I'm weighing in a little late on this one but wanted to make a few comments
regarding cables and connectors in hopes that a prior poster may not have
covered these thoughts.

With regards to single-shield coaxial cables, I think we are all aware that
shielding effectiveness can vary widely between cables of the same type.  I
have seen cable shielding as bad as 50% and as good approaching 90%.  Of
course, anything below 100% means that they are leaky - for either inward or
outward bound signals.  But one little cautionary indication as to the
quality of the cable and its shielding is stamped on the cable itself.  If
you see a description saying RG-xx "Type," you are basically dealing with a
cable of which the manufacturer is saying that it is like that specific type
of cable but does not necessarily meet the specifications one would expect.
So, if you want a cable to meet a certain quality, use a cable without the
"Type" designation.

In your posting, you mentioned the words "radio room."  That in itself is a
red flag meaning possible sources of strong RF.  In situations like that you
must make sure that any circuits requiring good isolation from other
interfering sources must contain adequate shielding be it circuit- or
cable-wise.  I think we all are aware of that consideration.  I have seen
instances of strong enough RF sources actually setting up circulating
currents in adjacent coaxial cable shields that couples the stray power into
the innocent cable and also affects the impedance characteristics of same.
This has also been seen in the larger solid copper transmission lines as
well as microwave waveguides.

With regards to connector selection, during my life in Colorado working with
the US government, I once attended a RF connector course at the (then) NBS.
It was quite an eye opener with regards to the connect-disconnect cycle life
of the typical coaxial cable connector.  If you want a reliable connection,
don't start with a worn connector.  And the choice of manufacturer of the
connector can be as important as well with regards to the construction and
plating materials used.  When I compare the typical easily-obtained "Jameco"
or "Marlin P Jones" type of connector to a better made product such as a
true MIL item on a network analyzer, one can see a noticeable difference
between the impedance and VSWR characteristics. It has been long known that
the simple nickel-plate connector can cause problems over time in an
installation from changing contact resistance and such.  The manufacturer
"RF Industries" still uses silver & gold plating on their types of RF
connectors as compared to the cheaper nickel-plated types and I have found
these to be very reliable with a cost slightly higher than the cheaper
nickel plated types.

Greg

Hi

Not quite sure how my name got into the thread title...

Coverage numbers aren't all they might be. A simple aluminum foil wrap lets
a vendor claim 100% coverage. The skin depth of the foil is near zero, so
it's not a big help shield wise at low frequencies. Same thing with labels,
just about any fly by night outfit can write nearly anything on a cable.

Skin depth at 1 MHz is about 0.1 mm (4 mils). At that point, shielding is
compromised. Depending on cable length / required isolation / optical
coverage you might want 5 or even 10 skin depths. It's a bit tough to find a
flexible lab cable with 1 mm (40 mil) shielding on it.

As you go down in frequency, at some point your only real solution is a
twisted pair. Before that point, multiple isolated shields can help (good
luck with connectors). At 1 Hz (1 pps ..) you would need 0.1 m of shield to
get to one skin depth. That's not a cable I have in stock...

Bob

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Gregory Muir
Sent: Tuesday, March 05, 2013 12:08 PM
To: time-nuts@febo.com
Subject: Re: [time-nuts] Don't use cheap cables -- a cautionary tale
(BobCamp)

I'm weighing in a little late on this one but wanted to make a few comments
regarding cables and connectors in hopes that a prior poster may not have
covered these thoughts.

With regards to single-shield coaxial cables, I think we are all aware that
shielding effectiveness can vary widely between cables of the same type.  I
have seen cable shielding as bad as 50% and as good approaching 90%.  Of
course, anything below 100% means that they are leaky - for either inward or

outward bound signals.  But one little cautionary indication as to the
quality of the cable and its shielding is stamped on the cable itself.  If
you see a description saying RG-xx "Type," you are basically dealing with a
cable of which the manufacturer is saying that it is like that specific type

of cable but does not necessarily meet the specifications one would expect.
So, if you want a cable to meet a certain quality, use a cable without the
"Type" designation.

In your posting, you mentioned the words "radio room."  That in itself is a
red flag meaning possible sources of strong RF.  In situations like that you

must make sure that any circuits requiring good isolation from other
interfering sources must contain adequate shielding be it circuit- or
cable-wise.  I think we all are aware of that consideration.  I have seen
instances of strong enough RF sources actually setting up circulating
currents in adjacent coaxial cable shields that couples the stray power into

the innocent cable and also affects the impedance characteristics of same.
This has also been seen in the larger solid copper transmission lines as
well as microwave waveguides.

With regards to connector selection, during my life in Colorado working with

the US government, I once attended a RF connector course at the (then) NBS.
It was quite an eye opener with regards to the connect-disconnect cycle life

of the typical coaxial cable connector.  If you want a reliable connection,
don't start with a worn connector.  And the choice of manufacturer of the
connector can be as important as well with regards to the construction and
plating materials used.  When I compare the typical easily-obtained "Jameco"

or "Marlin P Jones" type of connector to a better made product such as a
true MIL item on a network analyzer, one can see a noticeable difference
between the impedance and VSWR characteristics. It has been long known that
the simple nickel-plate connector can cause problems over time in an
installation from changing contact resistance and such.  The manufacturer
"RF Industries" still uses silver & gold plating on their types of RF
connectors as compared to the cheaper nickel-plated types and I have found
these to be very reliable with a cost slightly higher than the cheaper
nickel plated types.

Greg

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On 5 March 2013 17:08, Gregory Muir engineering@mt.net wrote:

I've recently been playing with some N adapters from various
manufacturers. I was particularly keen to find a decent male to male
adapter to include with a low-cost N calibration and verification kit
I am sellling.

http://www.vnacalibration.co.uk/sales/

If you read the specs on that, I state if the male to male adapter is
used, it limits performance. I've tried various makes, from the cheap
to fairly expensive (Huber and Suhner), although I am not considering
metrology grade devices, given the price I am selling the kit for.

I don't have all the data in front of me, but certain things stick in my mind.

1. The Huber and Suhner female-female N barrel was the most expensive
   of any I tried. The qualtiy of the threads was excellent. But when I
   stuck it on the VNA, the performance was really poor. I was intending
   to phone H+S to ask what was wrong, but before doing so I looked at
   the specification sheet. The spec was indeed poor, and the one I had
   met it.

2. Someone recently sent me a couple of RF Industries male to male N
   barrel adapters. RF performance was higher than I had seen on any
   connector of its type with the exception of metrology grade devices.
   However, it felt as though there was sand between the threads when
   tightening these up.

3. I found some pretty decent female-female Ns that were really cheap
   (< $5). RF performance was good, mechanically well made. They did not
   have the best RF performance of those I found, but they were cheap.

For a male-male adapter in that cal kit, I have settled on the Radiall
component for now, but I'd like to find a better one. The RF
Industries unit would do, as long as I supply a lubricant to make
screwing the things on better.

So to me, there does not appear to much correlation between cost and
performance. I've seen poor expensive items and fairly decent very
cheap items.

Dave

At what frequency do these "problems" appear?  And how bad it bad?

-----Original Message-----
From: David Kirkby david.kirkby@onetel.net
Sender: time-nuts-bounces@febo.com
Date: Tue, 5 Mar 2013 18:16:54
To: Discussion of precise time and frequency measurementtime-nuts@febo.com
Reply-To: Discussion of precise time and frequency measurement
time-nuts@febo.com
Subject: Re: [time-nuts] Don't use cheap cables -- a cautionary tale (Bob
Camp)

On 5 March 2013 17:08, Gregory Muir engineering@mt.net wrote:

I've recently been playing with some N adapters from various
manufacturers. I was particularly keen to find a decent male to male
adapter to include with a low-cost N calibration and verification kit
I am sellling.

http://www.vnacalibration.co.uk/sales/

If you read the specs on that, I state if the male to male adapter is
used, it limits performance. I've tried various makes, from the cheap
to fairly expensive (Huber and Suhner), although I am not considering
metrology grade devices, given the price I am selling the kit for.

I don't have all the data in front of me, but certain things stick in my mind.

1. The Huber and Suhner female-female N barrel was the most expensive
   of any I tried. The qualtiy of the threads was excellent. But when I
   stuck it on the VNA, the performance was really poor. I was intending
   to phone H+S to ask what was wrong, but before doing so I looked at
   the specification sheet. The spec was indeed poor, and the one I had
   met it.

2. Someone recently sent me a couple of RF Industries male to male N
   barrel adapters. RF performance was higher than I had seen on any
   connector of its type with the exception of metrology grade devices.
   However, it felt as though there was sand between the threads when
   tightening these up.

3. I found some pretty decent female-female Ns that were really cheap
   (< $5). RF performance was good, mechanically well made. They did not
   have the best RF performance of those I found, but they were cheap.

For a male-male adapter in that cal kit, I have settled on the Radiall
component for now, but I'd like to find a better one. The RF
Industries unit would do, as long as I supply a lubricant to make
screwing the things on better.

So to me, there does not appear to much correlation between cost and
performance. I've seen poor expensive items and fairly decent very
cheap items.

Dave

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

On 6 March 2013 11:33,  lists@lazygranch.com wrote:

My interest for VNA calibration and verification kits is probably well
outside most time-nut uses. I normally test N connectors from 50 MHz
to 6 GHz. 50 MHz is the lower limit of my HP 8720D (20 GHz) VNA and 6
GHz is the upper limited of my N calibration kit (HP 85032B).

To give a couple of figures.

1. Huber and Suhner female-female barrel, which is the most expensive
   I tried (15.26 GBP), has a worst case return loss of just 17.2 dB.
2. TE Connectivity the cheapest I tried at just  £2.84 GBP, had a
   return loss of 35.1 dB.
3. Telegartner, mid-priced at 6.68 GBP had a return loss of 38.9 dB.

So a mid-priced one was better than both the expensive and cheapest
choice. All are based on a saaple of two, and worst case results
quoted above. I did not find much variation between two of the same
manufacturer.

Although I don't have exact data in front of me, as I'd need to go
through the individual data files which I can't be bothered to do, all
were very good at low frequencies (say 500 MHz or less).

The worst case is not always at the highest frequency, although the
performance at 6 GHz is always worst than at 50 MHz.

All data was colllected by use of

• HP 8720D VNA within its calibration period.
• Agilent 3.5 mm test port cables on the VNA (over $5000 a pair)
• Metrology grade 3.5 mm to N adapters
• HP 85032B N calibration kit.

Since the N calibration kit was after the metrology grade 3.5 mm -> N
adapters, any issues with those adapters would be removed by the VNA
calibration routine.

I've seen some Americans make comments like "if it does not have
Ampenol printed on it, then avoid it". I'm not German, but the
mid-priced German made Telegartner F-F adapter was the best. See
pictures at the bottom of

http://www.vnacalibration.co.uk/support/NCV032B/HP/

For qualtiy of the threads, the Chinese made Huber and Suhner device
was the best, but it was by far the most expensive.

The 85032B comes with 4 APC-7 to N metrology grade adapters. They can
be assembled to make any N adapter you want (male-female, male-male,
female-female). These always provide better peformance than any
commerical grade connectors I mention above, but such devices are far
too expensive for the kit I use, which I sell at less than the price
of the adapters.

I would never use an adapter for a critical high frequency application
without testing it first.

BTW, I used to work in a Ministry of Defence calibration lab in the
UK. They always removed the rubber washers in N connectors. I notice
the metrology grade adapters don't have these, and Joel Dunsmore,
Agilent top VNA guru, reccomeneds in his book on VNAs to do the same.

Dave