Waveguide Bend Tuning
Waveguide Bend Tuning
(OP)
How do you tune swept waveguide bends to avoid standing waves when the frequency you're using differs significantly from the midband frequency of the waveguide? I've struggled with this question for months now- we certainly never hit on it in emag fields and waves in college, and no textbook I've seen addresses this directly. Worst of all, the official guidance put out for military waveguide (below) seems to contradict the physics as I understand it. This has been the standard reference for manufacturing military waveguide since the original 1963 document, and I can't believe it's wrong, but it certainly looks that way... Am I overanalyzing this, or have we been doing this wrong all these years?
The first question is- what wavelength should I be using for the calculations- the ?=c/f freespace wavelength (ala MIL-HDBK-660A) or the "waveguide wavelength" taking phase velocity into account, as seems to be a more appropriate course of action? I mean, that's how I'd do it for straight waveguide, I don't see why bending it would change this fundamental principal, yet there's the mil-hdbk right there saying to use ?=c/f...
The second question is what wavelength fractions should be tuned for- half wavelengths like MIL-HDBK-660A recommends, or not. Now, call me crazy, but it looks like tuning to ½? would create standing waves instead of canceling them out- wouldn't an odd ¼? or ¾? be the right wavelength to use. What's the right approach to take? And can we even use the simply mean length for these calculations or go more in depth analysis with the wavefront through the H-bend? I can't see it mattering much for an E-bend.
My final question is, does it really matter? Are my VSWR & losses in a 5? radius bend going to be negligible no matter how I design it? What about 1? or 2??
MIL-HDBK-660A: http:/ /combatind ex.com/mil _docs/pdf/ Hopper/MIL -HDBK/CI-6 60A-MH-482 2-4932.pdf
The first question is- what wavelength should I be using for the calculations- the ?=c/f freespace wavelength (ala MIL-HDBK-660A) or the "waveguide wavelength" taking phase velocity into account, as seems to be a more appropriate course of action? I mean, that's how I'd do it for straight waveguide, I don't see why bending it would change this fundamental principal, yet there's the mil-hdbk right there saying to use ?=c/f...
The second question is what wavelength fractions should be tuned for- half wavelengths like MIL-HDBK-660A recommends, or not. Now, call me crazy, but it looks like tuning to ½? would create standing waves instead of canceling them out- wouldn't an odd ¼? or ¾? be the right wavelength to use. What's the right approach to take? And can we even use the simply mean length for these calculations or go more in depth analysis with the wavefront through the H-bend? I can't see it mattering much for an E-bend.
My final question is, does it really matter? Are my VSWR & losses in a 5? radius bend going to be negligible no matter how I design it? What about 1? or 2??
MIL-HDBK-660A: http:/





RE: Waveguide Bend Tuning
RE: Waveguide Bend Tuning
Make sure no dead bee's are inside, for each one you lose one dB I've been told.
kch
RE: Waveguide Bend Tuning
Is there an optimal radius and bend length given a particular frequency or is it all something you've got to don your wizard hat and cape for and cackle wildly while dropping smoke bombs?
RE: Waveguide Bend Tuning
RE: Waveguide Bend Tuning
As far as VSWR, the bigger the bend radius, the smaller the VSWR--theory of small reflections helps you out.
RE: Waveguide Bend Tuning
While I'm a long wavelength guy (i.e. I'm moving up in frequency so I might be learning here), here-s my two cents on waveguides.
The 1/2 wavelength guideline seems to be semantics. I interpret their 'integral multiples of 1/2 wavelength' as meaning 'even multiples of 1/4 wavelength'. Avoiding quarter wavelength impedance transformations seems the goal in their guideline.
Paragraph 1.2.5 states that the concern is for two reflections - one at the far end of the bend and the next reflection at the source end of the bend. So traveling two paths (there and back) means you travel a full wavelength for minimum VSWR. If the length was a quarter wavelength multiple it would take four paths to get back in phase; with attenuation on each reflection that seems much more prone to VSWR.
Let me know if I'm all wet here - I love learning!