How to excite a waveguide ?

[StoppTidigare]

how to excite waveguide

Hi all, I wonder if waveguides and slotted waveguideantennas.
Should they be fed balanced ?

I've seen that one usally have some kind of hook into the waveguide to couple the effect from the coax. Does the shape of the hook, orientation determine if I am going to get a TE or a TM-mode ?

Kindest regards,
StoppTidigare

 

[flatulent]

You have to inject an electric feed (straight wire from the center conductor of a coax) where the mode you want to excite has the maximum electric field. Alternately, use the loop to inject a magnetic field where the mode you want has a peak magnetic field. If you want modes with more than one cycle of electric or magnetic field you have to put one feed probe at each peak.

The shape and size of the probe sets the impedance the coax sees.

 

[StoppTidigare]

Thanx flatulent ! And can anybody say someting about the balanced or unbalanced feeding of the waveguide ?
Which should be used ?

Kindest regards,
StoppTidigare

 

[eirp]

Hi, StopTigidare!

In practice, usually 2 types of WG feeds are used - both unbalanced due to coaxial cable nature which is mostly used to be a feeding line:

- probe. This basically excites E field and thus should be oriented in E-field peak of desired waveguide mode. Advatage is simplicity when built.

- "trapeze" (very close to loop mentioned by flatulent). It's usually more frequency-wide than probe and can cover all the band of dominant mode (in rect. WG 1:2 band TE10 - TE20). For good frequency response it should be carefully designed and optimized (you will need some cones when crossing from coaxial inner feed to trapeze).

Both of those feeds are well described in literature.
I have one project with trapeze transition in MWS, if you're interested, I can PM it to you.
I have also a nice MWS example of sector slotted 15dBi WG antenna.

BTW, why you are interested also in TM modes (or generally high-order ones)? They won't give you advantage in common applications...

Kind regards,

eirp

 

[StoppTidigare]

Hi all, and thanks for the replys.

Eirp, I didn't know there were any disadvantages with TM -modes could you explain ?

Kindest regards,
StoppTidigare

 

[g579]

If you excite TM modes, you may have some extra losses. And if near ecitation you place some elements they are difficult to consider too.
g579

 

[eirp]

For all interested - slotted waveguide sector antenna for 2.45 GHz band simulated in MWS..

I will post also comparison of measured and calculated radiation pattern later..

best,

eirp

 

[eirp]

Simulated (red curves) and measured radiation pattern in vertical and horizontal plane. Note that we haven't been enough far to fullfill farfield condition..

regards,

eirp

 

[goxy]

Thanks Eirp!

Waveguide can be excited and with simple pin but shifted because impedance adjustment like in posted example

 

[eirp]

Thanks Eirp!

Waveguide can be excited and with simple pin but shifted because impedance adjustment like in posted example

Hi, goxy!

Can you please upload your files again? Main .mod file is missing thus can't be opened...

Thank you,

eirp

 

[goxy]

I'm replace bad file above, I hope now it's OK.
Thanks :sm28: