Twist on BNC connectors to join VHF aerial cable?

Richard10002

Richard10002

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I need to join two pieces of VHF aerial cable, and I'm crap at soldering. I've tried soldering the wire onto the middle of a normal connector, but i get it all over the place, and the connectors wont join.

So..... having seen the Twist On BNC connector on the Garmin aerial thread, can I use two of these, (male & female), to join VHF aerial cable..... or even just TV aerial connectors??

Many Thanks

Richard
 
Yes - I've done this when joining the VHF aerial just below the deck with the cable through a deck gland(rather than using a deck plug, which I found very unreliable)
 
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or even just TV aerial connectors??

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Absolutely not TV components; wrong frequency wrong impedance, just wrong /forums/images/graemlins/grin.gif
 
Appreciate what you're saying and I'm sure you are right but, assuming they fit the diameter of the existing VHF aerial, does the metal of the connector have frequency, impedance, and so on?

This is me not understanding something which is probably obvious /forums/images/graemlins/smile.gif
 
Go to your local Maplin Electronics Shop..
Must be one in Manchester...
They've usually got anything you 'll need and they try to be helpful too!!!
Cheers Bob E.....
 
[ QUOTE ]
Go to your local Maplin Electronics Shop..
Must be one in Manchester...
They've usually got anything you 'll need and they try to be helpful too!!!
Cheers Bob E.....

[/ QUOTE ]

Several in Manchester, but I'm in Malta right now /forums/images/graemlins/smile.gif, and I've got a selection of connectors in my electrical box, of which some might do the job.

It's a 10 minute job if I can use what I've got - which would be nice.
 
interesting chat, though a little light on theory

Quite a nice description of connector impedance

In reality Richard, you could probably get a signal through a chocolate box if you wanted to. If all you used the radio for was to call the marina up from the pontoon..

Each error along the line will reduce the signal and lift the noise floor, with obvious consequence.

I deliberately did not comment on the screw on bnc jobbies you mention as I have no experience with this particular type of connector.

p.s. It is rare for you to <u>transmit</u> with your television.
 
Like PetiteFleur when I replaced my VHF aerial, I couldn't be faffed to drop a new coax down and soldering a connector at the top of the mast isn't on! I used a twist on connector with lots of self amalgamating tape over the join. It's been up for 3 years now - all ok.
 
Impedance of a cable is not the "resistance" which you may be familiar with from Ohm's law, it is a value that depends on the geometry of the cable (diameters of the internal conductor, the screen braid and the dielectric constant of the insulator separating them) but is confusingly expressed as "Ohms per metre."

A co-ax cable behaves like a transmission line, and this impedance must be matched to the terminations and to the electronics at each end and all joints must be matched to prevent reflected signals. This does not matter much in reception as all that will happen is that you lose some signal strength. But when you bang 25 watts of transmitted signal up the line it is a different story. Any mis-match and a proportion of the signal comes belting back down and can fry your output transistor.
 
OK - I now understand and get the message /forums/images/graemlins/smile.gif

How can you tell the impedance and frequency range of a particular connector, or type of connector and, as a result of the limitations, do I have to use the connectors where you solder the centre wire to the connector - please tell me there are alternatives /forums/images/graemlins/smile.gif
 
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interesting chat, though a little light on theory


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"To continue Jeff... Frequency domain limits are calculated from time domain measurement using the 0.35/Trise translation, which approximates the half-bandwidth power level for a sinusoidal signal. This does not typically directly relate to the system clock frequency."

That is like listening to Sir Humphrey in Yes Prime Minister - amusing, but meaningless


/forums/images/graemlins/laugh.gif
 
I thought you might like that, I couldn't follow it either lol.

to your question about the bnc connectors you have, probably safe to assume they are 50ohm, you can get 75ohm but these are seriously rare items*. It should be written on the bag if you got them from maplin.

I did not know how to tell the two apart so I googled, the page gives this..

BNC connectors exist in 50 and 75 ohm versions. Originally all were 50 ohm and were used with cables of other impedances, the small mismatch being negligible at lower frequencies. The 75 ohm types can be recognized by the reduced or absent dielectric in the mating ends. The different versions are designed to mate with each other,[1] although the impedance mismatch will lead to signal reflections. Typically, they are specified for use at frequencies up to 4 and 2 GHz, respectively.

NOTE. The physical sizes of the 50 and 75 ohm sockets are slightly different and can damage the "wrong" plugs.

Unfortunately they show no images to help differentiate the two types.

These screw on types you have, how do you connect the pin? is it crimped?



* I assume you bought them in maplin or a chandlers, which will generally mean as far as I am aware, you will have bought 50ohm, the chances of picking a 75ohm off the rack at maplin is very small indeed. In fact, going by the wiki description, if they plug into the top of your h/h vhf without any force then they are 50ohm, simple uh..
 
Impedance of a cable is not described as ohms per meter. (That is for DC)
All transmision cables have losses more at higher frequencies. Characteristic impedance is the ratio of current to volts of the system (source and antenna (or end use) that provides minimum loss.

Imagine you are transmitting hydraulic power in a fluid. Any given pipe will either burst with too much pressure or restrict flow if it is too small. So there is a preferred volume to pressure ratio to transmit the most power without loss.

It is a bit the same with radio frequency power except more critical. The cable has capacitance inner to outer which can tend to allow the power to be lost and the conductors have inductance which tends to restrict the current flow. The best compromise of these two factors for minimum loss is seen as current and voltage. So can be described in ohms. And in fact to the high frequency voltage and current it works like ohms resistance.

The characteristic impedance is described as the impedance the cable appears to be looking into an infinitely long length. In fact the impedance(Z) appears in a shoprt piece of cable at high frequencies and needs longer cable to exhibit characteristic impedance at lower freq.

The impedance of a cable is determined by the size of the conductors and the spacing.

Connectors for coax cable are designed to appear to emulate the impedance of the cable. By convention TV coax cable is 75 ohms Z while for radio transmission 50 ohms is used. This means that connectors for TV will slightly greater spacing or thinner conductors.

So to use a TV connector in a 50 ohm cable will provide a small part of the total cable with greater Z than the cable. Thjis will be a greater concern the more connectors are fitted and the higher frequency that is transmitted.

VHF marine radio is really only in mid range of frequencies that might use coax but top of the range for the smaller RG58 over a 15 metre run.

If you are using the RG58 to a typical mast head antenna then the losses in the cable will be vastly greater than any from a 75 ohm (TV) connector.
Considering that over 20 metres you could lose half of your transmitted power from cable loss alone. Yet people still use it and it works OK.

The cable loss will tend to attenuate the reflected signal when transmitting but a discontinuity of incorrect impedance will cause a reflection.

The use of a 75ohm connector IMHO will not noticeable reduce power out to the antenna or performance at VHF especially as compared to losses associated with the smaller RG58 cable of which our OP is concerned.

Modern transmitter amplifier throttle back with bad VSWR or are simply immune to the higher voltages in a bad load situation.

Regards olewill
 
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These screw on types you have, how do you connect the pin? is it crimped?

* I assume you bought them in maplin

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Now that I look closely, I have only got 1 x F Type connector, and it is a Twist On F Plug CT125, (which I think is for TV aerial cable, and plugs into the Triax amplifier box /forums/images/graemlins/smile.gif

I also have a male & female BNC pair, (clamp on I believe), but I dont know if they are 50Ohm or 75Ohm - as per the photo:

IMG_0147a.jpg


If we cant tell, I'll get a pair of 50Ohm from Maplins when I get home, and do the job next visit. By the looks of things, they only do clamp on in both male and female:

http://www.maplin.co.uk/search.aspx?Menu...=y&doy=12m3

Cheers for now

Richard
 
What about contacting this comany for bits. Push fit 50 ohm connectors certainly do exist and there is a good chance they will stock them.


Otherwise, if the connection is in a dry and protected place you could try a solderless bodge, but I would do a soldered bodge, it is not so hard! How about this for a plan


cut off 3 or 4 inches of coax and put aside

cut back outer insulation on the ends to be joined and peel back the screen

remove 1/2 inch of insulation from the centre conductors, twist together and solder. It really is dead easy provided the wire is clean and sound and you have a fine tip iron.

Now dismember the bit you put aside and salvage some of the white insulation that covers the centre conductor, trim to the length to fill the gap, split it lengthways so it will slip over. Mop up the blood and put some gaffer tape over the gash in your thumb.

secure the inner insulator with the minimum of thin insulating tape - don't overdo this, it is the one stage where you are most significantly changing the electrical characteristics of the cable

pull the screen back over

overlay some of the screen from the bit you put aside - just pile it on with lots of overlap to the remaining screen.

wrap the whole thing in tape - as much as you like this time!


Sounds complex but is dead easy really. The only way it can go totally wrong is if you allow any of the screen to contact the centre connector.
 
What a breath of fresh air, and essentially reinforcing what i said earlier, one is trying to fabricate a "pipe" for the signal to flow through, between the outer and inner "conductor"
stu
 
I have used Glomex screw-on coax connectors and they work. They are very easy indeed to connect and with a bit of self-amalgamating tape make a good connection without the risk (if you are as crap at soldering as I am) of damaging the inner insulator.
 
Dumb question time - if so much is lost from cable, is it OK to shorten the cable? My AIS/emergency VHF ariel only needs to be 5 metres long.
 
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