How to connect antenna base to powder-coated steel?

[Joerg]

Gents,

Ran into a wee snag during a design. The metal enclosure is plain steel
that is powder-coated. The guys at the metal shop said that they cannot
provide a nice connection surface for a bulkhead connector like this:

http://www.l-com.com/item.aspx?id=5295

I was hoping they could provide a masked nickel-plated area but ...
nope. They can only provide a threaded stud like those safety ground
ones. Not so cool at several GHz. Ok, we could make a wide shunt strip
for that but that's yet another custom BOM item. Corrosion is also a
major concern here since this gets deployed in coastal regions. So
straight bolting onto bare steel is out.

Anyone know of a clever way to handle this? If someone knows of a
cheaper bulkhead SMA feedthrough connector that would also be nice. Four
bucks is a bit steep.

 

[tm]

Gents,

Ran into a wee snag during a design. The metal enclosure is plain steel
that is powder-coated. The guys at the metal shop said that they cannot
provide a nice connection surface for a bulkhead connector like this:

http://www.l-com.com/item.aspx?id=5295

I was hoping they could provide a masked nickel-plated area but ...
nope. They can only provide a threaded stud like those safety ground
ones. Not so cool at several GHz. Ok, we could make a wide shunt strip
for that but that's yet another custom BOM item. Corrosion is also a
major concern here since this gets deployed in coastal regions. So
straight bolting onto bare steel is out.

Anyone know of a clever way to handle this? If someone knows of a
cheaper bulkhead SMA feedthrough connector that would also be nice. Four
bucks is a bit steep.

It's not such a good idea to use Ni plated connectors at microwave
frequencies due to intermod. Ni is a magnetic metal and can provide the
non-linear properties to allow mixing to occur. Silver, gold, tin, etc would
be better.

Can you install a dummy connector while the powder coating process is done?
At least clean off the inside surface so the lock washer can get a good bite
into the steel.

 

[Mike]

[...]

It's not such a good idea to use Ni plated connectors at microwave
frequencies due to intermod. Ni is a magnetic metal and can provide
the non-linear properties to allow mixing to occur. Silver, gold, tin,
etc would be better.

Can you install a dummy connector while the powder coating process is
done? At least clean off the inside surface so the lock washer can get
a good bite into the steel.

That won't last long. Steel will rust. Other metals will cause galvanic
corrosion. See

http://l-36.com/corrosion.php
http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm
http://www.corrosion-doctors.org/Definitions/galvanic-series.htm
http://www.mcnallyinstitute.com/Charts/galvanic-series.html

Mike

 

[Adrian Jansen]

Gents,

Ran into a wee snag during a design. The metal enclosure is plain steel
that is powder-coated. The guys at the metal shop said that they cannot
provide a nice connection surface for a bulkhead connector like this:

http://www.l-com.com/item.aspx?id=5295

I was hoping they could provide a masked nickel-plated area but ...
nope. They can only provide a threaded stud like those safety ground
ones. Not so cool at several GHz. Ok, we could make a wide shunt strip
for that but that's yet another custom BOM item. Corrosion is also a
major concern here since this gets deployed in coastal regions. So
straight bolting onto bare steel is out.

Anyone know of a clever way to handle this? If someone knows of a
cheaper bulkhead SMA feedthrough connector that would also be nice. Four
bucks is a bit steep.

If you can get a lock-washer to bite thru the powder coat on the
backside, that gives you DC connection. Then a stainless steel oversize
washer on the front side might provide enough capacitance for the GHz
frequencies.

I have also experimented with silver-loaded paint around and thru a
hole, but results are a bit variable.

Beware any connector exposed to salt spray, the corrosion will kill it.

 

[tm]

[...]

It's not such a good idea to use Ni plated connectors at microwave
frequencies due to intermod. Ni is a magnetic metal and can provide
the non-linear properties to allow mixing to occur. Silver, gold, tin,
etc would be better.

Can you install a dummy connector while the powder coating process is
done? At least clean off the inside surface so the lock washer can get
a good bite into the steel.

That won't last long. Steel will rust. Other metals will cause galvanic
corrosion. See

http://l-36.com/corrosion.php
http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm
http://www.corrosion-doctors.org/Definitions/galvanic-series.htm
http://www.mcnallyinstitute.com/Charts/galvanic-series.html

Mike

If the inside is exposed to the corrosive atmosphere, so will the
electronics. Or
maybe something is left out of the description of the requirements.

The gasket on the outside will seal against the powder coat and keep the
inside clean and
dry, right?

Nickel is not the right coating to use in a corrosive environment.

 

[Mike]

If the inside is exposed to the corrosive atmosphere, so will the
electronics. Or
maybe something is left out of the description of the requirements.

The gasket on the outside will seal against the powder coat and keep the
inside clean and
dry, right?

Nickel is not the right coating to use in a corrosive environment.

Any metal in a salt environment is trouble. Any penetration of an
enclosure is asking for breakdown of the sealant and capillary wicking
into the junction, then galvanic corrosion.

Joerg is an old ham. He understands the difficulty of sealing antenna
traps and tuners against the weather. The problem is especially difficult
in a salt atmosphere.

This problem is not easy. I'd budget a bit more for the enclosure, and
carefully examine the other end of the coax for similar problems.

Mike

 

[Martin Riddle]

You have to do it the second way, to provide a way to clip the return.

...Jim Thompson

I second the Nickel plate, mask and then Powder coat. It would be
cheaper than any alternative, even if it adds cost.

Cheers

 

[Joerg]

You have to do it the second way, to provide a way to clip the return.

That's what I had suggested but they can't do it. For a variety of
reasons we have to stay with this shop.

 

[Joerg]

Joerg,

Is it possible to use a threaded tube to, later, mount a weather tight box
that
contains or mounts the RF connector? The feed line can run through the
tube.

We don't have a lot of space below, maybe an inch. So we (likely) need
to connect to the bulkhead connector with an angled plug. And it should
be cheap, low in labor minutes.

 

[Joerg]

It's not such a good idea to use Ni plated connectors at microwave
frequencies due to intermod. Ni is a magnetic metal and can provide the
non-linear properties to allow mixing to occur. Silver, gold, tin, etc
would be better.

Yes, Ni is not ideal but in this case definitely good enough. I have
even used it on sensitive equipment in the lower HF range.

Can you install a dummy connector while the powder coating process is
done? At least clean off the inside surface so the lock washer can get a
good bite into the steel.

We could, but the other issue is that it's bare steel, no plating. In
coastal regions having a Ni bulkhead connector on that will look yucky
within months.

On thing we were thinking about are PEM nut kind of inserts. Big enough
in diameter so its thread don't get in the way, but it'll be tough to
find one flat enough. Maybe stainless.

 

[Joerg]

Actually, our electronics fare quite well. And they are coated.

Any metal in a salt environment is trouble. Any penetration of an
enclosure is asking for breakdown of the sealant and capillary wicking
into the junction, then galvanic corrosion.

Joerg is an old ham. He understands the difficulty of sealing antenna
traps and tuners against the weather. The problem is especially difficult
in a salt atmosphere.

This problem is not easy. I'd budget a bit more for the enclosure, and
carefully examine the other end of the coax for similar problems.

The other option would be to purposely insulate the bulkhead
pass-through. Then we'd have no corrosion there as long as nothing
crunches through the powder coating. However, then the coax continuin on
the inside will become part of the antenna and pick up all sorts of
clock harmonics and such. It's a transceiver so this can mess up the RX
part of it, IOW we could lose range. We may have to try that though if
nothing else pans out.

 

[Joerg]

If you can get a lock-washer to bite thru the powder coat on the
backside, that gives you DC connection. Then a stainless steel oversize
washer on the front side might provide enough capacitance for the GHz
frequencies.

I have also experimented with silver-loaded paint around and thru a
hole, but results are a bit variable.

Beware any connector exposed to salt spray, the corrosion will kill it.

And that's just the problem. The lock-washer DC connection would be good
enough because it is short. But it'll be eaten away by corrosion. Just
the air is enough to do that out there. Imagine a climate where every
day massive amounts of humidity condensate onto everything.

 

[Joerg]

you can't put the gasket on the outside and have bare metal under the
nut
on the inside ? cover every thing in in vaseline or maybe even epoxy
before
you assemble it to keep the moisture out?

The gasket on the outside would be no problem at all but vaseline would
just push corrosion out some more months.

or go crazy, solder it in before powder coating


doesn't seem bad, digikey want 5 times that

Well, at that price we might just make our own

 

[Tim Williams]

The other option would be to purposely insulate the bulkhead
pass-through. Then we'd have no corrosion there as long as nothing
crunches through the powder coating. However, then the coax continuin on
the inside will become part of the antenna and pick up all sorts of
clock harmonics and such. It's a transceiver so this can mess up the RX
part of it, IOW we could lose range. We may have to try that though if
nothing else pans out.

Hmm, you'd want to ground the coax to the nearest grounding stud,
preferably with a shield covering the section from ground to connector.
And probably a ferrite bead or two on the coax between the transceiver and
ground point.

Tim

 

[Joerg]

Philmore/LKG has them for $2 in quantities of 5-500. p/n 11315

Thanks! I'll ask them. Their web site is a train wreck, doesn't even
have a search function:

http://www.philmore-datak.com/connectors.html

Or, roll your own with an 11305, brass nuts, rubber washers for $1.10

That's what we'd do if it's too expensive otherwise.

Or, leave the powdercoating in place and put a large enough base on
the antenna that the couple of dozen pf between base and steel is
nearly a short circuit at several Gigs.

That is what I am trying to avoid because this would have to be fastened
to the steel plate again, with screws -> corrosion in four spots instead
of one.

[...]

 

[Joerg]

Hmm, you'd want to ground the coax to the nearest grounding stud,
preferably with a shield covering the section from ground to connector.

A stud is one option they gave us but at gigeehoitzes that means a
custom stainless of whatever strip from bulkhead to stud.

And probably a ferrite bead or two on the coax between the transceiver and
ground point.

That won't do any good at a few GHz. But one or two small loops will do.
Unfortunately they also pick up RF but we can keep them out of the worst.

Well, as John Larkin once said, on some projects packaging can become

 

[Mike]

On thing we were thinking about are PEM nut kind of inserts. Big
enough in diameter so its thread don't get in the way, but it'll be
tough to find one flat enough. Maybe stainless.

Watch out for galvanic corrosion with stainless against pretty much any
other metal. Where you develop more than about 0.15V is a potential
corrosion problem:

http://l-36.com/corrosion.php

See for example the effect of stainless against aluminum:

Why My Kenmore Washer Died

Mike

 

[miso]

Gents,

Ran into a wee snag during a design. The metal enclosure is plain steel
that is powder-coated. The guys at the metal shop said that they cannot
provide a nice connection surface for a bulkhead connector like this:

http://www.l-com.com/item.aspx?id=5295

I was hoping they could provide a masked nickel-plated area but ...
nope. They can only provide a threaded stud like those safety ground
ones. Not so cool at several GHz. Ok, we could make a wide shunt strip
for that but that's yet another custom BOM item. Corrosion is also a
major concern here since this gets deployed in coastal regions. So
straight bolting onto bare steel is out.

Anyone know of a clever way to handle this? If someone knows of a
cheaper bulkhead SMA feedthrough connector that would also be nice. Four
bucks is a bit steep.

Wouldn't you be using a COTS NEMA box? Maybe type 6?

You could simply skip the feedthrough and use a marine grade cable feed?
Then the metal is out of the equation.

http://images.cwrelectronics.com/product/images/14296.gif

There are also COTS waterproof cable feeds that I think seal better, but
they aren't stainless.

I've examined a lot of NEMA setups, and they tend to pigtail the coax
through a connector to avoid this issue. I've seen the feedthrough too,
but less often. Think of it this way. That connector is going to need
some weather protection on it. Goop, RTV whatever. The pigtail removes
one point of contamination.

 

[My Name Is Tzu How Do You Do]

Watch out for galvanic corrosion with stainless against pretty much any
other metal. Where you develop more than about 0.15V is a potential
corrosion problem:

http://l-36.com/corrosion.php

See for example the effect of stainless against aluminum:

Why My Kenmore Washer Died

Mike

Leave it painted. Mount the antenna INSULATED, and ground the antenna
with a wire, and ground the plate as well. To a ground rod.

 

[Tim Williams]

Well, as John Larkin once said, on some projects packaging can become

Tell me about it, we're packing a 50kW induction heater into the smallest
enclosure on the market: tabletop sized, just a little heavy for one
person to lift.

Tim