SMA connector mating with Antenna - Gap

[hcglitte]

Hi,

When looking into an SMA connector one can see the inner pin/hole surrounded by a white dielectric.
The same for the Antenna.

When these are mated, what "stops" the mating is these two white dielectrics touching each other.

If there is a small gap where these two dielectrics should touch, but still the pin and hole are mated, but not fully - will this be an issue?
I would think that the transmission line no longer is 50 Ohm in the gap area, and that this will cause reflections.

The threaded body of the SMA connector is too short when installed in the enclosure wall, hence the issue.

Another positive, perhaps, effect of not letting the inner dielectric touching each other is that the
mating force (when antenna is screwed on and secured tightly) will act on the SMA nut and the outside of the antenna connector
rather than the inner dielectrics which isn't that mechanically rigid.

[dmills]

What frequency are you running?

The discontinuity will be very short, and not be a big deal for most things, but the Metrology folks get very funny about connectors and will have 'connector gauges' to measure and verify pin position.

With SMA this is a particular issue when intermating with the high precision 3.5mm parts sometimes found on high frequency analysers, see here

https://www.keysight.com/main/editorial.jspx?cc=GB&lc=eng&ckey=1000003325:epsg:faq&nid=-32575.536880729&id=1000003325:epsg:faq for a little discussion.

Actually having the pin too far forward is the case that causes damage.

Regards, Dan.

[hcglitte]

The frequency is 869 MHz.

When perfectly mated, the male pin will go 2.0 to 2.1 mm inside the hole.
In my application it will go 1.5 mm in. So the gap is about 0.5 mm long.

So with respect to avoiding damage, this may actually be beneficial...!?

I can use another slightly longer SMA connector so this issue no longer exist.
However, I am actually thinking the 0.5 mm gap may be a good thing, since the customers are normal people who like to tighten it hard.

The question remains whether the gap will decrease performance significantly.
I would guess it doesn't since 0.5 mm is very short compared with the wavelength.

[Bud]

Do not use crappy Chinese ones and refer to the manufacturer specification for the connector SWR/RL (3-18GHz is typical frequency range)

[ejeffrey]

At ~1 GHz it will not make a big difference.

A transmission line has a characteristic capacitance and inductance per unit length.  The gap in the dielectric has the same inductance as the regular coax, but much less capacitance due to a dielectric constant of 1.  Lets treat the capacitance as zero.  This leaves you with an unmatched series inductor.  The inductance is about 300 nanohenry / meter, or about 150 picohenry for your 1.5 mm gap. At 1 GHz, that 150 picohenry is about 1 ohm reactance.

There is a second effect as well.  When fully mated, the center conductor ends up with a uniform thickness, as the back of the pin is the same diameter as the socket.  If you don't mate the connector completely, there will be a short area where the pin narrows before reaching the socket.  This increases the inductance slightly.  The inducance is logarithmic in the ratio of the conductor diameters, so this is a relatively small effect.

As you can see, that gap really matters if you are running at 10 or 20 ghz, where the mismatch caused would be many ohms.  But at 1 GHz, it isn't likely to be a big deal unless you are trying to do precisely calibrated measurements.

I wouldn't worry about customers over-tightening an SMA connector, at least if they are doing it by hand.  If you have ever used an SMA torque wrench, you will see that the design torque is higher than you can apply by hand.  If they are using a wrench that is not a torque wrench, you should stop them.  You should only tighten an SMA connector by hand or with a properly set torque wrench.  Also, despite some persistent myths, tightening to torque is not required to meet signal integrity performance.  Hand tightening is sufficient to reach the rated performance.  Tightening to torque is only helpful for vibration resistance, preventing the connection from loosening over time.

[hagster]

My advice is to use an SMA connector with the correct thread length so you can screw it in properly.

The arguments people have made about RF performance impacts are all good. More importantly though, if someone sells me a product where the exposed SMA thread is too short I will imediately think that the rest of the product is trash and either send it back or throw it in the bin.