Hi all,

I have to make up some highly accurate length RG-214 coax patch leads
for network analyzer interconnects up to 1.3Ghz. I have the cable and
the male N-type plugs to terminate them with.
So how best to trim this cable to maintain its 50 ohm impedance as
the plugs are joined to the cable? The plugs' center pins are
solder-type; the shielding Otoh is clamped in place. Are there any
fool-proof cutting dimensions to get the result right every time?
THanks,
Paul

There are quite a number of styles of N connector, and each is a bit
different. Cable stripping dimensions for a couple of the more common
ones are shown in the ARRL Handbook, and you might be able to find
instructions for your particular connector on the manufacturer's web
site. I'd say the most important thing is to make sure the center
conductor pin protrudes the correct distance, and make sure the cable
end of the center conductor pin is against the insulation. That is,
there shouldn't be any place where air is the only dielectric between
inner and outer conductors except along the large diameter portion of
the center pin.

My advice is based more on extensive experience with time domain
reflectometry than on a great deal of experience making precision cables
with N connectors. Hopefully someone with more actual connector
experience will comment further.

Roy Lewallen, W7EL

Paul Burridge wrote:
Hi all,

I have to make up some highly accurate length RG-214 coax patch leads
for network analyzer interconnects up to 1.3Ghz. I have the cable and
the male N-type plugs to terminate them with.
So how best to trim this cable to maintain its 50 ohm impedance as
the plugs are joined to the cable? The plugs' center pins are
solder-type; the shielding Otoh is clamped in place. Are there any
fool-proof cutting dimensions to get the result right every time?
THanks,
Paul

On Fri, 17 Feb 2006 17:35:47 -0800, Roy Lewallen
wrote:

There are quite a number of styles of N connector, and each is a bit
different. Cable stripping dimensions for a couple of the more common
ones are shown in the ARRL Handbook, and you might be able to find
instructions for your particular connector on the manufacturer's web
site. I'd say the most important thing is to make sure the center
conductor pin protrudes the correct distance, and make sure the cable
end of the center conductor pin is against the insulation. That is,
there shouldn't be any place where air is the only dielectric between
inner and outer conductors except along the large diameter portion of
the center pin.

My advice is based more on extensive experience with time domain
reflectometry than on a great deal of experience making precision cables
with N connectors. Hopefully someone with more actual connector
experience will comment further.

Thanks, Roy. This is *totally* uncharted territory for me so any
advice is welcome.
I guess even if I can't get any cross sectional drawings of the
N-types off the net I could perhaps measure the required cut from
probing the connectors themselves with a dial caliper depth gauge -
which I happen to have. That ought to be the most accurate way of
going about it, I guess.
Is the amount of braid one clamps up into the plug important? Does it
matter if it gets twisted slightly (where it's doubled back down over
the outside of the outer insulation)?
Thanks,
p.

Paul Burridge wrote:

Thanks, Roy. This is *totally* uncharted territory for me so any
advice is welcome.
I guess even if I can't get any cross sectional drawings of the
N-types off the net I could perhaps measure the required cut from
probing the connectors themselves with a dial caliper depth gauge -
which I happen to have. That ought to be the most accurate way of
going about it, I guess.
Is the amount of braid one clamps up into the plug important? Does it
matter if it gets twisted slightly (where it's doubled back down over
the outside of the outer insulation)?
Thanks,
p.

Details of how the braid is clamped will affect the physical ruggedness
of the assembly, but it shouldn't affect the electrical characteristics.
I don't understand about doubling it back over the insulation, though.

Remember that virtually all the current is flowing on the outside of the
inner conductor and the inside of the outer conductor. What you're
trying to do is maintain a constant impedance as the signal goes through
the connector. This means that at places where the dielectric is absent,
as in the mating area, the inner conductor has to become larger and
closer to the outer conductor. If the dielectric is absent at any point
where it should be present, you'll have a high impedance in that region.
Any place where the center conductor changes diameter, there has to be a
corresponding change in the spacing to the outer conductor and/or a
change in the dielectric. The connector is carefully designed to do all
this, but as you're well aware, it'll only work out as designed if it's
assembled correctly.

Roy Lewallen, W7EL

On Fri, 17 Feb 2006 17:35:47 -0800, Roy Lewallen
wrote:

There are quite a number of styles of N connector, and each is a bit
different. Cable stripping dimensions for a couple of the more common
ones are shown in the ARRL Handbook, and you might be able to find
instructions for your particular connector on the manufacturer's web
site. I'd say the most important thing is to make sure the center
conductor pin protrudes the correct distance, and make sure the cable
end of the center conductor pin is against the insulation. That is,
there shouldn't be any place where air is the only dielectric between
inner and outer conductors except along the large diameter portion of
the center pin.

My advice is based more on extensive experience with time domain
reflectometry than on a great deal of experience making precision cables
with N connectors. Hopefully someone with more actual connector
experience will comment further.

Roy Lewallen, W7EL

Roy makes some good points. However, for this application, I think
what is also important is that the cables be identical.

If you make the same mistake on all of them then they are still
"identical".

Lay them out and cut them all at the same time. To reinforce what Roy
said, one thing you absolutely -do not- want to happen is to have the
center pin protrude too far. If it bottoms in the mating connector on
your analyzer and damages it you will not be a happy camper.



Paul Burridge wrote:
Hi all,

I have to make up some highly accurate length RG-214 coax patch leads
for network analyzer interconnects up to 1.3Ghz. I have the cable and
the male N-type plugs to terminate them with.
So how best to trim this cable to maintain its 50 ohm impedance as
the plugs are joined to the cable? The plugs' center pins are
solder-type; the shielding Otoh is clamped in place. Are there any
fool-proof cutting dimensions to get the result right every time?
THanks,
Paul

On Sat, 18 Feb 2006 09:16:57 -0700, Wes Stewart
wrote:

Roy makes some good points.

He invariably does!

However, for this application, I think
what is also important is that the cables be identical.

If you make the same mistake on all of them then they are still
"identical".

Lay them out and cut them all at the same time. To reinforce what Roy
said, one thing you absolutely -do not- want to happen is to have the
center pin protrude too far. If it bottoms in the mating connector on
your analyzer and damages it you will not be a happy camper.

Okay, thanks, guys. I'm taking everything on board. If there's
anything else that comes to mind, I'd be keen to hear it.

Paul Burridge wrote:

Hi all,

I have to make up some highly accurate length RG-214 coax patch leads
for network analyzer interconnects up to 1.3Ghz. I have the cable and
the male N-type plugs to terminate them with.
So how best to trim this cable to maintain its 50 ohm impedance as
the plugs are joined to the cable? The plugs' center pins are
solder-type; the shielding Otoh is clamped in place. Are there any
fool-proof cutting dimensions to get the result right every time?
THanks,
Paul


See if you can find the datasheet for the connectors you have. they
will give you the information needed to assemble the connectors
properly.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida