How far can flex waveguide bend without adverse RF issues? - RF Cafe Forums

 Post subject: How far can flex waveguide bend without adverse RF issues?

Posted: Mon Dec 13, 2010 11:12 am 

since I've read that wave guide bends should have a radius of greater than two wavelengths, I was wondering why the bend radius limits of wr75 and most other sizes of flexible waveguide are less than two wavelengths, and I can't find any specifications addressing this.

Shouldn't the flex waveguide bend adhere to the two wavelength radius standard?

Here I will "paste" some of the literature I've read. This literature can be found at http://www.tpub.com/content/et/14092/css/14092_78.htm and many other places. It is from the NAVY Training guide (the "figures" did not copy and paste):

Waveguide Bends The size, shape, and dielectric material of a waveguide must be constant throughout its length for energy to move from one end to the other without reflections. Any abrupt change in its size or shape can cause reflections and a loss in overall efficiency. When such a change is necessary, the bends, twists, and joints of the waveguides must meet certain conditions to prevent reflections. Waveguides maybe bent in several ways that do not cause reflections.

One way is the gradual bend shown in figure 3-48. This gradual bend is known as an E bend because it distorts the E fields. The E bend must have a radius greater than two wavelengths to prevent reflections. Figure 3-48.—Gradual E bend.

Another common bend is the gradual H bend (fig. 3-49). It is called an H bend because the H fields are distorted when a waveguide is bent in this manner. Again, the radius of the bend must be greater than two wavelengths to prevent reflections. Neither the E bend in the “a” dimension nor the H bend in the “b” dimension changes the normal mode of operation. Figure 3-49.—Gradual H bend. 3-22

 Post subject: Re: How far can flex waveguide bend without adverse RF issue

Posted: Mon Dec 13, 2010 12:21 pm 

Greetings murraymt:

The rules of thumb are good for when you don't have the luxury of a manufacturer's spec to go by. If the company that makes the waveguide certifies that it will perform within some VSWR spec with a bend radius of less that two wavelengths, then I would go with that.

It's kind of like the air pressure spec printed on car tires. The tire might be stamped for 32 psi, but if your car manual says inflate to 30 psi, then that overrides the tire's broad application spec. The car engineers have designed the suspension/braking to perform maximally at their spec, not the tire company's (which is basically a rule of thumb type number for people who don't read their owner's manuals).

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 Post subject: Re: How far can flex waveguide bend without adverse RF issue

Posted: Mon Dec 13, 2010 1:02 pm 

Thank-you for the quick reply. I haven't found any specs that say it will perform within some VSWR spec with a bend radius of less that two wavelengths, or for any radius. They spec it as a mechanical tolerance which is why I asked here, I didn't know where else to get an answer. I suppose I could call various manufacturers, but it was more of a curiosity on my part.