wideband integrated feeding system for a type of spiral antenna

[sara67]

Hi guys

I'm working in a integrated feeding system for a type of spiral antenna

my antenna work very well , I have a return loss better than -10dB in 2-12 GHz
my Balun work very well too
but when I connect them together
my results are so bad and I dont have a wide band

Can anyone help me ?

 

[kspalla]

Check balun impedancece.
Balun could be 50 to 100 that meakes you blind for 50 ohm as other port is terminated with 50 ohm.
if balun impedance match with antenna then one should do not see bad RL

 

[sara67]

Hi
thank you
but my balun's impedance is 50 to 200 ohm ,and it have return loss better than 10dB in 200 ohm
input impedance of antenna is 200 ohm and it work really good in 2-10GHz
it is very amazing that when I coonect them toghether ,results are so bad
may there are another problem ? because I dont have mismatch

 

[kspalla]

I am sure that there is mismatch.
In a 50 ohm system, RL near 10dB is wide impedance range. its resistance can be between 25 ohm to 100 ohm and combination if imaginary part with practical impedance.
To get good RL you need to conjugate match R+ix of balun to R-jX of antenna, so work in impedance calculations to get good RL.

 

[sara67]

can I use of smith chart for understanding exact impedance in both?
I can't understand z smith chart
can you help me?

 

[WimRFP]

How did you determine that the antenna is working very well (simulation/measurement, what type of feed, reference impedance, etc).

Other thing can be that the balun becomes part of the antenna (that means there is common mode current at the balun that interferes with the radiation pattern). This all depends on the orientation of the balun with respect to the antenna. Posting some images may enable us to give better feedback.

 

[kspalla]

can I use of smith chart for understanding exact impedance in both?
I can't understand z smith chart
can you help me?

Unless you have good expertise in using smith chart,, its difficult to handle wide band problems.

 

[sara67]

How did you determine that the antenna is working very well (simulation/measurement, what type of feed, reference impedance, etc).

Other thing can be that the balun becomes part of the antenna (that means there is common mode current at the balun that interferes with the radiation pattern). This all depends on the orientation of the balun with respect to the antenna. Posting some images may enable us to give better feedback.

I simulated my antenna and balun in CST software
my antenna was analysed by S11 parameter
both have return loss better than -10dB in a wide band
I share some pic of my results
1_ s11 in balun
2_ s11 in antenna
3_ s11 when balun connected to antenna

 

[kspalla]

you are almost there, try to do some tweaking, adjust balun or add some patches
Better next time that save in PNG format than bmp which consume more memory.

Regards,

 

[sara67]

this results is not good for me , because I must obtain a wideband antenna with balun
I try a lot ,but they are not good
I dont know , why ?!

 

[kspalla]

This is real world. Things are ok and do not get worry.
See below link.
https://www.rfcafe.com/references/electrical/vswr-mismatch.htm
RL of balun and antenna are 10dB means vswr around 1.93 and resultant could be 1.93*1.93 =3.74 which is 4.8, vswr of 4.8 mean RL is near 5dB and the same you are getting in cascade simulation result.
So better focus the points to minimize those or target to get RL beter than 16dB for balun and antenna then you will get cascade RL better than 10dB.

Regards.

 

[WimRFP]

As also mentioned by kspalla, cascading a network with low loss but reasonable VSWR (your balun) with a load with reasonable VSWR (your antenna), can result in bad overall VSWR. So you should optimize for better VSWR for both antenna and Balun.

There is more that goes wrong: around 3.25 GHz, RL(antenna) = 9dB, RL(balun) = 25 dB, RL(ant+balun) = 4dB

A lossless network with RL=25 dB and a load with RL=9dB, would result in a worst case RL=7.9 dB, though your simulation shows 4 dB.

Did you simulate the balun together with the antenna in one EM simulation (so not a simulation based on S-parameter data)? If so, check for possible electromagnetic interference between the balun and the antenna itself. In other words, the field of the antenna may interact with the balun and that will change the antenna's impedance (and radiation pattern). This does not solve your problem as RL of both antenna and balun is not good enough.

If you did overall simulation based on S-parameters (this assumes no electromagnetic interaction), something is wrong with your reference impedance used during the EM simulation.

 

[sara67]

I really appreciate your help Kspalla and WimRFP
you are right
I think , I must try for better s parameter in both
I tryed and optimized them before, but I think those were not enough

 

[aniakhan]

Hi guys

I'm working in a integrated feeding system for a type of spiral antenna

my antenna work very well , I have a return loss better than -10dB in 2-12 GHz
my Balun work very well too
but when I connect them together
my results are so bad and I dont have a wide band

Can anyone help me ?

hi,
sara can u plz guide me something about baluns desiging.i have designed wide band crossed bow tie antenna having impedance of 100 ohm now for feed structure do ineed baluns???if yes then how to design a balun for wide band anteena ,specfication and refrences if possible plz help me