Low Loss Coax for Test Equipment

[RCHRDM]

Can someone please recommend the best low-loss coax for connecting RF test equipment together (with N connectors)?

Richard

[beanflying]

The Best for what will be the obvious question from others?

Rough idea of any power requirements and frequencies would help narrow it down. But from your Type N's I gather to are up and over 1GHz?

[TheUnnamedNewbie]

What frequencies are you working at? What do you consider best? Phase stability? Loss? Robustness? Flexibility?

At work we use mostly H+S Sucoflex 103 and 102 for the generic low microwave stuff. 102 has slightly higher loss but is a thinner, more flexible cable. Both are available as stock product with N connectors.
For phase stable stuff we (like pretty much everyone, I guess?) use Gore Phaseflex.

For the very low level signals we tend to stick to RG316 and RG58 for most stuff. RG316 is tiny and so flexible.

[tautech]

You might find this chart useful for guidance:
As attached.

[mzzj]

Luckily OP just wanted best without price constraints as Gore phaseflex cables are about 1000 usd per piece 

tautech, link, not image 

[beanflying]

I remember speccing Teflon Encapsulated Goretex O'rings on a few specialty jobs in my former life - so glad I was selling not buying 

[TheUnnamedNewbie]

Luckily OP just wanted best without price constraints as Gore phaseflex cables are about 1000 usd per piece 

Where can I get those? We pay a heck of a lot more per cable than 1000 USD ;p 1000 bucks a cable is what we spend on a lot of our non-phase-stable cables.

[Berni]

Sure Gore Phaseflex or Huber+Suhner Sucoflex

But the question is if you are prepared to pay the associated price tags.

If you want the absolute best in performance you will need rigid coax bent into the form you need.

[mzzj]

Luckily OP just wanted best without price constraints as Gore phaseflex cables are about 1000 usd per piece 

Where can I get those? We pay a heck of a lot more per cable than 1000 USD ;p 1000 bucks a cable is what we spend on a lot of our non-phase-stable cables.

Mouser
https://www.mouser.fi/Gore/Wire-Cable/Cable-Assemblies/RF-Cable-Assemblies/Phaseflex-Series/_/N-bkrix?P=1yym9e5Z1z0zkrm

2.92mm connectors are lot more spendy tho

[radar_macgyver]

If you want the absolute best in performance you will need rigid coax bent into the form you need.

For phase stability, yes. One can get lower loss, but using Heliax. It's awful stuff to work with though - has a will of its own and refuses to bend the way you need it to. Of course, there's also flexible waveguide for even lower loss.

[RCHRDM]

Thanks for the replies!  I am connecting test equipment together over about three feet or less (e.g., signal generator to spectrum analyzer or oscilloscope).  I am checking the calibration of the spectrum analyzer and oscilloscope at present.  Frequencies are below 500 MHz and power might be up to 10 dBm.

Richard

[RCHRDM]

I am working mainly with ham radio equipment.  "Best" is lowest attenuation.

Richard

[RCHRDM]

How about RG142?

Richard

[Berni]

For those sorts of things good ol RG58 tends to be plenty good enough, as long as its good quality cable and not some cheap Chinese crap that has only a few strands around it as shield. At these sorts of distances and frequencies the attenuation is almost nothing. That power rating is also handled just fine by any cable out there.

But if you do want a nice low loss cable a commonly used one is LMR240. Gets you less loss and handles things into the GHz range much better, but will be a good bit more expensive while having the poor flexibility of a decently sized garden hose.

Cables that get you low loss, high frequency operation and high flexibility all at once tend to be more expensive proprietary things from people like Gore

[RCHRDM]

I checked.  RG-213 has lower loss than RG-142.

Richard

[mzzj]

I am working mainly with ham radio equipment.  "Best" is lowest attenuation.

Richard

I have some cut-offs of "perfect" cable for you:



oops. any other requirements than lowest attenuation? 

[don.r]

I am working mainly with ham radio equipment.  "Best" is lowest attenuation.

Richard

I have some cut-offs of "perfect" cable for you:

<img>

oops. any other requirements than lowest attenuation? 

A family of mice could make a nice home in that cable.

[mzzj]

Heliax also comes in 5" diameter but I have never seen that 

Ham radio fellas are  building notch filters from similar cable to the one in picture ie ~2" foam insulated.
For a good notch you need really low loss cable. 

[RCHRDM]

Wow!  Nice cable!

Richard

[tautech]

Thanks for the replies!  I am connecting test equipment together over about three feet or less (e.g., signal generator to spectrum analyzer or oscilloscope).  I am checking the calibration of the spectrum analyzer and oscilloscope at present.  Frequencies are below 500 MHz and power might be up to 10 dBm.

Richard

Up to 1 GHz these 1M long ones have been fine for me:
https://siglentna.com/product/bnc-cable/

Sure you can buy Met grade cables but unless you're chasing every 0.1dB why ?

[RCHRDM]

I am doing something kind of basic, which is to check the accuracy of my test equipment.

I have an Agilent E4400B signal generator.  I checked its output with an HP 432A for which I have three working sensors.  The E4400B seems to be accurate when I measure 2 dBm at 50 MHz and all three sensors agree.

When I make this same input to an HP E4406A Transmitter Analyzer, the measurement is about 1.95 dBm on its spectrum analyzer.  This is pretty good, I should think.  The other day when I tried this, I did not get as good a result and wondered if it was the coax cable.  I have a couple of other instruments (LeCroy WaveRunner 44Xi-A oscilloscope and a JDSU JD747A Signal Analyzer) that I would also like to check. 

I just tested again, and the spectrum analyzer measurement on the JD747A is way off (like 12 dBm!) presumably because I don't know how to adjust the settings correctly.

I am not an electronics engineer and have the equipment for ham radio work, mainly.  I am still learning!

Richard

 

[tautech]

 

I just tested again, and the spectrum analyzer measurement on the JD747A is way off (like 12 dBm!) presumably because I don't know how to adjust the settings correctly.

Yes just might be input attenuation settings.

Absolute amplitude measurement precision is another rabbit hole most don't want to go into, if I'm getting within a dB or 2 it's near enough to tell me I'm thereabouts.

Even Sharirar on the Signal Path checking accuracy to 3 GHz on a SVA1032X using reasonable quality SMA cabling and a SMA-N adapter was getting ~0.5dB losses.

Grab a few cheap BNC cables using the chart I posted earlier to give guidance on coax type and frequency specs and for your work you'll probably find them quite OK.
I got a customer 50 odd BNC cables just from an Aliexpress supplier and apart from their crimps being a little 'how ya doing' suited their needs well.
You don't need to spend big bucks for 90% of uses.

[RCHRDM]

Thanks!  That's good to know.  I don't need to obsess about it!

I learned this about RF wattmeters (like the Bird 43) that have about 10% accuracy.  As you say, that is good enough to know if there is a problem or not.

I have a few Chinese cables with N-connectors.  The crimps failed on a couple of them and the connectors fell off!  I'll get a few good quality cables.  I figure it is also a good idea to avoid adapters (e.g., N to BNC) and go straight to the device under test, if possible.

Richard

[tautech]

Thanks!  That's good to know.  I don't need to obsess about it!

I have a few Chinese cables with N-connectors.  The crimps failed on a couple of them and the connectors fell off!

 
Yes this you have to keep an eye on as there's a lot of subtle variability in measurements and a just poorly adjusted crimper can let you down here.

I figure it is also a good idea to avoid adapters (e.g., N to BNC) and go straight to the device under test, if possible.

Yes in the perfect world we'd all have the correct cable with the right ends but again for hack around work you will need some adapters and cheaper cabling.
Don't overlook all quality connecters/cables have limited mating life cycles and such good stuff needs be reserved for bestest work.
For instance the front panel bulkhead connector into quality equipment is often preserved with a sacrificial adapter, say a N-SMA then you are able to work with SMA cables that are quite cheap and typically much more flexible.

[RCHRDM]

Good idea!  I have a miniVNA Pro, which is a small VNA, as you may know.  It has SMA connectors.  I bought some short cables (1 foot) with BNC or UHF connectors for testing.  I thought this would save the SMAs from getting yanked around.

I should do something similar for the bigger test equipment, as you suggest.

Richard