Monopole radiating in a waveguide - how much energy propagates along the waveguide?

When a monopole radiates in a waveguide, it sends energy not only along the waveguide. Can I count on about 25% of energy propagating along the waveguide?

Depending on the cutoff frequency of the waveguide, you can may be able to count on 0% propogating!

What's the dimensions of everything and the frequency? What way is the monopole placed? All these things are critical before anyone has any hope of answering your question.


A coaxial to waveguide transisition is a standard piece of microwave equipment. It might be that is what you want, although you think you want a monopole.

A "monopole" stuck into a waveguide (assuming along the centerline to excite a TE10 mode) will have a certain efficiency related to the impedance match over your operating bandwith and the conductor and dielectric losses of the actual feed. So, on average, assume 0.95 of the power gets thru (assuming a well matched launch).

Then the waveguide goes in two directions. If you do nothing about that, half the power goes one way and half the power goes the other way, so you will get something like 0.4 of the incident power actually going along the waveguide.

If you try hard to launch a wave in only one direciton (using a backshort, or some sort of multiple point coupling mechanism) you would get a little more loss in those mechanisms, so maybe 0.9 of the incident power will propagate.

This is all modified, as drkirby says, by the actual waveguide cross sectikonal dimensions which if too big or too small will cause higher losses.

[QUOTE=biff44;1104268]
Then the waveguide goes in two directions. If you do nothing about that, half the power goes one way and half the power goes the other way, so you will get something like 0.4 of the incident power actually going along the waveguide.

I imagine reflections of waves sent at various angles to the long axis of the waveguide. I wonder how they interfere and therefore if in the far field inside the waveguide there are maxima and minima of intensity of EM radiation. I do not expect the textbook case of two plane waves propagating at an angle θ to the axis of the waveguide.

I think the question unclearer than necessary.
- What do you exactly want to achieve?
- In any case, if the "monopole" is placed inside a closed waveguide, all energy is either propagated in the waveguide or reflected back to the driving port. Impedance matching can minimize reflections, presumed you are working above the wave guide cut-off frequency.
- In the general case, the "monopole" will excite different waveguide modes. It's your job to adjust it's position and length to get the modes you want.
- An industry standard coaxial-to-waveguide transition can be understood as a monpole as well. It's usually placed at a waveguide end, so the wave is only transmitted in one direction.

Let's say I want to transmit EM energy from the Tx monopole to an Rx monopole via waveguide. My question is about what can be expected in a realistic case - the Tx monopole radiates in various directions with respect to the axis of the waveguide. All these waves INTERFERE - sometimes constructively, sometimes destructively. Are there known results regarding where along the waveguide I can expect maxima of intensity of E and H?

It sounds to me you need to Google the coax to waveguide transition. One each end of a bit of waveguide will do what you want I think.

This looks fairly informative, though I don't know how accurate the information is. It's not an area I know much about.

http://www.qsl.net/pe1cqq/pdf/trans.pdf

There are loads of commerical companies selling these. I'm sure you can find out what the loss is.
Dave

You'll find a number of basic cases of "wave excitation in waveguides" discussed in theoretical electrical engineering and RF engineering literature. For arbitrary antenna geometries, an EM solver should help.