Reciprocity of Antennas? Always? Some considerations t.b.d.
You might all have read the chapters on antenna
transmitting and receiving reciprocity, almost always
explained via an easy dipole or horn antenna.
Most of antennas are not that easy and especially
Antenna Arrays with complex feeding networks will
require - at my opinion - more considerations in order
to declare reciprocity.
For example:
If a feed network is corporate (parallel) and each branch
of the feeding network is completely asymetric, let s say composed
by dividers, the antenna will NOT receive the same amount of RX
energy, right ?
But at the same time, some of the energy will be back scattered
(due to no good S22 of the feed network) and appears back at
the antenna array elements.
NOW HERE is my problem to understand: Am I right that this energy
will at least change the near field solution of the antenna?
Or will it even have an influence on the radiation pattern in RX itself?
Think about other configurations: Leaky Wave Antennas, Traveling Wave
Antennas (Wire, Slot,..).
Give me some new ideas how to solve my problem and I am glad
to give you support , too!
RadioCC
Yes and always reciprocal, as long as no intelligence or circulator or active circuit present.
you can take antenna or array of antennas, reciprocal (means s21=s12) is satisfied in all conditions.
Hi,
thx for your answer.
This does only partially answer my question.
Let s assume the feeding network has an element
providing S21=S12 , because it is passive, but
S11= zero , however S22 is not entirely matched,
so unzero.
Now is some of the RX energy backscattered
to the radiating elements ?
I have difficulties to understand this, sorry.
When an antenna is used for receiving, half of the energy is reradiated. Another thing to consider is the radiation resistance compared to the ohmic resistance. A ferrite core loop can be used for receiving, but would burn up when used for a kW transmitter.
.. half of the energy is reradiated ? I guess this is a missunderstanding.
Half of what energy? The plane wave keeps ideally unlimited energy,
and if the antenna is perfectly matched to the receiver network, why
should half be reradiated?
Please precise.
thx.
They have to be reciprocal to maintain thermodynamic equilibrium.
In other words if you had an antenna, any antenna, connected only to a matching resistor, of temperature 300K, pointed at the ground which had a temperature also 300 degK then you'd expect the resistor to stay at the same temperature.
It can only do that if the characteristics of the antenna are perfectly reciprocal.