Author Topic: BNC connectors impedance mess1 help!  (Read 2532 times)

Offline Greekman

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BNC connectors impedance mess1 help!
« on: September 29, 2017, 11:28:25 AM »
guys i am totally perplexed.
I am shopping for BNC connectors and i am trying to avoid getting the 75Ohm ones.

But i see pics in sellers' listing that are advertised as 75ohm, or they are suggested for 75Ohm applications,  but they look like the 50ohm ones.
i.e.
http://www.ebay.com/itm/253102363852
http://www.ebay.com/itm/271750686777

furthermore I shopped some connectors from a local electronics shop that stocks only TV items. And they also look like 50Ohm ones
Maybe it is they that they use the wrong ones?
http://www.cctvinstitute.com.br/bnc-75-or-50-ohms.html

I base my shopping on this pic
https://en.wikipedia.org/wiki/BNC_connector#/media/File:BNC_50_75_Ohm.jpg



Offline Carl

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Re: BNC connectors impedance mess1 help!
« Reply #1 on: September 29, 2017, 12:51:28 PM »
  A BNC connector is just a BNC connector ,though designed as a 70-75 ohm cable connector it has no real impedance by itself.
The difference can be the diameter of cable it will fit though no real problem exists as long as the cable fits in the connector.

  ALSO ,as I am here now, the 70-75 ohm cable will work great on 50 OHM radios , only a 'perfect' SWR match will mean 1.5 to one and
remember that a VERTICAL ANTENNA typically has a 31 OHM impedance while a dipole has 70 OHM impedance and when you trim the antennas
for perfect SWR , you are actually de-tuning the SYSTEM (antenna/coax) and inducing LOSS just to satisfy your quest for a perfect match.
  So SWR may look better ,but your antenna is NOT BETTER. SWR is not an indication of antenna effectiveness,it is just a sign of
impedance match between the radio and antenna/coax system.

BOTTOM LINE:
Just make sure the coax you use PHYSICALLY fits the BNC you chose and don't worry about the impedance lable on the fitting.

Offline DonC

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Re: BNC connectors impedance mess1 help!
« Reply #2 on: September 29, 2017, 02:11:51 PM »
Since he beat me to the punch, I'll echo Carl! I've been using BNC a lot while hooking up some CCTV for the Church. Take it from a semi-newbie, you don't need to pay as close attention to SWR as you think. Remember if you cut your wires and mess it up, it's more difficult, (and costly), to fix.

A good case in point: During Field Day 17, I was using a Ham Stick Dipole. It might've looked neat as a portable antenna setup. When I hooked it up to the SWR meter, it gave me a 4.6:1 SWR on 40m. If I had taken what the meter said and not bothered with it, I wouldn't have made the amount of contacts I did! (After hooking it up to the autotuner, it got a 1.7:1 SWR.

Back to your situation, don't worry so much about SWR. And like Carl said, use what you can get. I wouldn't lose sleep between the difference of a 75ohm v. 50ohm connector. You're gonna give yourself a headache for nothing. As long as it fits on your coax, you should be fine!

Good luck!
-Don

Offline armymars

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Re: BNC connectors impedance mess1 help!
« Reply #3 on: September 29, 2017, 07:26:21 PM »
  The OD of RG 58 and RG 59 are the same. What's different is the center conductor. The larger the center conductor The lower the impedance. So if your using 50 ohms coax, it's 50 ohms. Just make sure the socket will fit the center conductor. 

Offline Greekman

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Re: BNC connectors impedance mess1 help!
« Reply #4 on: September 30, 2017, 02:38:33 AM »
armymars,
thanks for the distinction. guess that explains why they use 50ohm stuff?

carl and DonC,
you know I got scared away from the BNC connectors in a Greek FB HAM group.
yet i find them very convenient for a second option use as they can be had in styles that allow assembly with no tools, or minimal soldering (like the ones I linked to), and their socket on feature i handy sometimes and they can take soem cable twisting.
I do not know if I have told it again, but once i traveled for work somewhere no one stocked any radio gear. And was left with a broken mobile cable the very same day. So I try to keep easy to use connectors at hand.

Now on the SWR issue. I was not referring to it specifically, although it is makes no sense in introducing losses in a field antenna especially when you are running a handheld.

Also, Carl. When you say all verticals are 31ohm, does this include a DIY 1/4 wave groundplane, or a mobile antenna mounted on a chassis adpter with extra groundplane kit?

Offline Carl

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Re: BNC connectors impedance mess1 help!
« Reply #5 on: September 30, 2017, 03:29:35 AM »
Here is a quote from an article:

  Experimentation in the early 20th century determined that the best POWER HANDLING capability could be achieved by using 30 Ohm Coaxial Cable, whereas the lowest signal ATTENUATION (LOSS) could be achieved by using 77 Ohm Coaxial Cable.  However, there are few dielectric materials suitable for use in a coaxial cable to support 30 Ohm impedance.  Thus, 50 Ohm Coaxial Cable was selected as the ideal compromise; offering high power handling AND low attenuation characteristics.
 
  With 50 Ohm Coaxial Cables being the best compromise solution, practically any application that demands high power handling capacity, i.e. 100 watts or more, will use 50 Ohm Coaxial Cable.  A good rule of thumb is that any device that functions as a transmitter or transceiver tends to use 50 Ohm Coaxial Cable.  This includes devices such as CB/Ham Radios, Broadcast Radio/TV Transmitters, Wi-Fi and Cellular Phone Repeaters and 2-Way Radios

Back to me : I see that 70 ohm coax is lower loss and you may find that most repeaters using hardline, concidered the best,low loss feedline,are using 70 ohm hardline as it induces LESS LOSS THAN 50 OHM COAX. It really matters little that you use 50 or 70-75 ohm coax and an SWR METER does not indicate loss,only match impedance as most antenna/coax is mismatched anyway as a properly cut quarter wave vertical antenna is 31 ohms (until the owner introduces loss buy cutting it(or radials) to a non quarter wave length)   And the typical, 1/4 wave each side,dipole has an impedance of 71 ohms until someone mistakenly trims the length and detunes the antenna,thus introducing LOSS so as to see a lower SWR..

  You see ,cutting an antenna to adjust SWR only degrades the antenna performance to achieve a SWR reading that really represents little and costs loss in performance ...I only 'trim' an antenna when SWR in above 1.7 to limit heat buildup in the transmitter. As long as an antenna is within reasonable SWR range to trim it beyond it's measured length for lower than acceptable SWR only fools the meter by introducing loss in the antenna/feedline/ground system

  A 70 ohm dipole should not show below 1.5 to one SWR unless loss due to cutting the antenna or otherwise inducing LOSS to the system.

Last thought:
 Just do what you want to do. 50 or 70 OHM coax will not cause the losses that cutting an antenna to satisfy an SWR meter will introduce.

Here is a link to read.:

http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/


Offline Greekman

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Re: BNC connectors impedance mess1 help!
« Reply #6 on: September 30, 2017, 02:11:44 PM »
what I get from all this.
- 50Ohm coax is a useful compromise and no god given rule as most of HAMs preach
- if i want to minimize line losses (mounting a larger antenna to a handheld radio) I better choose 75ohm line.
- ideally there should be a un-un (?) between ANY antenna and feed line

Offline armymars

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Re: BNC connectors impedance mess1 help!
« Reply #7 on: September 30, 2017, 02:36:16 PM »
Greek Man,
  As far DIY 1/4 wave verticals are concerned if you mount the radials at about 45* you get a 50 ohm impedance. More or less.     

Offline Carl

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Re: BNC connectors impedance mess1 help!
« Reply #8 on: September 30, 2017, 02:38:45 PM »
what I get from all this.
- 50Ohm coax is a useful compromise and no god given rule as most of HAMs preach
- if i want to minimize line losses (mounting a larger antenna to a handheld radio) I better choose 75ohm line.
- ideally there should be a un-un (?) between ANY antenna and feed line

OK ,I know it can be a real change from what most people say,but I use good 50 or 75 ohm coax that I can solder on my connectors. I solder as
a matter of choice as a good crimp-on is fine for most,just not the way I choose to do it.

  I use a balun or un-un for HF but have not found it needed on VHF/UHF ,though some antennas may work better with one.

Offline Carl

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Re: BNC connectors impedance mess1 help!
« Reply #9 on: September 30, 2017, 02:40:39 PM »
Greek Man,
  As far DIY 1/4 wave verticals are concerned if you mount the radials at about 45* you get a 50 ohm impedance. More or less.   

And an inverted "V" can also become 'close' to 50 ohms also. The rules of antennas can be confusing.