in message
...
A vertical conductor, a metal tube, running diametrically across a
large magloop with the tuning capacitor at the top, has no effect
either on radiating efficiency or the radiation pattern. It may as
well not be there apart from providing a prop or support for the
antenna itself. It will, of course, need to be well-insulated from the
high-voltage parts.

In general, provided the longest dimension of foreign metalwork is
much less than 1/2-wavelength, and there is not much of it, negligible
loss will be induced in it. This is due to the very low loss
resistance of most metals.

Metalwork in the vicinity of beam antennas may upset only the
radiation pattern.

Short lengths of foreign metalwork in close vicinity of tramsmission
lines, such as open-wire and ladder lines, will cause neglible loss
but will slightly reduce the line impedance Zo which might affect the
L and C settings of the tuner.

But beware of "semi-conductors", like damp wood such as foliage and
tree trunks, in the near-fields of antennas and lines. Their higher
impedance provide a better power match to the 'generator'.

You can have as much foreign polyethylene as you like.
----
Reg.

"John N9JG" wrote
This is useful information to know. I don't know why the section
about small
loops in the Antenna Handbook doesn't mention this. BTW, how far
does the
near-field extend?

======================================

It's because neither the authors nor the editors were aware of it.

The near-field on the surface of an antenna's conductors starts off at
a very high value. The far-field is zero at zero distance.

The near-field fades into the distance just the same as the
ar-field - only much faster. It also has a radiation pattern which
depends on an antenna's structure.

For argument's sake, for an isotropic antenna (which doesn't exist),
at a distance of 1/2/Pi = 1/6 of a wavelength from the antenna, the
near-fields and far-fields are equal to each other but differ in
phase. They both exist together and their effects on foreign bodies
are inseparable from a measurements point of view. They exist
seperately at all distances only as a mathematical fiction.

The near-field of open-wire transmission lines is sensibly zero at a
distance of 4 or 5 times the wire spacing but the radiation field is
finite and very small. The radiation field increases with wire spacing
and is a maximum when spacing is about 1/2-wavelength which nobody
ever uses. This accounts for the lack of use of low-loss open-wire
lines at UHF and higher frequencies.

Back to Magloops where the measurement unit is loop diameter rather
than wavelength and the near-field is mainly magnetic. The magnetic
field falls off at a rate proportional to the cube of distance. I
would be very happy with a distance of 1/2 the loop diameter or more
between the loop and large foreign bodies such as the ground.

Highly conductive metallic bodies merely slightly detune the loop with
little additional loss.
----
Reg.