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Two of four SMA-BNC pigtails purchased in July are already failed
One is exceedingly affected by proximity and orientation,
presumably weak shield attachment.
The other shows as shorted on Smith when 50 Ohm load attached,
but correctly showing open, perhaps damaged center insulation
near crimp ferrule.
Connectors are crimped, so will cut off BNC and dedicate to some jig.
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I was fortunate enough to get some high-quality PTFE (Teflon) coax (RG-174 sized) SMA to BNC jumpers - and these hold up much better, probably because they have a small piece of heat-shrinkable tubing at the interface between the connector and the cable which distributes stress over a larger area. "Heavy" use of these good cables does eventually fatigue the shield or center conductor and make them unreliable - but they are still quite rugged.
If I could, I would put more heat-shrinkable tubing on them, but this is difficult with the already-attached connectors. One thing that I have done on cables like this is to put a shaped blob of "Shoe Goop" or equivalent adhesive to form a flexible "boot" at the transition point: A bit uglier than heat-shrink tubing, but similar in effectiveness in de-stressing the cable at that point.
73,
Clint
KA7OEI
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On Sun, Nov 3, 2019 at 04:07 PM, Clint Turner wrote: If I could, I would put more heat-shrinkable tubing on them, but this is
difficult with the already-attached connectors.
I have used two alternatives: wrappable heat shrink and Silicone Rubber Self Fusing Tape. The tape come in different thicknesses and widths. A life-saver during some high-altitude balloon missions, because it goes on by itself in the field. Not as much luck with the wrappable stuff, but if you have some shrink tubing with a high shrink ratio, you can pre-wrap with other material, the slide a larger sleeve over. I have done this, and admit that it can look clunky, but function wins over form for me here. -- On the banks of the Piscataqua Rich NE1EE
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I was fortunate enough to get some high-quality PTFE (Teflon) coax (RG-174
sized) SMA to BNC jumpers - and these hold up much better, probably because
they have a small piece of heat-shrinkable tubing at the interface between the
connector and the cable which distributes stress over a larger area These have heat shrink (one layer) twice the length of crimp ferrules; 34cm is longer than typically needed, so splurged another ~ US$6.50 on four more with 15cm length. A source for durable pigtails would be good to know.
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I finally mounted my Nano onto a piece of aluminum. I have very little use for SMA connectors and the cal kit supplied. Now I am doing most everything with BNC but can also used N connectors. Hope the pictures I uploaded made it and can be viewed. I also added a strain relief for the USB cable.
The toughest part of adding the N connectors was to get a pair of short semi-rigid coax SMA to SMA jumpers close to the same length so I didn't over stress the SMA's and the PCB on the Nano.
I scavenged the aluminum, the N connectors and the sr coax from some old piece of HP test equipment. I found that careful use of an Exacto knife makes it possible to modify lengths of sr coax with SMA's.
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Very well. So is your calibration kit now type N, SOL and Thru or Type BNC?
Alan
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Hi Alan
Yes, If I was to use type N, which is not very likely, I have an HP 85032 Type N cal kit. For BNC I just made a short. My interests are HF so leaving the male BNC open is good enough and I use the 85032's load thru a BNC adapter. Thru a good Amphenol adapter the 50 ohm load is very good. There is no electrical length issue with the load. I am all set. I need no other software and just Nanovna Sharp. I down loaded a version of "saver" and it only seems to present more graphical representation. I take out the s1p file and that's it. From the s1p file I convert the mag and phase angle of the reflection coeff to any other parameter I need using a spreadsheet. I am pretty much only interested in Z and RL. I have used the nano with an HP 8721A directional coupler measuring S21 to measure RL. It works very nicely.
Pete
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On Fri, Nov 8, 2019 at 12:08 AM, WB2UAQ wrote: The toughest part of adding the N connectors was to get a pair of short
semi-rigid coax SMA to SMA jumpers close to the same length so I didn't over
stress the SMA's and the PCB on the Nano. I will shortly repackage my nano with N connectors. I have found a couple of bulkhead types in my junkbox. They are for 1/2" coax, but can be adapted for some 1/4" PTFE coax from the same source :) I want to use a diecast box (e,g, Hammond) to house the nano if possible. Although the shells of both SMA connectors on the nano are shown connected to ground on the schematic, they appear to be soldered to separate areas of ground plane on the component side of the board. I see from your pictures that the shells of both the N bulkhead connectors are connected to each other via the the aluminium chassis. Have you noticed any deleterious effects as a result of mounting them in this way? Other posters have isolated the shells of the bulkhead connectors from each other, either by mounting on an insulating panel, or by cutting separate lands on single sided pcb material. Has anyone got any information on which way to go with this before I start cutting metal?
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Hi Guyla
Yes, I read that. But it does not say *why* the grounds for the N connectors are kept separate.
Any ideas?
73
Nick
G3VNC
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If you float either or both N connectors from the shielded enclosure, you largely
defeat the purpose of the enclosure.
My advice? Bond both connectors firmly to the enclosure, and run coax cable
within the enclosure from each N connectors to its existing SMA connector on
the nano. Then test to see how it works out. If unhappy, try putting ferrite "beads"
on both coax lines.
One problem area I see relates to the USB cable. That, too, should be treated
in a similar manner, and a shielded USB cable should be used between the
enclosure and the external computer. And don't spare the ferrite beads on that,
either, both inside and outside the enclosure. The trick might lie in finding a
USB bulkhead thru connector that can be bonded to the enclosure and which
effectively bonds the shield of at least the external cable (but preferably both)
to the enclosure wall.
Dana K8YUM
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All network analyzers that I have worked on (repaired, calibrated and use nearly every work day) since 1977 (8409, 8505, 8753, 8510.......) have the test ports connected to and thru the same front panel (built in or on the S-parameter test set). I see absolutely no reason to isolate them on the nano. I have not used the isolation part of the cal yet but isn't that the reason for doing isolation?
I thought about mounting my unit in an enclosed box as well but I had this old HP radar test set with everything right there and ready to go plus I do every thing at HF and have not seen any leakage problems there.
K6JCA's set up looks nice but I again I see no reason to separate the connectors. It might even lead to cross talk when doing S21 creating a ground path loop.
Last night, after finally getting a second HF antenna lashed up before the winter snows, I checked the isolation between the two antennas using S21 for the first time (except for a quick functional check when I rec'd the nano in late August). On 40 meters there is only 20 dB of isolation and on 20 meters and 160 meters there is about 29 dB of isolation. For each band I retuned the transmatch on the one antenna that needs a transmatch. When I de-tuned the transmatch even by a small change, the coupling dramatically decreased showing the impedance match was critical for max radiation from one antenna into the other.
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On Mon, Nov 11, 2019 at 07:24 PM, WB2UAQ wrote: All network analyzers that I have worked on (repaired, calibrated and use
nearly every work day) since 1977 (8409, 8505, 8753, 8510.......) have the
test ports connected to and thru the same front panel (built in or on the
S-parameter test set). I see absolutely no reason to isolate them on the nano.
I have not used the isolation part of the cal yet but isn't that the reason
for doing isolation? Thank you Pete and Dana for your helpful replies. I connected the two supplied SMA cables to the nvna and calibrated from 10kHz to 1500MHz. With both ends terminated in 50 Ohms the nvna cannot measure any difference in isolation with the grounds of the load ends floating or connected. In the attached picture the isolated plot is green and the connected plot is red. Both are averaged over 100 scans. Worst case isolation is 45dB @ 1.5GHz limited by the nvna itself. See also post #6867.
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On Tue, Nov 12, 2019 at 02:35 PM, Nick wrote: In the attached picture the isolated plot is green and the connected plot is
red.
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Thanks for doing the test Nick! I guess we are all having a great time working with this little unit. I was considering buying an 8753 and retrofitting it with an after-marked LCD display but I think for what I am doing now the Nanovna is all I need. Just wish it could measure higher impedances more accurately but when that is needed I have used my old General Radio bridges.
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I am thinking that the point of putting the nano in an enclosure is to stabilize the
poor isolation, not necessarily to eliminate it. But when the nano's leak is stabilized,
the ISOL CAL should then enable good correction.
This assumes that what goes outside the enclosure is done with good quality
cables (at least with respect to leakage) that are terminated. It would probably
help to terminate the ends in items that roughly duplicate the impedances that
the DUT will provide, although this is admittedly not very practical in most
cases.
BTW, SMA(M) cables that are left dangling with nothing connected can exhibit
wildly variable leakage from the ends. The center pin is coupled capacitively
to the nut, whose contact to the main body of the connector can range from only
fair to non-existent, depending on how it is laying (i.e., on what forces are acting
on the nut and from what directions, etc). RF leakage of this kind leads to RF
currents on the outside of the cable, which can resonate at various frequencies.
radiate to nearby cables that are similarly indisposed, etc. So even if you can't
spare good terminations, at least screw each cable end onto something that is
well-shielded for the isolation CAL step.
Dana
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Hi I'm a newbie when it comes to the NANOVNA but over the years I have been involved with many different aspects of radio both amuter and professional.
SMA connectors are not intended for repeated connection and reconnection, there is little or no strain relief with SMA connectors, use good quality connectors and cable, use large radius no sharp bends in test environment.
When the integrity of a connection is of concern (SMA connectors on NANO) and the connectors have to be connected or reconnected many times, for example in testing and re testing/integration it is common to fit additional sacrificial connectors to those on the unit (I have). This is to prevent damaging the connector's and can be replaced and still preserving the integrity of the main connector.
--
Raymond G8KPS
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On Tue, Nov 12, 2019 at 09:37 AM, WB2UAQ wrote: " Just wish it could measure higher impedances more accurately " ----------------------------------------------------------------------------------------------------------- An HP8753 would not have helped you to measure higher impedances, I used one for years in my workplace and once you got too far off of 50 ohms the accuracy declined. In general, I believe that is true with all VNA's with a 50 ohm input impedance. The HP8753 also started at 300 kHz which meant my company had to purchase a second VNA for lower measurements. I'd love to own a HP 8753 but I also know I can't afford the maintenance costs that would eventually be required. - Herb
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Oristo
Rich NE1EE
Gyula Molnar
