TE10 standing wave in a waveguide

I want to excite a standing TE10 wave in a waveguide (i.e. a wave that is standing in the longitudinal direction). I want to use eithe a short dipole or a half-wave dipole. How should I place it in the waveguide? What will be the direction of the H field of the standing wave?

if you drill a hole in the middle of a long waveguide and put a radiating element into it, you will get a wave propagating to the left, and another one propagating to the right. There is nothing "standing" about the wave, though. To make a standing wave, you would need to terminate both ends of the waveguide in something (like a short circuiting wall of metal). Then you will get waves travelling toward the ends, and reflecting off of the ends, thereby causing a "standing wave" pattern.

Yes, I understand that the waveguide has to be terminated. Will the H field of the standing wave be perpendicular to the axis of the waveguide?

IF you can excite only the TE10 mode, then yes...the magnetic fields will be perpendicular to it.

The problem is that in the vicinity of the launch mechanism, there will actually be many modes excited--mostly evanexcent modes that do not propagate. So right around the probe, there will be lots of fields, but some small distance away, they will all have died out in amplitude.

How can I excite only the TE10 mode in most of the waveguide?

htg said:

How can I excite only the TE10 mode in most of the waveguide?

Click to expand...

Just use a standard coax to waveguide transition as described here:
https://www.qsl.net/pe1cqq/pdf/trans.pdf

Thanks for the link, but it is not very applicable to my situation. I am going to use either a short dipole or a half-wavelength dipole at 150 MHz in a properly large waveguide. The question is how to position it.

There are pretty standard ways to do that. TE10 has an E field maximum halfway from either of the smaller walls. So you just put a centerpin from a coaxial connector at that point. If you want efficient transfer of power, you need to place a backshort a little less than a quarterwavelength from the center of that probe in the z direction.

It looks like things are significantly frequency-dependent here. So I still wonder what to do at 150 MHz. I am thinking of placing a dipole 1/4 wavelength from one of the ends of the waveguide. I didn't think about using the center pin of a coax at 150 MHz. Should I?

No, you probably want to use a 4" diameter copper pipe as a center pin (maybe even bigger) at 150 MHz. thats very low frequency!

The point is that I think about using balanced input to a dipole, which I want to place in the waveguide. What about such a solution?

What is about the full band ( Wide band) impedance matching.
A head disc is reuired at the tip
can anyone have calculations about that

htg said:

The point is that I think about using balanced input to a dipole, which I want to place in the waveguide. What about such a solution?

Click to expand...

as Biff44 and one other hinted at, ... waveguides are usually ALWAYS fed by monopole probes, in all my work on waveguides I have never seen one fed any other way. That is unless the RF exciter is in the WG is say a Gunn Diode or other
active device.

I have to ask, htg, do you really understand how huge a WG is at 150MHz ???
do you realise that you are looking at something approximating a large air conditioning duct in dimensions ? ~ 1 metre x ~ 1.5 metre x whatever length you desire

cheers
Dave
VK2TDN

htg said:

It looks like things are significantly frequency-dependent here.

Click to expand...

No. Things just scale with frequency. The microwave waveguide feed concept works perfectly fine for you, just scales in frequency.

You dipole idea makes no sense to me. The coax to waveguide probe linked above will create the proper mode much better than your "dipole in a box".

volker_muehlhaus said:

You dipole idea makes no sense to me. The coax to waveguide probe linked above will create the proper mode much better than your "dipole in a box".

Click to expand...

Why would the center pin of a coax work better than a dipole?

htg said:

Why would the center pin of a coax work better than a dipole?

Click to expand...

The "center pin of a coax" is much more than an arbitrary coax pin. It is carefully placed at the right distance from the metal back side (short) and with the proper dimensions, to excite the desired waveguide mode and obtain best matching.

Your antenna is designed to radiate in free space, not in a metal waveguide. It will work terrible in a waveguide, which is a completely different environment with "ground" all around the antenna in the near field.

How long should the protruding center pin of the coax be?

htg said:

How long should the protruding center pin of the coax be?

Click to expand...

an electrical 1/4 λ
and as Biff44 said above, for 150-200MHz, the diameter of the centre pin will be substantially bigger in diameter than the centre conductor of a say RG213 coax cable

cheers
Dave

Lets take a step back here. Even though you are very low in frequency, and the waveguide will be huge, in order to propagate a TE10 mode you need the height to be smaller than the width of the cross-section.

Scope this page out:
**broken link removed**

The biggest waveguide they list there is WR-2300, which is too small for you. The lowest frequency WR-2300 will propagate is 320 MHz. Since all of this stuff is linear with frequencies vs dimensions...a waveguide twice this size WILL propagate 150 MHz.

So, lets say you had a waveguide that was 46 by 23 inches in cross section. That would work. You are going to place a probe, centered in the 46 inch wide wall, pointing into the waveguide. At 150 MHz, 1/2 wavelength will be a little smaller than 46 inches (study waveguide theory to see the explanation).

Any probe that you place pointing into the waveguide will, obviously, have to be shorter than 23 inches. If it was 23" long, it would short out at the other end!

In addition, since we know that the 1/4 wavelength in the guide is smaller than 46/2 = 23 inches...that the electric field from bottom to top of the waveguide is all one "sense". In other words, the electric field never swaps from + to - as you travel from the waveguide bottom to the top wall. It is all one polarity. So if you tried to put a dipole into the waveguide, which has one arm +, and the other arm - in relative sense, half of the dipole would be fighting the generation of the TE10 mode. In face, if you place it right in the middle of the guide, it should completely null out the creation of a TE10 wave.

You need a probe that is shorter than quarterwavelength, and one that is all the same polairty.

Can I make a waveguide from a copper-clad PCB laminate?