Are there any advantages to using rigid RF coaxial cables?

Question

While watching this Signal Path video, I came across this shot:

I saw few rigid coaxial cables there and so I started wondering what might their advantages be. I could only find disadvantages:

• They're expensive
• Limited in available angles 45° or 90°. Otherwise they're not easy to bend compared to the other cable types
• Sold in sections so we may need to always join them
• Prone to corrosion

The only advantage seem to be higher power capability.

So, are there any advantages to using rigid RF coaxial cables?

Answer 1

So, are there any advantages to using rigid RF coaxial cables?

First, the product you show might be semi-rigid rather than rigid cable.

In either case, the main advantage is lower loss, because of the solid outer conductor (as compared to the braided o.c. used in flexible cables). The solid o.c. also reduces radiation, which can be important if there are sensitive receivers nearby.

In a production environment the rigidity of the cable is itself an advantage because it means the system will always be assembled with the cable arranged in the correct way, rather than however the assembly worker can make it fit. The rigidity of the cable also improves phase stability.

Most of the disadvantages you cited can be mitigated, or don't even apply when manufacturing in volume.

Limited in available angles 45° or 90°.

Not true at all. There are numerous shops that will bend these cables to whatever angles you like, following your drawings.

Sold in sections so we may need to always join them

Not true. You can have these cables assembled at whatever length you like. Certainly they can be fabricated up to at least a meter in length, which is plenty for the type of system you showed in your image.

Prone to corrosion

The outer surface is typically tinned copper or aluminum. Either of these materials is likely to be present in numerous other locations in an electronic assembly, so these cables will be no more prone to corrosion than, for example, the surfaces of any PCBAs used in the product.

Answer 2

From a mechanical point of view, a rigid (or semi-rigid) cable has the advantage that they will support themselves. That means that you don't have to worry about them getting pinched or stuck between things when they shift or move during assembly or transport.

A rigid/semi-rigid cable also has performance advantages. As it doesn't move or flex or vibrate, the electrical length will be more stable over time than with flexible cables. This can be important in applications where length matching or calibration is important, and is one of the reasons that test&measurement equipment is full of these rigid sections.

As others have said, you will have these manufactured by your suplier (or inhouse) to the exact shape and bends you need.

From an assembly point of view there is another advantage: they can result in fewer errors (but I don't think this is a significant factor when choosing for them). As they are made to the exact size, they will usually only go in one way, and you can't by accident connect to the wrong port within the device.

I think they might also be cheaper at high frequencies, since the solid nature of the outer conductor is easier to manufacture than the very tight weaves needed for coaxial cables operating at many tens of GHz.

Answer 3

I expect the isolation is also better, and the crosstalk is lower. Thus your ultimate system dynamic range will be better, which may be crucial for battlefield survival or to detect more planets around red dwarfs

If you want a system that will NOT outgas from plastic cable coverings, the volatiles not being set free to condense on your billion dollars of optics, or condense on relay contacts, then semirigid or rigid is your friend.

Also the rigid may use very sparse spacers inside, allowing mostly air/vacuum as the dielectric and thus LOWER losses.

Answer 4

Lower passive intermodulation, phase stability, and better shielding. Lower insertion loss, better temperature range, better dielectric and ... just everything.

But notice that your reference url shows a device in which two of the original semi-rigid cable assembly parts (W4 and W5) have been replaced with colorful flexible coax: flexible coax is available in better grades than it was when the 8445B was new.