waveguide excitation through coax

[buyan]

For a coax to inject an electric field to waveguide, normally how much will the shape and size of the probe affect on the transition loss?

thanks for help~~~~

 

[biff44]

The length and diameter of the probe determine the imdedance match. The distance of the probe to the backshort determines the center of the operating frequency band.

 

[buyan]

thanks for help.
BTW, is there any formula to calculate the impedance matching based on the given probe's size and shape? or is there any rule to follow?
^__^

 

[biff44]

With modern emag simulators, you should be able to get a good 20% bandwidth design in short order. Just play around. If you need wider bandwidth, it gets harder. The widest bandwidth I ever saw was an HP waveguide to coax transistion that was a series of waveguide steps up to the probe and then a reduced height waveguide section a little less than a quarterwave long to the backshort. The probe was coax fed from the top of the wavedguide, the center conductor went across the waveguide, and shorted into the bottom of the reduced height section.

 

[buyan]

Hi Biff44, thank you so much.

Now my concern is still in narrow band, not wide band.

In my case, I have a transition from coax to waveguide through a probe. Let's say the probe size and shape have been changed a bit, will it affect the insertion loss (from the coax to waveguide) much?

I know the impedance will be changed. Now I am worring is how much it will change. Or is there any formula to calculate the change?

 

[XuQing]

Hi,
HFSS is very accurate for this problem.
good lucky!

 

[fekete]

Hi,
HFSS is very accurate for this problem.
good lucky!

Guess what? - So do all the other real 3D simulators! What is your point?
I don’t think somebody should spend ~ $ 42K on a program just to design coax 2 waveguide transitions.

 

[buyan]

Hi,
HFSS is very accurate for this problem.
good lucky!

Yeah, I know HFSS can do it.
But now my HFSS cannot work. Also I think there must be some rule or formula to follow except simulation results.
Anyway, thank you.

 

[biff44]

I have never seen a simple closed form solution to that problem, without going to the EM simulator. I agree, HFSS stinks and is WAY overpriced. I haven't used it in years, but remember a tendency to not converge on these types of problems, waveguide is big, probe is small, so you need a very fine grid to get correct simulations, etc. The main problem in forming the closed form solution is that you are impedance matching between a TEM coaxial 50 ohm impedance (V/I) to a TE mode waveguide impedance (E/H), and the EM simulators handle this sort of problem in stride.

You could probably arrive at a good design empirically, especially if the frequency is not too high. I would get a good waveguide load and hook it and a waveguide straight section to your transition. Then I would hook the coax line to a network analyzer and watch the coax port's impedance. Sweep the frequency to see where it seems to resonate (where the backshort is the right distance from the probe). Then at the center of resonance, see if it is 50 ohms. If it looks higher than 50 ohms, I would make the probe a bigger diameter. If it looks lower than 50 ohms, I would make the probe a smaller diameter. After you exhaust the diameters in your tool box, try making the probe a little longer or shorter. When you are done, if the center frequency is off, move the backshort around, closer for higher frequency, farther away for lower frequency. When it looks like 50 ohms and on-frequency, measure whatever dimensions you have!

 

[amicloud]

I think you should use HFSS with parametric optimization.
define 3 parameters,the diameter and the length of the probe,and the length
to the shorted waveguide face.then parametricly optimize it.
perhaps in one day ,you will get good result.

 

[XuQing]

If you haven't HFSS, you should make a probe with adjustable length and a adjustable short. Then do it by test.That's not time consuming to reach your desire.

fekete,all 3D simulator can provide the good results. I am familiar with HFSS and I know it's expensive if u buy HFSS only for coax to waveguide transition. But I don't know what 3D simulator is the cheapest?

 

[fekete]

fekete,all 3D simulator can provide the good results. I am familiar with HFSS and I know it's expensive if u buy HFSS only for coax to waveguide transition. But I don't know what 3D simulator is the cheapest?

Sorry I jumped a little bit earlier, but there is almost no design post in which somebody does not drop in something like: use HFSS, or use CST – like we don’t know that an EM software that cost as much as a luxury car, should be able to solve at least a simple transition problem. In fact, even in this topic, after three of us basically said that yes, we know that HFSS can solve this and we don’t care – another one jumped in and told us again that HFSS can solve this problem! You’ll see soon a CST fan will show up.
Don’t get me wrong, I like both MWS and HFSS (version 9 at least) and a few other solvers, but this wasn’t the place for that.
I don’t want to do the same mistake and divert this topic, so I’ll open a new one about simulators and prices.

 

[newbie101]

Hi All,

I plan to feed my antenna with a coaxial cable. The input impedance of antenna at feed-point is 50ohm.

1) Should I use a coax to waveguide connector (i.e. connecting the waveguide to antenna feed-point)?
2) How can I be sure that the coax-to-waveguide connector will allow all frequencies without any distortion etc?
3) How do I include this in a simulation with HFSS? (does anyone have an example file in HFSS? Please upload it so i can find out how it is done?)

Thanks,
newbie101