Connectors for VHF antenna cabling

boggybrn

boggybrn

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www.messingaboutinboats.co.uk
I finally got the mast up on Saturday, but was somewhat surprised when I came to reconnect the cable for the VHF antenna.

Firstly the plug just fell off the cable coming out of the mast. There was no sign of it ever being soldered on... Secondly the socket that this is meant to plug into is a completely different type! The plug is a PL259 but the socket looks like an N Type.

Last season (my first with this boat) the radio seemed to work OK - at least I could hear the coast guard, and was able to talk to the local marina. However come to think of it there was a lot less chatter going on than with the radio on my prevous boat.

So... For this co-ax to co-ax joint what are my options?

The N type(?) connector also seems loose, so I'm inclined to replace that. Can I just use a pair of 50ohm BNC connectors with a female to female adaptor?

The connection is inside the boat as it is a keel stepped mast. So there is no need for it to be weatherproof.
 
You have a couple of options. You could use a BNC pair - male and female. You could use two PL259's and a PL258 barrel connector. Or you could use a TNC kit, TNC's are the threaded equivalent of the BNC bayonet connector. All will do the job. The PL259 option is robust but larger than the other two options. The BNC is more vulnerable being a bayonet connection.
 
Both suggestions sound like easy ways of doing it, but as I understand it for efficient transmission you need to be careful about matching the impedance of cable & connectors.

[Edit] Thanks SaltyJohn. I was referring to the previous two posts, I'll go with one of your suggestions.
 
Even holding them close to each other may give you a signal (leaky feader is a funny thing) but to get the best you need proper connections, any matching set of 50 ohm impedance connectors. Domestic TV are 75 ohm so will give you a loss. Also watch out BNC in particular are available in both 50 and 75 ohm as are some of the other connectors. My gut rection would be for a male to female BNC but you may prefer a screwed connector.
 
For the benefit of an ignoramus like me, why wouldn't connecting the core and shield using a block connector work, providing it could be kept dry? Or alternatively, soldering the cores together, binding in tape and then soldering the shield-obviously not easily disconectable. I believe that I read on here some time ago that this was regarded as good practice in military situations?
 
It will work, and will give acceptable results but won't be the best you can do. The main difference will probably be on Transmit, i.e. you won't be able to put the full power of the transmitter into the aerial. You may also get more noise and so miss out on low power received signals. Keeping the non-co-ax bit as short as possible will help. Crimping bootlace terminals onto the wire ends instead of poking them into the terminal block will help as well.
For "normal" yachtie vhf use, I doubt you'll notice the difference.
 
The braid, or ground-shield conductor, is grounded and completely surrounds the centre core so that the cable doesn't pick up electrical noise.
If you are going to join coax without connectors it's better to stick to this configuration. So, join the centre core and then insulate it and then join the braid and then insulate that.
For connections, rather than joints or splices, better to use proper 50ohm connectors as discussed above.
 
why wouldn't connecting the core and shield using a block connector work, providing it could be kept dry?
*************************************************

Shhhhhhhhhhhhhhhhhhhhh!!!!

You will now get 10,000 reasons how it will wreck your radio, soak up the signals, and not be PC... However it is what I have done for 5 years where the connection is in the heads.

I still get my signal to places others can t reach, the SWR is better than just acceptable and I dont even shield the block with foil. Why not try it and see ????

I find it more reliable than an expensive solution out on deck!
 
It is best to use good quality connectors such as BNC, which tend to have a rubber gasket inside, to seal the cable. Ingress of damp into the cable will allow the braid to corrode, which increases the loss and eventually causes failure.
Using non co-axial connections can increase the loss radically and in extreme cases can either damage the transmitter or cause it to reduce transmit power further.
Having more than one join is more than twice as bad, as you can get standing wave effects between the two discontinuities.
It may appear to work adequately, but let you down 15 miles offshore.
Bear in mind that even a perfect, brand new 15m coax will typically lose 30% of the power.
Whether you consider degrading your primary distress comms 'acceptable' is up to you.
 
[ QUOTE ]
For the benefit of an ignoramus like me, why wouldn't connecting the core and shield using a block connector work, providing it could be kept dry? Or alternatively, soldering the cores together, binding in tape and then soldering the shield-obviously not easily disconectable. I believe that I read on here some time ago that this was regarded as good practice in military situations?

[/ QUOTE ]

Because you'd then create an impedance mismatch which creates reflections.

Large enough reflections may damage the output PA of the transmitter, will definitely reduce the power transferred to the antenna and will reduce the power received at the receiver LNA.

You'd also create a reflection using 75 Ohm Belling-lee connectors in a 50ohm system. (plus Belling-lees are crap)
 
[ QUOTE ]
For the benefit of an ignoramus like me, why wouldn't connecting the core and shield using a block connector work, providing it could be kept dry? Or alternatively, soldering the cores together, binding in tape and then soldering the shield-obviously not easily disconectable. I believe that I read on here some time ago that this was regarded as good practice in military situations?

[/ QUOTE ]

At the moment I use a 'torpedo co-ax' connector. Looks like the metal barrel part and internal of a TV co-ax plug but is double ended and conatined inside two halves of the plastic torpedo - so you connect cable to each end with screws and threaded caps to ground the shielding. All covered in jolly good old black tape.

This will be replaced later by the new antena and plug combo that Saltyjohn supplied me some weeks ago. (antena with PL259 connector)

I had to find a way after clouting a harbour wall with mast head when laid along deck ! It ripped of the antena leaving bracket and cable only .. so my spare office antena was fitted and spliced to the remaining cable - worked fine.
 
The amount of actual loss in transmitted power or received microvolts if you use a incorrect impedance join is very small. The actual amount depends on frequency and so is very small at HF a bit more at VHF and a lot at UHF and microwave. As already stated the thiner RG 58 cable can lose 30% of the power on a typical tall mast long wiring run.
So yes a correct 50 ohm connector is best a 75ohm TV connector would be barely measurably worse and wires into a chocolate block connector perhaps measurably worse. Note in actual use you would never notice the difference. NB the op hardly noticed the difference with no connection.

What is significant is the possibility of moisture getting into the cable causing a short life. I would suggest the op get another N type connector to make a proper pair. Not that it makes much difference but to be able to feel he has done a proper job. olewill
 
Proper connectors are the only way to go, crimped or soldered properly and the over taped with self amalgamating tape if exposed. Using "choc" block is a definite no-no, it might work for a bit but eventually it WILL fail and then you have no antenna, also bear in mind that on some of the higher frequencies, GPS for instance, the signal passes down the dielectric (the plastic bit in the middle) not the copper, acts like a wave guide, the copper passes the power up to the antenna to drive the circuits up there. So a choc block is highly unlikely to work.

Also choc bloc is crap at keeping RF noise out and as what your wanting is a specific RF signal, you will lose what you want usually in having the squelch upt far too high.
 
In the overall expense of boating why do 'we' do so many things in a bodged way?

For the sake of approx £5 for proper connectors and a little time soldering why mess around with choc-blocks, etc and risk a failure? It's bound to fail when you need it most - Murphy's Law?

W.
 
[ QUOTE ]
In the overall expense of boating why do 'we' do so many things in a bodged way?

For the sake of approx £5 for proper connectors and a little time soldering why mess around with choc-blocks, etc and risk a failure? It's bound to fail when you need it most - Murphy's Law?

W.

[/ QUOTE ]

Agreed - but trouble is trying to solder outside in the wind etc. I have a good iron - but it doesn't like being cooled by those northerlies !

I prefer fittings that use screws personally - there are plenty of same type fittings in both solder and screw style without resorting to choc-blocks.
 
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