Electronics > RF, Microwave, Ham Radio

Must-have RF "Plumbing" for the lab

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0culus:
This year I'm planning to embark on some ham related projects that I haven't quite decided yet. But in the meantime I am looking to acquire the tools I need.

For those of you with more experience, what are some "must have" items for making connections that no RF lab ought to be without? There's dizzying arrays of attentuators, pads, splitters, combiners, and more. I have a few attenuators already.  Also, eventually, I want to get into microwave, though I likely will want to stay at or below 18GHz to at least help keep cost under control.

Even further, are there things that are worth building yourself for the experience?

Thanks!

T3sl4co1l:
Definitely a set of cables, tees, adapters and so forth for whatever your equipment mostly has.  BNC is probably your first choice, followed by SMA.  Difference being, BNC is very common on test equipment and lots of hardware, handles reasonable power, but tops out in the low-GHz; while SMA is good to higher frequencies, maybe more vulnerable (smaller connector, more wear on the thread and nut?), and less power handling.

Adapters and fittings are readily available either way.  Attenuators, splitters, directional couplers...  Incidentally, watch out for 75 vs 50 ohm BNC, always triple check what you're getting.  They're mostly compatible but not always, and obviously your SWR will be worse with dissimilar devices.

When you get to microwaves, probably better to buy the hardware for that on an as-needed basis ($$$!).  Wouldn't want to buy up a dozen microwave-capable cables and connectors, just to step on and kink and break them doing pedestrian tasks!

Regarding self-made things, depends.  I keep a stock of PC mount BNC connectors handy for projects.  Easy enough to solder one onto a proto board (edge launch style) and get something almost as good as a proper metallic cylinder/box module.  Whatever you want, you can make: filters, splitters, couplers, mixers...  If nothing else, do a few to get a feel for how difficult it is to make a well-balanced coupler or mixer or whatever!

Tim

dmills:
And **REALLY** watch out for 75R vs 50R N types!
They do not mix, and a 50R Male into a 75R female will make for a poor fit for any future 75R males.

You can quickly wind up with a LOT of money in RF connectors, and inter series adaptors, so one trick is to mostly pick one connector (I suggest SMA as being reasonable at low power) and adapt everything to it, slightly more loss but it turns an N^2 problem into a 2N problem.

My top tip, some small tin boxes with a couple of whatever your favoured connectors are and some feedthru caps fitted are surprisingly useful for screening prototypes, well worth keeping a few around.

When buying adaptors and the like, go for the used silver plated variety rather then the new overly shiny nickel ones, used Amphenol, MA-Com, Huber-Schurner, Radial are generally better in all ways then new ali-express.

T3sl4co1l:
Speaking of nickel plating, it's awful for PC mount parts, but nearly unavoidable. >:(  Takes forever to tin.  Tin-plated connectors are almost nonexistent, at reasonable prices anyway.

That's a definite advantage to SMA, gold-plated connectors are cheap and plentiful, also in edge-launch types.  No solderability problem there!

Anyway, nickel plating is undesirable for low noise or low distortion RF applications, because its surface oxide tends to be nonlinear.  In a mated connector, there's probably direct metal-on-metal contact somewhere, but the nonzero voltage drop across those contacts means there's some voltage drop across the rest of the mating area, including nickel oxide contacts, which draw nonlinear currents.  These currents are small relative to the dominant (metallic contact) current, but that still leaves some.  AFAIK, this is usually in the -80dB (THD/IMD) sort of range, not something you'll notice casually, but annoying in a sensitive measurement.  Silver plating does tarnish, but the tarnish is "wiped" easily (scraped off the connectors are mated), which helps.  Best of course is gold, which doesn't tarnish at all.

Tim

dmills:
The real way to wind up dumping money into the rf connector and cable money pit of course is to buy a VNA, at which point you can see the mismatch and poor return loss....

They also come with weird precision metrology connectors like APC7 and precision 2.92mm which tend to be both delicate and more money then god (And do NOT get me started on the cal kits).

Seriously you think the 6.5 digit voltmeters of the volt nuts are expensive? I give you the Gore "Phase stable Test port extension cable", costs more then audiophile speaker wire, but is actually (sort of) justified.

Regards, Dan.

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