use of receive antenna in HFSS simulation model and radiation box

1. If I am going to have a receive antenna in simulation model, why do I need a radiation box? The radiation box measures the power radiated, gain, etc....in the far field, but what if I have an antenna in far filed to measure all that?

2. Also, the minimum radiation box size of lambda/4 is not in the far field since minimum distance in far field is rff = 2 * (D)^2/lambda, where D is largest dimension of antenna.

Take for example lambda = 327mm and D=150mm,
lambda/4 = 82mm, and
rff = 138mm

So, basically the radiation box where radiation is measured is in the near field?! I am getting far-field measurements in HFSS when I simulate with a lambda/4 radiation box, are they not correct measurements?!

3. Final Question, what if I have a receive antenna in HFSS, how different will it be from the transmit antenna as far as setting up the source gap, etc...

I'd appreciate it if you hit all my 3 questions.

Thanks.

1. HFSS does not consider how you are using any given structure or antenna, it only needs physical parameters to compute a 'radiation pattern' using their tetrahedron based algorithm.

2. Quarter lambda **away from the structure** is sufficient to compute the developed fields that determine the radiation pattern. I have modeled a number of designs (including several 1/2 wavelength end fed dipoles) and the results match up with real-life range plots ...

3. Should not make one iota of difference. Recall the Principle of Reciprocity applies here as it does to all linear time-invariant networks. (Otherwise maybe you can cite the law in physics that gets transformed from one 'power flow' direction to another?)

Now, if you are building an actual transmit antenna, voltage breakdown (of gaps) and self-heating (due to high circulating currents) must be considered. Otherwise, losses (resistive) in the antenna contribute to the Noise Figure of your antenna for receive.

Jim

RF_Jim,
Thanks for your quick reply.

Re #2, I understand quarter lambda may be sufficient to determine radiation pattern and measure power radiated, etc...but what about the Far field equation rff = 2 * (D)^2/lambda, how is this equation considered or not considered to determine far-field radiation?

Re #3, my question really is: how different do I set up a receive antenna in HFSS? Do I set its power input to zero? How do I determine power received at antenna port 2? Is the power received at antenna 2 considered also power radiated from antenna 1?

Thanks.

RF_Jim is correct. As for #2, if you recall classic electromagnetic theory and the Sommerfield equation, it will be noted that the far field is nothing but a simplification of the near field as the phase curvature flattens (becomes planar) the further you get from an antenna for a given target size. A wave (Electric field) travels per e^(i*k*r)/(i*lambda*r)) which is referred to as a spherical wave in the very near field. However, if you approximate the the spherical phase with a linear phase, you are in the far field.

As for #3, use HFSS to determine the gain of your antenna (both Tx and Rx) and then simply use the Friis Equation to determine antenna coupling for given orientations (assuming a free space and no ground bounce environment of course ;))

Have Fun

tallface65,

Re #2, you are basically saying that lambda/4 radiation box calculates far-field radiation by approximation. Correct? How do you approximate the spherical phase with a linear phase, do you set r = infinity?

Also, why not have the radiation box bigger, though? Or would that make the simulation run a lot longer or be computationally expensive.

Re #3, for rx antenna, power at source gap (for dipole antenna) should be zero, correct? Also, for Friis equation, is power transferred the power accepted at tx antenna, not the power incident? Also, could S21 plot in HFSS get me the Power received at rx antenna? Why or Why not?

Thanks and sorry for all these questions at once, but I really need confirmation of concepts I think I got right in my head.

re #2: no, there is no approximation. rather the far field transformation is an approximation of the near field transformation for the field distribution and phase at a particular distance from an antenna
re #3: you can use S21, but for antennas that are separated by large distances, the calculation time is exceedingly large.

Best advice is look over the Green's function transformations for near/far field calculations from a near field distribution. Even in measurement, most people measure the near field of an antenna and use the Green's function transformation to determine far field performance.

Have Fun

Hi!
How can I use receiving antenna in simulation and get amount of the power received from transmitting one? Would you please send an example or instruction to my email pls

I am trying to figure this out too. Someone please post an example, it would be very helpful!

Is your transmitting antenna and receiving antenna in the same design? If so, once the transmitting antenna is excited by port, the radiated field will excite another antenna. If you have two dipoles, you will excite the transmitting antenna with lumped port and another dipole will have 50 ohms or whatever your terminating impedance is.
If two antennas are in different designs, your receiving antenna can be excited by linked far field or near field source of the source antenna. If you know your magnitude of incident electric field you can use incident wave for excitation.