hfss1
If I put a ground plane under the feeds (see attachment) and use 2 lumped ports (1 for each feed) then I get different results. I'm trying to make the short dipole a 2 port antenna because I'm trying to make a matching circuit in ADS. The antenna must have a matching circuit because it's a short dipole. I will extract the matrix data (S, Z, Y, gamma) and use the results for ADS, but I need to make the dipole a 2 port antenna.
The attachment : there is not ground around the dipole. There are 2 feeds where each has a lumped port. A ground plane surrounds the antenna.
Are you driving the two ports out of phase when calculating the radiation pattern? If you want to measure the input impedance then you must drive it through a balun.
Yes, I'm driving the ports out of phase. I saved all the fields and exported them to ADS. I'm going to create a single ended 50 ohm matching network in ADS. With these matching network values I'll use lumped RLC elements in HFSS to simulate the antenna. I'll add a halfwave transmission line to act like a balun for the input? Not sure here...
Is this the correct approach?
I don't know why I need the balun to get the impedance characteristics.
Maybe the inductance of your feed trace is messing up the result. It sounds like you have a good approach. Maybe try broadside coupled lines (one on bottom of substrate) instead of edge coupled. It may give a more balanced feed and the poles will be concentric around 1 axis. When you import into ADS you will need a balun or a some way of measureing differential s-parameters. You have to drive both ports to measuere the input impedance. If you meausure Zin through an ideal balun, calculate a single ended matching network, you can make it differential by mirroring the components (the shunt ones are 1/2 the single ended value).
I do not think the difference is because the port but due to the ground plane.
If you put a ground plane close the dipole will be shorted out. Try to use the
same port(lumped) and make a parametric analysis as you close the ground
plane. You will see the variations. By the way, there is not such thing as dipole
over ground plane. If the separation is about lambda/4 or a multiple then you
have a dipole with a reflector, or an antenna with a reflector for that matter.
But the closer you get the lower the return loss will be.
Thank you for your input. I found that the approach is correct. I'm having trouble with the matching network though. I matched the circuit in ADS and added Lumped RLCs in HFSS, and found that the S parameters match in both ADS and HFSS, which is great.
However, when I varied the L & C values I found the S parameters for HFSS and ADS to vary by as much as 100MHz for anyone who cares.
I DON'T UNDERSTAND WHY THE GAIN IS SO LOW (-30dB) FOR THE ANTENNA EVEN THOUGH THE DIPOLE IS MATCHED?
that is because the efficiency of short dipole is very low witch depends on it's short length(l/lambda). that is why the gain of a short dipole is very low even though the matching network is perfrct. but i think your approch is not the rigth one. because for a short dipole (and ofcourse for a broad band matching) the active elements (usually FET) is used. this precedure compensate the low gain of the antenna.
I'm not sure if I agree with Maryam because I calculated the radiation efficiency from a text book. efficiency = (radiation resistance) / (rad resistance + ohmic loss) .
Usually radiation resistance is low because short dipoles are made with 1% or less of a wavelength, like car antennas (only 7% efficient), but I'm using 10% of a wavelength.
From my calculation I believe I should see an efficiency of 97.7%
Maybe someone can help.
Electrically small antennas give high capacitive reactances with small resistive parts.Though you can tune out the reactances by inductors and capacitors, etc., the small resistive part cannot be compensated and may require a transformer in the matching network.hope this helps.