WATERPROOF SMA CONNECTION

[scanfor]

I have a wet application where a sensor is mounted in an underground pit that can fill with water after rain.

The sensor is fitted with a waterproof SMA connector, so the moisture doesn't enter the housing of the sensor.

However, the coupling between the antenna lead and the sensor is getting wet and corroding.

Is there a way of waterproofing that point of connection?

I've considered some form of conformal coating, but I'm hoping there is a more elegant and field-repairable option.

Any help would be greatly appreciated.

Scott

[Poroit]

G'day Scott,

Can you Heat shrink it and then wrap it with Self Amalgamating or Denso Tape?

[scanfor]

Thats certainly one option, and would probably work.
My main issue is that ham-fisted gorillas may be working on these and based on past experiences, they sometimes don't take as much care as they should, so I'm looking for a "foolproof" solution.

[richard.cs]

SMA isn't optimal for ham-fisted gorillas anyway. Are you sure it can't become N type?

[dietert1]

Some sensors come with a pigtail in order to locate the first connector in a better place.

Regards, Dieter

[Poroit]

G'day Scott,

Can we see a pic of the sensor?

[Kean]

There are some SMA rubber boots available on the market.  Not sure how waterproof they are, and if they will protect from submersion.

e.g. https://gammaelectronics.net/products/sma-rf-weatherproof-boot

[richnormand]

"There are some SMA rubber boots available on the market.  Not sure how waterproof they are, and if they will protect from submersion.

e.g. https://gammaelectronics.net/products/sma-rf-weatherproof-boot"

Looks like the beginning of a good solution Kean. Assuming the case and connector are actually waterproof I would take the rubber boot and fill it with RTV silicone.
After putting it in place I would make sure all the seams are full and oozing some RTV out. Let it dry and cut the excess.

being RTV you could dismantle it in the future if needed.

[Solder_Junkie]

You can use either "non acidic" silicon rubber solution, or liquid electrical tape. The silicon rubber solution is specifically manufactured to be safe on electrical equipment, as opposed to the bathroom type which will at least tarnish, or worse still, corrode metal.

Liquid electrical tape is rubber dissolved in a strong solvent, which comes in a jar or tin with a brush in the lid.

With either of these, you can over wrap with self amalgamating tape for physical protection, don't try to coat self amalgamating tape with liquid electrical tape as the solvents dissolve the rubber tape.

Use a torque spanner to tighten the SMA plug (8mm, 0.5Nm), they are widely available and not expensive.

SJ

[Smoky]

These SMA connectors have silicone rubber gaskets in them:



Maybe SV Microwave has a similar SMA connector to use with your specific cable

[tooki]

These SMA connectors have silicone rubber gaskets in them:



Maybe SV Microwave has a similar SMA connector to use with your specific cable

All SMA connectors have a gasket, since it is required by the standard. See page 78 (PDF page 84) of the attached PDF, which is the standards for nearly all coaxial connectors.

However, that only describes how the mating interface is sealed. (The gasket is also why there’s a defined torque for SMA, since overtightening will affect performance slightly as the gasket deforms.) An ordinary SMA connector could still allow water into the back of the connector where the shield is crimped. And of course it won’t protect the connector body itself from corrosion.

[scanfor]

Ha, yes less than optimal!

These need to be small diameter to fit through a hole in concrete, so I think the N types would be too big unfortunately

[scanfor]

I'll get an in-situ photo and post that.
It might help to describe the issue better.

[scanfor]

 This is a new sensor in the lab, before it's been installed.

The enclosure is IP68 and can be submerged.
There is no ingress into the enclosure, but when submerged the SMA connector corrodes and fails.

Regards
Scott

[shabaz]

.."non acidic" silicon rubber solution

By chance, I was looking for such a thing the other day, and I wanted it to be available in a small tube, and cheap. A low-cost, easily available option is Loctite 5660, it doesn't emit acetic acid, and, can confirm it is odorless when applying. My use-case was not under-water, so I can't comment on that.

[Bud]

Paint the connection in place with Plasti Dip, covering the crimp too.

[EEEnthusiast]

Hot dipping in plastic.... Or silicon sealant

[Smoky]

Butyl adhesive would work very well. It's used to install windshields.

Smear it over the entire fitting after it's installed.

It remains soft and can be removed with mineral spirits.

[Kean]

I typically use 3m Scotch 23 Self-Amalgamating Tape to cover outdoor connectors.

Based on that photo I think you will have difficulty properly sealing it from submersion by wrapping with rubber tape.
I suggest using a generous amount of something like Sikaflex, or other neutral cure silicone.
Plastidip maybe would work, but from memory it doesn't make that tight a seal with some surfaces, so it may allow water to wick in.

Maybe you could attach the antenna cable semi-permanently before installation, and use something like that IP68 SMA boot I linked earlier to help seal the antenna end of the cable.

[Poroit]

G'day Scott,

I suggest get rid of the connector on the enclosure by using a Nylon IP68 Cable Gland and 500mm of RG58 Cable.(Pigtail)

Then fit N Type Male to Antenna Cable and N Type Female to the Pigtail.(Hopefully this won't affect your Signal Strength)

Wrap those connectors with Self Amalgamating or Denso Tape.

Your Gorilla's should be able to handle N Types.

RFI  and Rojone were always a good source in Oz.

[tooki]

I suggest get rid of the connector on the enclosure by using a Nylon IP68 Cable Gland and 500mm of RG58 Cable.(Pigtail)

I was just gonna suggest almost the same thing -- use a cable gland to exit the enclosure, with enough length to relocate the antenna connector to above the water line, if the boxes don't fill up to the brim. If they do, then I'd seal the mated inline connectors using one of the methods described by others.

OP said they can't use N connectors due to size restrictions.

[Poroit]

Oops....sorry I missed that about N Types.

How about TNC Series?

[max-bit]

SMA is basically a "delicate" connector (even in the best design).
It is much better to use a TNC connector or simply an N connector.

[Poroit]

G'day max-bit,

The OP stated that N type will not fit through the access hole in the concrete pit.

TNC may also be to large...... but we wait for response.

[max-bit]

In this case, I would use SMA but wrap it with self-vulcanizing tape.

Unless it is to be disconnected frequently...
Then good SMA and hope for luck that nothing will happen.