Joe is right...
Maxwell's equations relate time dependant electrical and magnetic fields. If B
is changing with time, E cannot equal zero everywhere in space and if E is
changing with time, B cannot be zero everywhere in space (otherwise you can not
fulfill the two equations for the "curl" of the fields). So, no B without E and
reverse (if there is some time dependance which is the case for hf).
If you calculate the field strength for a given current distribution, you can
make some simplifications that lead to field components changing with the
inverse of the cube of the distance (1/r^3), field components changing with one
over the square of the distance (1/r^2) and components changing with 1/r (the
far field radiated by the structure). For short distances to the antenna, the
1/r^3 and the 1/r^2 terms are the dominating ones and form the near fields.
The difference between an elementary electrical and a magnetic dipole is that,
for very short distances to the antenna ("reactive near field", less than about
1/6 wavelength), the leading term (the 1/r^3 term of the field expansion) is a
purely electrical field for the electrical antenna and a purely magnetic field
for the magnetic antenna.
So, the most efficient coupling for the magnetic antenna would be a conducting
loop (if properly oriented), but a straight conductor for the electrical
antenna. There is some limited control for this coupling to nearby conductors
by "switching from electrical to magnetic", but the next important term for
rather short distances is the 1/r^2 term of the field expansion, and both the
magnetic and the electrical elemetary dipoles show both an electrical and a
magnetic field component with this 1/r^2 behaviour. So both antennas can couple
to straight conductors or loops via this part of the field in the same manner,
only the absolute amount of coupling (coupling constant) might be different.
Conclusion: you have some (limited) control on the 1/r^3 part, which is purely
magnetic for a magnetic antenna and purely electrical for an electrical
antenna, but you will have this "1/r^2 coupling" for both a magnetic and an
electrical antenna, meaning you can not generally eliminate the coupling to
nearby conductors just by using a "dominantly magnetic antenna" or a
"dominantly electrical antenna".
Vy 73
Ralf, DL6OAP
Am 01.06.2013 um 19:46 schrieb "Joe Subich, W4TV" <li...@subich.com>:
>
> Whether the field is predominately magnetic or electric, it must
> still couple to the environment around it or the antenna *will not* radiate.
> whilst the same folding that is responsible for the low
> feed impedance and low efficiency will result in more cancellation
> close to the antenna than at greater distances, the antenna will
> still have *significant coupling* to any conductive element in its
> immediate environment (near field). If that were not true, yagi
> antennas would be impossible and phased arrays would be easy to
> design because one could ignore mutual impedance.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 6/1/2013 6:23 AM, David Woolley (E.L) wrote:
>> Joe Subich, W4TV wrote:
>>
>> The near field (basically within the range where it follows an inverse
>> cube law) can be predominantly magnetic or predominantly electric in
>> nature. Whilst the ratio of electric and magnetic fields in the far
>> field is constrained to 377 ohms per square, that is not true in the
>> near field.
>>
>>>
>>> No. If the field is not able to couple to nearby objects, it is not
>>> able to radiate (couple to) distant receivers. The statement flies
>>> completely in the face of physics.
>>>
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