Twist on BNC connectors to join VHF aerial cable?

now i'm slightly confused... One person says standard tv co-ax is 75 ohms and another says 75 ohms is rare to find. which is it?

Also, if you've got a cable that is of the wrong impedance and a pair of connectors half way along it. Is it better that the connectors are matched to the cable or to the radio?
 
When I said rare to find I was not talking about 75ohm equipment, I was talking specifically about 75ohm BNC connectors as almost all solutions requiring BNC will be 50ohm. This is not based on fact, but personal experience of buying BNC connectors, I have never seen a 75ohm one on the hanger in maplin nor ever been asked in an electronic outlet if that is the type I require, I am always given 50ohm.

75ohm is generally TV stuff, and TV has built up it's own assortment of connectors over time and sticks to them, as far as I have ever seen, thus 75ohm BNC in my opinion is only used for specialist jobs, where a standard TV plug just wont hack it.

I will be happy to be corrected, like I said, this is based on my own observation of never seeing a BNC used in a 75ohm application.

Sorry to confuse matters.


Backward EDIT, (to stop thread drift)>>> [ QUOTE ]

75ohm BNC connectors are relatively common in professional video equipment (among otheer places). 75 ohm plugs have smaller pins, so will generally go in 50 sockets without damage, but not generally v/v.

[/ QUOTE ] That makes sense; and clarifies why <u>I</u> have never seen one in use.
 
75ohm BNC connectors are relatively common in professional video equipment (among otheer places). 75 ohm plugs have smaller pins, so will generally go in 50 sockets without damage, but not generally v/v.
www.rswww.com will sell you what you need, they do all sorts.
They might even send to Malta or wherever.
An advantage of the clamp type bnc's is that they are pretty watertight if done right, and water ingress, leading to corrosion is the major cause of excess los in coax.
The key to good soldering is a small hot iron, most of the 12V ones are pretty poor, also try to avoid lead free solder!
 
If you use 75ohm cable you should use 75ohm connectors. However, you can't use 75 ohm cable for vhf transmission, you need 50 ohm cable and connectors.
Personally I wouldn't use BNC connectors to connect cables together - they are bayonet connectors developed for quick release and reconnection as in laboratory test equipment. There is a threaded version, TNC, which might be better but the best is two F male connectors and a connecting double female F connector. In rugged applications I think two PL259's and a PL258 barrel connector is unbeatable, but it's a chunky connection and difficult to hide under headliners.
 
TNC are used in the aero industry. Really play havoc with my thumb muscles when I had to undo / do up racks of 60 of them, used to bring tears to my eyes, literally. Good stuff though.

In my personal experience on a boat, I have never seen a bnc come loose.

And not to flame you in any way, not my intention, I would question availability if you needed a replacement shoreside in a hurry.
 
TV coax is 75 ohms same as ethernet and vhf transmitter is 50 ohms.

Both easy to find in the right places try Maplins if no other source.
 
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www.rswww.com will sell you what you need, they do all sorts.
They might even send to Malta or wherever.

[/ QUOTE ]

This guy, (near the main concentration of chandlers in Gzira/Sliema),

http://www.fabian.com.mt/

seems to have the market sorted in Malta - he has loads of little "things" attached to boards on a wall in his little sales room, and seems to be a font of knowledge.

However.... when I told him what I wanted to do, he frowned at my not wanting to solder anything.... "even with TV cable, you should solder the conductor". He didnt have twist-on BNC connectors for that reason, (so he said), so I bought a fuse box and some fuses.
 
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He didnt have twist-on BNC connectors....., so I bought a fuse box and some fuses

[/ QUOTE ] That's an interesting solution, did it work /forums/images/graemlins/grin.gif
 
In my experience the guys at Fabian know what they are talking about. They supply to electronic professionals not just boaties/marine work BUT the reality is 98% of BNC connectors (50 or 75ohm) installed today are crimped on. The secret is careful cable preparation and a correctly matching crimping tool. Quick and easy when the tools are at hand. Soldering is fiddly and prone to cable and connector damage if not done correctly.

Once you have satisfied yourself about the question of connector and cable impedance matches I think the main issue is protection from moisture. If the join is internal and unlikely to get wet (specially with a good covering of self amalgamating tape) then screw/twist connectors will work a treat.

The fact is connectors are always the weak link in the chain whether they be for joins - avoided if possible - or the unavoidable connection to the radio or antenna itself. How many radio problems have been resolved by simply unscrewing the antenna connector, cleaning it and the cable threads and re-connecting....uncountable.

My working background is in TV studios where 75 ohm BNC connectors have been the norm for 40yrs. In analogue TV times the signal bandwidth was about 10MHz and mixing 50ohm and 75ohm made no discernable difference to quality but the different central pin size did promote poor connection after repeated use. When digital TV was introduced the bandwidth of the signal went to 270MHz (well 270Mb really) and all of a sudden those old cables with mismatched connectors were useless even though to the eye they were perfectly good cables. Now HD TV needs 1.2Gb and soon 3Gb so we are dealing with microwaves where a rogue small length of the braid of a co-ax is like a tuned antenna.

The moral of the story is that anything with a higher frequency than DC is BLACK MAGIC.
 
Richard, follow Whipper_Snapper's suggestion here. You are buying an SWR meter anyway to check it? Forget adding connectors, you REALLY don't want connectors in your aerial lead. With care (practice on a few gash bits first or find a friend on another yacht who has experience) you will find that the sort of join like this is reliable and low-loss (low-reflection). I have made hundreds of connections from VHF to X-Band, from coax or various types, via stripline to waveguides (I have even cast waveguides myself in a foundry) so I am speaking from experience.
 
OK Makes sense and sounds pretty straightforward - my problem with soldering the usual VHF connectors, (259 or something), is that I get the solder on the prong, and it doesnt fit the other connector. One blob of solder, and some tape, seems a lot easier.

Not too sure about how to use the SWR meter - where/what does the pigtail plug into? Given that I have 2 cables: one from the aerial, and one from the radio, what am I testing, and how?
 
[ QUOTE ]
OK Makes sense and sounds pretty straightforward - my problem with soldering the usual VHF connectors, (259 or something), is that I get the solder on the prong, and it doesnt fit the other connector. One blob of solder, and some tape, seems a lot easier.

[/ QUOTE ]
I don't like the sound of 'blobs' of solder. To make an acceptable solder joint first select an iron of the right size. The bit (tip) should be a fair bit bigger than the workpiece or it will cool too quickly but not so big that it is awkward to use. Clean the iron and 'tin' the bit. If the bit has eroded you might have to file quite a bit off -- copper dissolves (slowly) into solder which is why copper bits become deeply eroded. Some bits are iron-clad; you shouldn't file those or they go back to copper and start to erode.

The solder that goes onto the work comes from the solder wire, not from the iron. The iron is used just to heat the work, not to transfer solder. The only reason for putting solder on the bit (tinning) is to wet the bit so it transfers heat nicely -- it is NEVER used to carry solder to the work. Vital.

[ QUOTE ]
Not too sure about how to use the SWR meter - where/what does the pigtail plug into? Given that I have 2 cables: one from the aerial, and one from the radio, what am I testing, and how?

[/ QUOTE ]

Have a read of this.......

http://highfields-arc.6te.net/beginner/swr.htm

What you are testing is the 'perfection' of the components and wiring downstream of your radio transmitter. In a perfect system all of the transmitter power goes down the cable and is launched into the air by the antenna. No system is perfect and at every discontinuity some power is reflected back. The reflected power sets up a 'standing wave', much like the pipe in a cathedral organ, which we can measure using a 'Standing Wave Meter' = SWR meter. Sometimes called a Voltage Standing Wave Meter because actually there are two standing waves - current and voltage - but we usually only measure the voltage. For your purposes, VSWR = SWR. What we are trying to do is to get the SWR downstream of the transmitter to better than 2.0:1 Try for between 1.5:1 and 1.8:1 To do this, take the SWR meter and put it close to the Tx, using a patch lead (supplied, or you must order one) connect the SWR meter to the Tx output. Now connect your aerial downlead (the lead that normally plugs into the Tx) into the other side of the SWR meter -- the meter should be clearly marked for you. Now follow the instructions supplied with the meter and the article above. If your SWR is too high you don't know where the problem is yet and that gets more tricky. Aerials themselves do fail and you cannot usually test them with a dc meter. I am ordering a dummy load to put on my masthead so I can take the aerial off, put on a dummy load (which looks like a perfect aerial) and then if the SWR is still too high I know it must be a problem in the cabling or connectors. Obviously you need to have the right connectors -- it is a tricky making decent RF joints on top of the mast.

Does that make sense? It's not up to text-book standard so fire away if you don't follow it.
 
Very clear.

Sorry about the "blob" thing. Just my way of saying things /forums/images/graemlins/smile.gif

The principles of soldering are fairly straightforward. It's just that it usually doesnt work for me - which is probably lack of practice.

Thanks for taking the time!
 
Well, most people tend to 'blob' solder -- it is the most common error aside from dry-jointing -- which are often not actually 'dry' but crystalline due to moving while the solder is still in a eutectic phase.
 
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