Cleaning SMA connectors.
(June 27 2013)

Background.

At the Orebro EME meeting 2013 it was discovered that the "normal" cleaning process with isopropylic alcohol is inadequate. It is not enough to blow dry gas for a couple of seconds to remove the liquid. (Other names 2-propanol or isopropanol)

The instructions from Agilent http://na.tm.agilent.com/pna/connectorcare/Cleaning.htm say: "Let alcohol evaporate" but it does not say how one can ensure that the drying time has been adequate.

Experiments.

Figure 1 is a recording of the measured NF of a L LNA in front of which two short RG400 cables with SMA connectors are connected.

The measurement starts after midnight at a room temperature of about 26.5 degrees. After about 200 data points the SMA connection between the two short cables is opened and the male and the female connectors are cleaned and then dried for 30 seconds in the air stream of the table fan which is a part of the NF measurement setup. The NF has risen from 0.3455 to 0.3615 when the measurement continues. The NF then goes down to about 0.3490 dB and stays there for several hours, but at about sample 2500 it rises to 0.359 dB and then slowly rises to 0.360 dB. At about sample 7500 the two cables were disconnected and baked for 5 minutes at 175 C in our domestic oven. The cables were then put back connected in the same order. It is obvious that the bad influence of the isopropanol has disappeared. The measurement was stopped in the afernoon after about 14 hours at a temperature of about 27 C. The NF is then about 0.3458 dB.

The SMA connectors have O-rings that make them hermetically sealed. The isopropanol would stay inside the connector/cable for a very long time and move around inside driven by temperature gradients. 5 minutes at 175 C is adequate to dry it out.

Figure 1. The effect of isopropanol in a connector pair. See text.

Figure 2 shows an experiment where the connector between the two RG400 cables are wetted like during normal cleaning and after that the two cables were left lying on a table for 5 minutes far away from the table fan.

Before "cleaning" the NF was 0.3433 dB. After cleaning with 5 minutes of waiting time, the NF is about 0.004 dB higher and unstable vs time. At about sample 410 the two cables were removed and left on the table for 10 minits in still standing air. That brought the NF down to 0.3445 dB. The two cables were then placed in the oven at 175 C for 5 minutes which led to a NF of 0.3440 dB. That was unexpected, loosening and again tightening the three SMA connectors restored the original NF.

Figure 2. Some drying experiments. See text.

The conclusions of the experiments in figure 2 are uncertain. A total drying time of 15 minutes might be adequate. Contact problems may occur randomly in SMA connectors but I do not see it often. The experiment was therefore repeated and extended. See figure 3.

Figure 3. Some drying experiments. See text.

In figure 3 the initial NF is 0.3435 dB. After a first cleaning with 5 minutes of drying the NF is about 0.349 dB at about sample 500. 10 more minutes of drying time between about sample 540 and 640 does not restore the original NF. At about sample 870 another 10 minute drying period is finished. The NF is about 0.3447. The connectors have been loosened and retightened several times between sample 880 and 1200. The cables were again placed in the oven for 5 minutes at 175 C. After that the NF is 0.3429. That is 0.0006 dB below the initial value as one should expect because of the decreasing room temperature in the evening. 0.0006 dB corresponds to 0.04 K. With 0.14 K per degree for Te one can conclude that the room temperature has dropped by 0.3 degrees.

Conclusions.

It is important to make sure connectors are clean and dry. If two SMAm-SMAm adapters are used to connect a LNA to an antenna through a relay, the degradation that would be caused by newly cleaned connectors could be something like 0.05 dB. That is 3 K and enough to cause a noticeable loss of S/N in a 1296 EME system.

The above experimens seem to indicate that one should wait a couple of hours before connecting SMA connectors after cleaning. At elevated temperature a much shorter waiting time would be adequate.