"Moding" in microwave transmission lines

Waveguide moding is where a waveguide has resonances at frequencies other than the intended one. A rectangular guide has horizontal and vertical prop modes, plus all their harmonics. Google waveguide moding.

A PCB trace, like a microstrip, has a resonant mode based on the gap between the trace and the ground plane, but it's generally too high frequency to worry about.

Coaxial cables have radial resonance modes that can be annoying. As frequency goes up, the cable has to be smaller to avoid modes, but small coaxes get lossy, so very high frequencies don't work on coax.

Thanks, John. I'm still confused as to the etymology of the term, though. That particular aspect is still puzzling me....

It's from "propagation mode" which suggests that different electric and magnetic field arrangements can transmit a wave.

The term is pretty general. It can refer to broadcast transmission, sound waves, earthquakes, fiberoptic transmission, waveguides, coax, all sorts of stuff.

"Moding" usually means some bad thing has happened because an undesired mode is active. As in magnetron moding.

Samtec has a good pdf on moding.

I'll look it up. Thanks again.

Fortunately my RF interests have always been confined to the sub

100Mhz range - "practically DC" as they say. It's only because I was tipped-off that a book called Planar Microwave Engineering is the "AoE of RF" that I ordered a copy and came across this term, which the author seems to assume the reader will already be familiar with. However, since I've previously been rooted to the domain where lumped element theory is sufficient, I've never encountered it.

resonance modes, mode-ing, moding ?

So you use wave-guides rather than coax.

Neither have I. But it has been around for a while. Alan Blumlein died in 1942 in a bomber that crashed during an H2S flight test. You should at least have heard about it. John Larkin probably hasn't.

Displaying a _repeating_ 100 GHz signal on a _sampled_ oscilloscope should not be too hard. Sample the signal with 9 to 9.99 GHz clock and the waveform can be displayed after 10 to 1000 cycles. The essential thing is having a Sample/Hold circuit going from sample to hold in less than 1 ps. The actual oscilloscope needs to be able operate at only 10 MHz - 1000 MHz.

Generating microwaves and using waveguids is nothing new, a lot was demonstrated before the year 1900. Signal generation and detecting up to 60 GHz (5 mm) was demonstrated in that era.

See Wireless World: "Victorian microwaves" Sep 1979 p 93-95

Sadly much of this was forgotten for half a century.

Just couldn't resist could you?

What was I supposed to resist? John Larkin doesn't seem to know about wave-guides either, and he is supposed to be a university-trained electronic engineer, even if the university was only Tulane. It is a bit surprising.

No, droning lame insults is his only joy. He doesn't design electronics.

Blumline was brilliant. His death changed the world.

But waveguides aren't useful for transporting wideband, DC-coupled, time-domain signals, which is what we do. Nowadays really fast signals use light.

Nowadays?

Fiberoptics and multi-GHz e-o modulators are fairly recent gadgets.

There have been free-space fast optical modulators around for a long time, Pockels and Faraday and Kerr type things, but mostly as phyics novelties.

Many-GHz and constellation modulation both need time-domain analysis, namely an oscilloscope.

Waveguides are quite good high-pass filters. Some even use them as part of EMP protection. A 2 m high and 1 m wide and tens of meters long waveguide can be used as a corridor into a Faraday cage room. It is simultaneously attenuating LF/MF/HF energy, where most EMP energy is.

John Larkin experiences helpful comments as droning insults. What he hears is "you might have done it better" which he can't believe.

None that interests John Larkin, who only pays attention to his own creations

John Larkin's ideas about signalling are somewhat idiosycnratic. Fibre-optics links have replaced microwave links on recent years, but microwaves were once the backbone of terrestrial coomuncations systems

RADAR was once a physics novelty/

John Larkin doesn't know any other way of looking at them.

Waveguides are dispersive, so distort wideband signals. Worse than microstrip or multimode fiber.

That's another reason to use an oscilloscope, with a coaxial cable connector, to look at wideband signals. A waveguide is a highpass filter, an ugly one, and coax (or microstrip, or fiber) is a lowpass filter, a pretty good one.

Singlemode fiber is astounding. There are a lot of optical wavelengths in 100 km.

Buried strip line is not dispersive. Microstrip - on top of the board - is, but seems to get used anyway.

You use the measuring equipment you understand - or think you understand - and allow for it defects (if you know what they are.

All true, But there were cross-country microwave links long before fibre optics got popular.

The bits that Fred Bloggs may have read might be.

was pretty speciic about wave theory, but that was published as phsyics.

Sturgeons Law says that 95% of everything is rubbish. This forum does seem to under-perform.

Buried strip line is not dispersive. Microstrip - on top of the board - is, but seems to get used anyway.

You use the measuring equipment you understand - or think you understand - and allow for its defects (if you know what they are).

All true, But there were cross-country microwave links long before fibre optics got popular.