I have designed a multiport antenna. Where as, I have 2 dB Gain but realize gain is -6 dB. Point to be noted, S11 is really good. I have -25 dB return loss at my desired frequency.
In realized gain antenna losses are counted that's why your realized gain is some what minimum. The reason behind could be that your antenna is mismatched.
check antenna impedance weather it is matched or not?
The only difference between the gain and realized gain is that- in realized gain return loss is taken into account. Since you have -25 dB return loss, a difference of 8 dB is not practicable. I would suggest you to redraw the structure and re-simulate. Thanks.
M. Ababil Hossain Yes, as I have really good return loss that makes me surprise why Gain and realize gain has so many difference. I tried to do resimulate but no difference. As I mentioned , it is a multi port antenna, I guess isolation could be a reason for bad realize gain.
Debraj Kar , yes, I am sure your input port power is going somewhere else. Plz check that.
Debraj kar , sir I too faced this problem sometimes even if I get good return loss the gain and realized gain are less. sometimes even the return loss just crossed -10dB level it showed good gain and realized gain. may be all of your input power is not fully accepted by your radiator as stated by M. Ababil Hossain sir.
To figure out the reason, the number of ports should be known. When you measure the gain what about the other ports ?, are they connected to matched loads or left open.
Checking on the near fields around the structure can give you some hints where the energy is heading to.
Besides the energy transfer to other points, maybe dielectric losses are eating up your energy. Isn't it the case?
A good return loss simply means you are getting the power in. To get the gain you need to get the power radiated. The missing power may be lost in ohmic and dielectric losses or in coupling to the other ports. Does your simulation give gain or directivity? Also check your measurement and your simulation. Don't automatically believe either especially the simulation.
The difference between gain and realised gain is power going somewhere else other than radiation. In your case about 86% of the power is going somewhere else. As you have a good S11 your missing power could be going out of the other ports, if they are not isolated. Look at the other S-parameters. The feeds for other modes need to be isolated from each other somehow, either by feeding at nodal points or using filters, if the frequency is different, or by using a switch. The only other thing I can think of is that your structure is very lossy. This does happen with antennas that are less than 1/10 wavelength long. Antennas this small need to be highly resonant to radiate much and the high currents generate a lot of heat in the antenna.
totally agree with M. Ababil Hossain Marcelo B. Perotoni and Malcolm White
try checking the ports.. u could identify the problem
Have you looked at the radiation patterns, both simulated and measured?
The gain and the pattern are linked. Does your simulation tell you gain or directivity? These are only the same for a 100% efficient antenna.
A. C. Marvin
The Gain referred to above is directivity. The Realised Gain is actually what is properly called gain, which includes the effect of circuit losses and mismatch losses.
Do you have a multiport antennas? What about isolation between ports? the realized gain also takes into account the mutual coupling
Well, as pointed out by collgues, the simulation can be completely different from realized design, simply when some practical consideration are not well defined, for instance the ports if they are more then two the losses and the coupling are real problems, I would add in the design, is they gain obtained for far field or near fied, i mean the box of radiation can cause miss concordance ! thanks to all.
Malcolm White and all, Yes, the problem all about is mutal coupling. It is multi port (4 ports with 90 degree phase shift) antenna. The isolation in between port 1 and 3 has very bad because of very narrow space in between port 1 and 3.
The realised gain is what you actually get with the actual mismatch. It takes into account total antenna efficiency not radiation efficiency.
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