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Failed BNC opened

Hi,

As mentioned in another thread, a BNC connector failed me on a recent activation. I had attached it myself, and handled it carefully when in use. It showed no sign of damage, so I was curious to know what went wrong. Sorry, nothing dramatic, but I thought I’d share, and would welcome comments or advice.

Here it is, before I opened it up, showing two layers of heat shrink sleeving to give mechanical support. I have experienced tearing of the braid close to the crimp after much use, but not in this case:

Sleeving removed, and crimp sleeve filed through:

Actually, it was the centre conductor that had broken, right against the crimp:

The cable is RG58A/U, which has a stranded centre conductor.

So, what was the cause? What can I do to reduce the chances of this happening? The connectors are of good quality, designed for RG58A/U and the crimp tool has the correct dies. It almost has to be something I’ve done, or not done!

Suggestions welcome!

TIA
Adrian
G4AZS

Just a thought, you might have pinched the inner strands when the inner dielectric was stripped away ?
thus creating a weak spot. It is unusual, the only other form of disturbance I can think of is counter rotation of the BNC inner pin against the coax itself causing it to shear.

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Shame the nice BNC connector is it is not as resilient as the often derided but tougher for SOTA PL-259. The braid on my friend David’s (CT1DRB) BNC went O/C today. David bodged it and stayed QRV.
One reason why I favour the rear entry antenna position on the good old FT-817.

73 Phil

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I think Jonathan is on the right lines with pinch or rotation damage.

In the past I have commented about the unreliability of BNC connectors when doing ultrasonic surveys on site. Some people denied that there was a problem, but the above is exactly the sort of thing that I have experienced repeatedly with commercial leads intended for the job. I am with Phil on this, use the good old PL-259 and the rear socket.

Brian

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There is a multitude of different connectors available for RF use. I use all BNCs on everything now for SOTA apart from on my larger PA. PL259’s are just a tad on the heavy side for some people I guess. If they did a BNC with a cable screw in option like certain PL259 plugs I would be very interested.

Jonathan

Hi Jonathan
Yes, they can be reliable but don’t believe they are designed for the sort of use we put them through and other uses - as Brian G8ADD has found.

The good old PL-259 “UHF Plug” - very sceptical about that term…

73 Phil

PS ,Weight not a problem when just carrying one plug!

Selection bias Brian. :wink:

True, but if its at the top of the fishing pole it is, I saw an article in SPRAT this month where the “designer” had used a 259 on the balun box at the top. I learn’t that lesson a few poles ago !

I have seen a few home-brewers use RCA phono plugs before with good results up to 30MHz. Simple plug and socket combination.

Jonathan

Well, there are such things:
http://cpc.farnell.com/1/3/twist-on-bnc-connectors

I have used them in a former life with IBM network controllers. They only work with solid centre conductors, as the centre also “screws on”. Very quick to fit, but I don’t like solid centre coax where it is frequently moved around.

One difference I can think of between BNC and PL259 is that in the latter the centre conductor is soldered at the tip of the plug, which means that there is some protection afforded by the “tube” it passes through, and a greater length before the cable grip, which provides some flexibility. Whereas in a BNC, the connection is more rigid right close to the cable grip, which gives no protection against differential movement of inner and outer. If that makes sense :smile:

Anyway, thank you all for the comments, I’ll ponder…

Adrian
G4AZS

Maybe, but I used a pulsed ultrasonic tester on concrete in the lab and on site for over thirty years and the BNC patch leads between the “PUNDIT” and the leads failed on a regular basis. I think they are OK in the shack where they will be left alone for long periods but they are not suitable for use in the field where the leads will be repeatedly coiled and uncoiled and the plugs repeatedly installed on and taken off the sockets - and I am talking about commercial leads with thick rubber protection over the plug and end of the cable, not flimsier home-brews with a bit of tape or thin heat shrink to protect the join. They are just not man enough for the job.

Brian

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Thanks Adrian, I have seen them before but not on a cable :confused:

I’ll give it ago sometime. RG174* is the medium here though.

Jonathan

It’s good that you found and solved the problem Adrian.

I also subscribe Jonathan’s counter rotation of the inner pin against the coax itself theory.

This type of BNC connectors I recently bought in Pamplona might be easier to connect, diagnose in case of fault and repair than the crimped ones you used.

However, I use a total of 5 PL-259 in my SOTA kit:
the one connected to the back of my FT-817,
the one connected to the transmitter SO-239 connector of my MFJ Antenna Tuner,
the one connected to the antenna SO.239 connector of my MFJ AT
These 3 are never moved nor disconnected as they are inside the wooden box containing the preconnected rigs.
See:

One short length of RG58 comes out of the box with another PL-259 permanently connected to a “barrel” SO-239+SO.239 adaptor and covered with insulating tape. You can see it on the top left picture of my 23/05/2015 post.

All I have to do when activating SOTA is connecting one single PL-259 from the antenna I’m using that day to the barrel adaptor.

The only PL-259 I use to connect and disconnect to the barrel has needed repair in both cases, the one I use with the mobile whip antennaes (on a RG213 cable) and the other one I use with the 7m long GP (on a RG58 cable).

Best 73 de Guru

I’ve 400+ activations using BNC with one cable failure (intermittent break RG174) and one BNC failure after tripping on the cable. If you were having regular failures on BNC connectors and I’m not then I’d be looking for what else was common to your setup when you were having failures.

2 Likes

Yes, That’s all I use on my portable equipment. Simple, reliable and light.
All my other gear is BNC. The only 259 socket in my entire collection is the one on the back of my FT7. That has a 259-BNC adaptor.

I’d say the issue with Adrian’s failure was a nicked inner core whilst stripping. I’ve seen it hundreds of times in installations that I’ve had to commission.

72

1 Like

I’m with Jonathan on this one, SO239/PL259 are just a tad bulky.

My FT817 is fine, I’ve never had a problem with the connectors but I have repaired a friend’s FT817 where the central pin on the SO239 had been broken off the PCB. An easy fix but there’s very little mechanical support of the centre conductor. I’m always super careful when plugging cables into the back of my FT817! I think that a bit of rough handling or some sloppy soldering on the PL259 pin could soon cause connector failure.

I’ve had one BNC fail, the coax (RG-174) had been rotating about the middle pin causing an open circuit. The BNC was replaced FOC by the manufacturer. On the whole I think BNCs are fine for SOTA, it’s certainly the connector that I have standardised on.

Two of my MTRs use RCA/Phono connectors, I know it’s common for people to replace these with BNC but I like to keep things as the designer intended. I believe that quite a bit of thought went in to using RCAs on the ATS series rigs. For one thing, if you’re out hiking for a number of days and need to pick up a new antenna connector for whatever reason, you’re more likely to source a phono (RCA) plug than an RF connector. Also an RCA jack weighs quite a bit less than a BNC so reduces the rig weight and will have less leverage on the case (a bigger issue if using Altoids tin). Some people reckon that RCAs disconnect too easily but I have not had a problem. The losses / mismatch etc is also really not an issue - I managed some >3000miles QSOs on Sunday through RCAs adapted to BNC on RG-174 with about 2w out from the rig. My usual MTR antenna has an RCA plug straight on to the RG-174. I like the price of RCA connectors :smiley:

I was also laughing to myself reading that piece in SPRAT, that device looks so heavy!

  • Colin

Well… Mountain Topper Radio uses RCA and that works fine. :slight_smile:
Also Heathkit QRP’s rigs used to use it in the 80 and 90’s (HW-7, 8 and 9).

I use RG-174 plus Amphenol BNC’s and it seems to work FB. The only issue that I ever had is a broken center conductor (coax cable).

This one uses a sleeve and a PVC sleeve from Sotabeams

I don’t use PL-259 any more and my SOTA kit is a all-BNC one. The only exception is the KX-3’s 2m transverter which uses a SMA…

73 Pedro

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I’m 100% BNC and N type on all my gear!

N at home for powerful stuff, BNC (Crimp or Pressure Sleeve) for SOTA and lightweight things at home.

I do not use the front BNC on the 817 as I wish to preserve it, so instead stick a high quality adaptor on the back of the rig to get from PL259/SO239 to BNC Female.

We have had these debates before, everyone has a different view on this. I suppose it all boils down to what you’re happy with. If i could be bothered, I would change all my BNC to TNC, however that is alot of patch cables that need modding.

Adrian, I am glad you cracked it, last time I had issues, I thought it was the connector, I opened up the plug and found the plug was fine, it was the water and blackened copper that had caused me the issues!!

73

Matt G8XYJ

2 Likes

Heathkit’s HF 100 watt radios also used to come with RCA sockets as standard. They were used for interconnections on Collins equipment too.

Although they are considered an audio connector, RCA sockets were apparently originally intended as microwave connectors and if the right series of plugs and sockets are used for the cable, their impedance characteristics are good. The modern type using a kind of mud as the insulator would not qualify though. The ones on radios mentioned above had ceramic insulation.

I use BNC for everything - I would replace the SO239 on the back of my rigs with a BNC if it was easy. As it is, there is a BNC adaptor almost permanently installed. It is only removed when I use a tuner for HF or an amplifier for a vhf band.

@G4AZS
Perhaps you have scratched the wires of the center braid when removing the dielectric as mentioned by G2HFR. Such scratches can cause the wires to break later. (Same mechanism as cutting glass: Create defects inducing a break.)
I use a snap-off blade utility knive to carefully cut the dielectric without hurting the center braid.

I have a BNC male - BNC female right angle adapter permantly attached to the front BNC socket. This not only protects the front socket of the FT-817 but also the coax is less disturbing when using the set dial below the socket.

73 de Michael, DB7MM