Extending VHF aerial cable

[narooma]

I need to move a vhf aerial and need to extend the cable.
Is soldering recomended? If not, there appear to be various options on connectors. Which is the best?

 

[Piers]

The best is to renew the whole cable to the required length. Connections reduce the gain by approx 3dB per connection. Others on this forum have advised not to solder.

 

[wooslehunter]

Best option as said is to renew the whole cable. However, if this is not an option, use a proper connector such as a BNC type. Even better is a TNC which is just a threaded version. Make sure you get a 50ohm connector though that's intended for a VHF radio. There are lots of 75ohm versions around & it's alomost impossible to tell the difference. It's a good idea to wrap with self amagamating tape to make a better moisture seal.

If a connector is losing 3dB at 156MHz it probably the "bit of wet string wrapped around another bit of wet string" type. 3dB equates to half the power so if you are transmitting 25W, then 12.5W is dissipated in each connector. Remember there's one at the back of the radio. A decent BNC will lose around 0.2dB at 3GHz. 0.2dB equates to 5%. At 156MHz, the figure will be far better.

 

[Salty John]

Yes, get a TNC pair. Loss will be almost undetectable (under 0.2dB) and the joint will be secure.
An N type is another alternative. You can also use a pair of PL-259's and a barrel connector (PL258). This is a big bulky connection, however.

 

[philip_stevens]

If you really have to connect an extension to your VHF aerial cable, you could do as I have done (with no noticeable reduction of signal) and connect with a Shakespeare connector.

You can get them internet from Westmarine - item C

 

[john_morris_uk]

[ QUOTE ]
The best is to renew the whole cable to the required length. Connections reduce the gain by approx 3dB per connection. Others on this forum have advised not to solder.

[/ QUOTE ]I think your decimal place is in the wrong place. As someone else has pointed out, 3db is 50% of the power and that isn't the correct loss figure for a BNC at 156 Mhz!!

I have no idea why people would suggest no soldering the aerial co-axial cable. In fact I will suggest that it SHOULD be soldered.

TNC or BNC connectors will be more than adequate for the job. If you do a good job, the losses will be undetectable as far as you are concerned. (The difference between TNC and BNC is that TNC is (T)hreaded and BNC is (B)ayonet.) You do need to make sure that you fit them correctly. A steady hand and meticulous attention to detail helps. You need to strip the coax in exacly the right way, splay the braid out evenly and neatly, make sure that the compression washers are in the right place and solder the inner pin carefully and then file off the excess solder from the inner pin with a needle file to do the job properly.

I am sure that someone will provide a link to the diagram.

Silver plated BNC/TNC connectors are best.

 

[narooma]

Impressive replys, thanks to all. Would have done the whole replacement thing but I seem to remember the cable came attached to the aerial which means opening up and (presumably) resoldering?

 

[MM5AHO]

I've made soldered connections in RG58, RG213, RG214, Mini8 and other coax cables, and the measured loss is not different than a connector joint using BNC, or N types. (Don't even bother with PL259s, there do not have uniform impedance and varying frequencies, and can be lossy).
Strip back the outer cover, exposing the braid, and fold that backward. Strip a little of the inner conductor of each end, and make a twisted joint, then solder it. Now carefully wrap this exposed joint in insulation tape (PVC), not overlapping the inner insulator, just the copper. Build this up to the thickness of the inner insulator.
Now wrap the braids over each side such that they overlap (you should have cut this long enough), and solder quickly with not too much heat as to melt the inner. Doesn't have to be fully round the circumference, three spots at 120 degress around will do.
Now wrap the braid with self amalgamating tape, overlapping the outer cover each end of the joint.
Aim to keep the spacing between the inner conductor and the braid even across the joint. Its this spacing (and to some extent the type of dialectric) that gives the coax its impedance, and you don't want this to change across the joint.

Works for me.

 

[john_morris_uk]

[ QUOTE ]
I've made soldered connections in RG58, RG213, RG214, Mini8 and other coax cables, and the measured loss is not different than a connector joint using BNC, or N types. (Don't even bother with PL259s, there do not have uniform impedance and varying frequencies, and can be lossy).
Strip back the outer cover, exposing the braid, and fold that backward. Strip a little of the inner conductor of each end, and make a twisted joint, then solder it. Now carefully wrap this exposed joint in insulation tape (PVC), not overlapping the inner insulator, just the copper. Build this up to the thickness of the inner insulator.
Now wrap the braids over each side such that they overlap (you should have cut this long enough), and solder quickly with not too much heat as to melt the inner. Doesn't have to be fully round the circumference, three spots at 120 degress around will do.
Now wrap the braid with self amalgamating tape, overlapping the outer cover each end of the joint.
Aim to keep the spacing between the inner conductor and the braid even across the joint. Its this spacing (and to some extent the type of dialectric) that gives the coax its impedance, and you don't want this to change across the joint.

Works for me.

[/ QUOTE ]I agree that this could work very well. I have joined co-ax this way as well and some copper foil to supplement the braid can help if you can get hold of any.

If you do it neatly, the impedence jump will be minute and the loss will not be noticeable.

The impedence of the cable is a function of the diameter of the inner, the diameter of the outer 'cylinder of braid' and the dielectric constant of what is in between.

The very lowest loss cables have solid copper outer, a solid copper inner and air with 'spacers' to keep the inner in the middle. The whole thing is then pressurised with dry nitrogen. (Pressurised so you can monitor for leaks - because leaks mean possible water igress and loss...)

This latter note is trivia and entirely irrelevant to the average yachtsmen so I am sorry if I have bored you...

Of course a whole new decent bit of co-ax cable would be the Rolls Royce solution, but in reality you will be fine joining in any of the agreed ways above.

 

[dsw]

I HAVE SOLDERED THE JOIN IN MY COAX AND USED A QAULITY HEAT SHRINK THEN A PIECE OF THICK FOIL ON THE OUTER COVERD AGAIN WITH HEAT SHRINK AND ITS BEEN FINE FOR TWO YEARS PLUS.

 

[john_morris_uk]

[ QUOTE ]
I HAVE SOLDERED THE JOIN IN MY COAX AND USED A QAULITY HEAT SHRINK THEN A PIECE OF THICK FOIL ON THE OUTER COVERD AGAIN WITH HEAT SHRINK AND ITS BEEN FINE FOR TWO YEARS PLUS.

[/ QUOTE ]Didn't quite catch that - can you say it ANY LOUDER??

 

[hightech]

Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer (jacket). It is used as a high-frequency transmission line to carry a high-frequency or broadband signal. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.

i.e. the signal goes down the plastic bit in the middle as a wave and each connector creates a VSWR loss as the signal traverses across. Connectors matched both for impedance and size are important to minimise such loss. Threaded connectors are top of the list for minimizing loss.

So use good quality cable and good quality connectors, choc block and taping of wires together is fine for DC, NOT for HF and above signals.

 

[ex-Gladys]

Exactly... Better to think of Coax as a bendy wave guide than as a cable

 

[john_morris_uk]

[ QUOTE ]
Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer (jacket). It is used as a high-frequency transmission line to carry a high-frequency or broadband signal. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.

i.e. the signal goes down the plastic bit in the middle as a wave and each connector creates a VSWR loss as the signal traverses across. Connectors matched both for impedance and size are important to minimise such loss. Threaded connectors are top of the list for minimizing loss.

So use good quality cable and good quality connectors, choc block and taping of wires together is fine for DC, NOT for HF and above signals.

[/ QUOTE ]

What you say is true, but if you read my post, you will see that what I was suggesting is that you can get away with joining the co-ax by soldering the inner connector and then moulding tape arround to insulate and then immitating the outer with foil and soldering neatly and carefully.

At 156 Mhz if you do the job well, it will be ok. You will also note that is suggested TNC or BNC inline as a better solution.

I used to work in Microwave Noise measurement and so I slightly resent the patronising nature of your post. If you want to discuss tranmission line theory I am happy to oblige, but we can also talk about realistic losses in coax and joins at 156 Mhz as well!

 

[john_morris_uk]

And where did I suggest chocolate block connectors and tape?

Only slightly miffed - please read what I say and not what you think I said?

John

 

[MM5AHO]

This thread could get really distracted and off the point...
picking holes in every technical detail ...

(its not a cylindrical sheath, its a tubular one.... HF OR broadband - could it not be both HF and broadband?.... connectors do create loss, but with some connectors its as insignificant as the loss in the cable.... threaded connectors .. the physical means of attaching one to another doesn't really determine efficiency...)

But the original question is a practical one - "Can I get away with joining coax?"

Answer is "Yes, though this isn't the way a purist would want to do it." /forums/images/graemlins/smile.gif

 

[john_morris_uk]

I agree!