A few words of advice regarding adapters !
Every RF Lab needs them and has them but the importance of adapters is always underrated ! a adapter can make or break a measurement so choosing the right adapter and buying quality is essential!
First off you can break your adapters in different categories.
The first category are support adapters, used only for DC or low frequency (such 10MHz reference input, sweep or control voltage connections or DC Bias ) loss or reflection coefficient is not of importance. However, stay clear of the really cheap stuff because the very cheap adapters have poor mechanical tolerances and may damage a good adapter if they are mated together. If the adapter is hard to plug into a known good connector, don't force it, get rid of it, it's not worth it.
Category two is general purpose, usually for frequencies up to 18GHz or higher but for applications where losses or repeatability are secondary, LO Signal lines or interstage connections would fall into this category, scalar measurements as well.
Category three are precision adapters, usually adapters with high repeatability and low reflection coefficient, everything you use around a VNA or to calibrate equipment should be in this category. Precision adapters should be visually inspected and gauged on a regular basis!
I recommend color coding your adapters for easy discernment, pick your colors and use a permanent paint, marker or color tape.
Also for precision adapters, make it a habit to keep the protective rubber boots on when not in use to keep dirt and dust out and reduce the risk of nicks and scrapes.
Adapters should be stored laying down, an adapter standing up unprotected is a invitation for dust, dirt and solder splatter!
Precision adapters should always be bought new since its very hard to judge a used adapter, visual inspection and gauging goes a long way but to fully test a adapter you need a VNA with time domain and that's something only a few have.
Sources for adapters:
As tempting as it may sound, avoid eBay unless you are buying support adapters our you buy from a trusted and well known source. I can recommend Testparts or Western Test Systems, Phil Schnabel is very knowledgeable and has a good lab so you can be assured you are getting quality.
For new adapters go to Aeroflex/Weinschel (limited selection but good and fairly cheap), Sun Moon Electronics (wide variety, cheap), SRI Connector Gage company (good assortment, high quality, minimum order) Maury Microwave (good stuff very high quality, check their clearance listings for bargains) or simply Digikey (they got a bunch of RF Stuff as well, no min order, fast delivery).
I have found Sun Moon Electronics to be a very good source, they represent INMET and some other suppliers and they are one of the very few that actually stock everything they have on their website, they are very well assorted and have prices and stock qty right there on the website, its a small company but they are good.
Lothar