Best connectors for 5.8GHz [OT]

G.W. Haywood ged at jubileegroup.co.uk
Thu Oct 6 13:21:21 EDT 2005


Hi there,

On Wed, 5 Oct 2005 Beat Meier wrote:

> A little bit off topic ...

Yeah.  Sorry. :)

> What is the best connector for 5.8GHz i.e. 802.11a?

It depends...

> I've heard that type-n is bad because of reflections at this frequency.

Type N was originally designed for use up to 5GHz but improvements
in design have extended its use to much higher frequencies.  There
are bad type N connectors out there.  There are bad eggs too - you
just have to be careful which ones you buy, and like eggs, it's not
really obvious unless you know what you're looking for.  Thesedays I
doubt you will have trouble at 5GHz with type N.  See for example

http://ece-www.colorado.edu/~kuester/Coax/connchart.htm

for a brief introduction to some connectors.

> so the advise was it would be much better to use
>   pigtail X -> SMA , antenna cable SMA -> N-type instead of
>   pigtail -> n-type, antenna cable n-typ -> n-typ

I don't really understand your descriptions here.  You want good kit
and a minimum of changes in sizes, types etc. but sometimes you can't
really avoid changing from one type of connector to another, perhaps
because the antenna is supplied with a type N connector and the radio
device, whatever it is, comes with a TNC.  Drives you nuts.  If the
overall length of cable is more than a couple of metres you probably
want the best cable you can afford, which probably means it's big and
can only be terminated with type N.  Incidentally it's "N-type" for a
semiconductor material, and "type N" for a connector.

> Any comments, experiences?

The main thing you need to bear in mind is that the cable and all the
connectors are acting as transmission lines.  This means that you are
working with electromagnetic waves and their funny little ways.  Lots
of the funny ways are similar to those of sound and light.  Where you
get a density change in the medium that's transmitting the waves, you
can get reflections.  That can be bad (e.g. in cables) or it can be
what you want (e.g. in a mirror).  You can also get refractions, which
again can be what you want (binoculars) or can be bad (er, binoculars).

Think of coaxial cabling as like plumbing.  For optimum transmission
the water needs a nice smooth bore to run along, with no restrictions
or kinks or very sharp bends.  Well, the bends are a bit different but
I won't dwell on it.

Properly designed *and fitted* coaxial connectors will appear to the
electromagnetic waves which travel along them as a continuation of the
cables to which the connectors are joined.  Seamless.  Invisible.

More specifically, the characteristic impedances of the cables and the
connectors will be the same.  That means effectively that the ratios
of the inner diameter of the outer conductor and the outer diameter of
the inner conductor will be the same and the dielectric constants of
the insulators between the conductors will be the same.  Or at least
nearly the same.  If there are differences, they will be small and
there probably won't be step changes in them unless there has been
some careful engineering - which is beyond the scope of this author.

It doesn't much matter about the outer insulators except perhaps to
prevent water ingress.  From the point of view of *joining* cables to
connectors and to each other it also doesn't matter so much about the
'losses' in the parts, but basically you don't want any losses.  That
means that the parts at these frequencies will tend to be be big, like
the type N connectors that you've been (perhaps wrongly) warned about.

The conductivities of the metals are important, particularly those of
any surface coatings.  Gold, silver and other precious metals can be
used to minimise corrosion and these materials are often used in high
performance connectors.  Stay away from chromium-plated parts if you
can, since the reason that chromium plating looks so nice is that the
metal is covered with a very thin, hard coat of oxide.  Not good for
conducting electricity.  The joining of the metal parts is important
from the point of conduction of the electricity (yep, even though it's
waves, it's still electricity) and also to prevent corrosion, which can
under some circumstances be especially nasty between different metals.
This 'galvanic corrosion' happens when any materials with different
affinities for electrons are placed in contact.  Like those nasty old
Leclanche cells.  Crimped connectors can be good, and done properly
they're supposed to weld the metal parts together so their electrical
properties are very good.  Done badly, they're awful.  Soldered joints
can be good if done properly, but it can be almost impossible to tell
a good one from a bad one if you're new to the techniques.  Sheesh.

Even if the connectors and cable are good, badly terminated connectors
and badly made joints can undo all that goodness and the reflections
set up can in some cases cause damage.  You're unlikely to suffer any
damage from this cause using the sort of power outputs that we use in
wireless gear for data networks.  (Lightning strikes are different! :)

HTH, there's a lot more on the Web - try Google.

73,
Ged.



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