Monday, April 26, 2010

SMA Port Savers

The SMA connector is excellent miniature RF connector used mainly for internal interconnects in low-power HF-to-Microwave applications. One thing to keep in mind is that this connector is not recommended for applications where repeated connect-disconnect cycles are required. In fact the SMA is rated for only about 500 mating cycles (for a high-quality one and as low as 50 cycles for the lower quality) and only if a great care is taken during mating. The connector should be tighten to a specified maximum torque using a proper torque wrench (torque is determined by the type - brass or stainless steel). Usually, the source of failure is the female connector (speaking of "delicate":-). The pin-receptacle contacts in the female connector are easily damaged or worn off . This makes the SMA connector unsuitable as "port connector" in lab instruments or any front-panel/real-panel applications where heavy use is expected. APC, N type of BNC/TNC connectors are used instead. My personal preference is to use 1) N-type and 2) BNC. The TNC is actually better than the BNC - it provides better mechanical support and more reliable connection but it is less popular, requiring frequent use of adapters and/or custom test cables.
A possible solution to the SMA's durability problem is the so-called "port saver". This is an "in-series" SMA adapter - male-to-female. It is connected to the port connector and provides a "disposable" extension - after the adapter's front-end is damaged or worn off, it can be quickly replaced (and at a low cost too). This way, the actual front panel port sees very little use and there is no need for a complicated and time consuming replacement procedure.

Unfortunately, the "port saver" is not very well known/used adapter and it might be difficult to obtain. Thanks to eBay I was able to get a bunch for a couple of dollars a pop! An even better choice would be the stainless steel version. When disconnecting devices, 5/16 wrench should be used to prevent the port-saver from unscrewing.

Saturday, April 17, 2010

N2PK VNA - SMA calibration standards

For my home-brewed SMA calibration standards I used the construction method, previously used for my BNC standards.

The Open, Short and Load standard are made of Male SMAs for PCB. The back side of the center pin of all three connectors is trimmed down to ~0.5 mm - the height of a 0603 SMD component. In addition, the SMA test fixture has its center pin also trimmed at 0.5 mm for improved solderability to the DUTs.

For the SHORT standard, I used a small teflon washer (cut from the center dielectric of a MIL grade coax) placed on the center pin. The thickness of the washer is ~0.3-0.4mm. This washer serves as an insulator in order to raise the actual "short" to the very tip of the 0.5 mm center pin stub, (the back plane is covered with silver bearing solder to short the pin). For the LOAD standard I used the same Vishay FC series 50.0 ohm (0.1%) high-frequency precision resistor as the one in my BNC standards set (Digikey p/n FC0603-50BWCT-ND). This type of resistor is the preferred component for the LOAD - it is designed with maximum precision and minimal reactance at high frequencies.

For mechanical protection, RF shielding and easy handling, I used sleeves made of brass tubing (Stock #135, K&S Engineering, 3/8 x .014") to cover the back side of each connector. Each sleeve is 1 cm long and it is soldered on the inside wall to the 4 ground pins of the SMA connector. The end cap is made of closed-cell foam and it is held in place by the heat-shrink tubing. The OPEN standard also has an additional rear RF shield made of a tin-plated brass disc, soldered to the inside wall of the sleeve. The OPEN is more sensitive to stray RF than the others and must be fully shielded. Such shield could be installed on the LOAD as well. The SHORT does not need one as it is self-shielded.

In addition, I made a set of female standards in order to cover DUTs with any port gender. I used PCB female SMA connector (round base) and the brass sleeves were made of Stock #134, K&S Engineering, 11/32 x .014"

The complete SMA calibration kit. Heat-shrink tubing is used for color-coding and it gives a nice and smooth finished surface, protecting the brass from tarnishing.

Saturday, April 10, 2010

New Callsign - AE1S

My original Technician class callsign was KB1FZA. After I upgraded to Extra class I decided to get a shorter 2x1 callsign (I didn't trust the systematic change to come up with a "good" callsign) and opted for a Vanity Callsign. My first choice was AE1S - it all made sense - these are my initials. At the time AE1S was assigned to Joseph Craddock and I got AE1Z - it was pretty close to what I wanted (Z almost looks like a mirror image of S :-). I must admit - I had a really nice run with the AE1Z callsign.
Sadly, Mr. Craddock became a Silent Key on March 22, 2008 (R.I.P.). FCC was made aware of this at the end of 2009 and canceled his callsign. There is a two years waiting period mandated by FCC before the callsign is released in the general pool. This waiting period ended on March 22, 2010 and once the callsign was available, I immediately applied for it.
My application was granted today and my new (and probably last) callsign is now AE1S.
My last contact as AE1Z was HK1NK on Apr 3, 2010, 21:20 UTC on 10 meters / QRP.
It is time to start a new logbook! The change is a bitter-sweet one! AE1Z signing off!