Author Topic: Patch leads BNC  (Read 1482 times)

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Offline DeathwishTopic starter

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Patch leads BNC
« on: July 10, 2015, 05:55:17 pm »
I have always thought that you should only use 50ohm RG58 cable for leads from one piece of kit to another, but I keep seeing RG59 cable listed as patch leads in such places as RS, what would be the difference in the two from say a sig gen to a scope or other stuff.

what is annoying me is that I can pick up a RG59 cable for about £1.50 but a similar one in RG58 is around £8.
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Offline Paul Moir

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Re: Patch leads BNC
« Reply #1 on: July 10, 2015, 06:37:00 pm »
Impedance mismatch.  RG-59 is 75 ohms while test equipment is 50 ohms.  RG-59 is for video, which is far more popular than radio / test equipment, so it costs less.  (And I would argue that they're likely not using the best wire anyway.)

Having an impedance mismatch will cause reflections and losses.  Also note that while 75 ohm BNCs and 50 ohm BNCs look generally the same, they're a little different. 

EDIT:  It's pretty popular to make your own BNC cables.  It won't really save you money but you can make the lengths you need out of decent wire & connectors.
 

Offline DeathwishTopic starter

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Re: Patch leads BNC
« Reply #2 on: July 10, 2015, 06:52:33 pm »
I knew it was 75 ohm on the RG59 as I used to use it for satellite cable when i couldnt get my hands on FT100/125 stuff occasionaly. It just got to me that some places were trying to pass it of as patch leads for interconnecting other things and wandered what the problems would be.

I used to make my own cables way back but not anymore having lost the crimper etc, and silly places like maplins want to charge as much for 1 metre of RG58 as a ready made cable for 1.65 on ebay so it really isnt cost effective anymore for me.
Electrons are typically male, always looking for any hole to get into.
trying to strangle someone who talks out of their rectal cavity will fail, they can still breath.
God hates North Wales, he has put my home address on the blacklist of all couriers with instructions to divert all parcels.
 

Offline tggzzz

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Re: Patch leads BNC
« Reply #3 on: July 10, 2015, 07:25:29 pm »
I have always thought that you should only use 50ohm RG58 cable for leads from one piece of kit to another, but I keep seeing RG59 cable listed as patch leads in such places as RS, what would be the difference in the two from say a sig gen to a scope or other stuff.

what is annoying me is that I can pick up a RG59 cable for about £1.50 but a similar one in RG58 is around £8.

Use 50ohm cable for 50ohm equipment. Use 75ohm cable for 75ohm equipment.

If one end is 50ohm and the other 75ohm, put a 50/75ohm impedance matcher in the relevant place with the relevant orientation[1]. A wideband passive resistive 50/70ohm matcher will lose ~6dB power. If you can't tolerate 6dB loss, then there are narrowband and active alternatives.

Be very careful of inserting an Xohm cable into a Yohm socket; frequently the centre conductor and/or dielectric diameters are different, which might require "undue" force.

[1] use http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm for the resistor values, e.g. 45, 86, 2380 ohms. I've made one to connect a 75ohm SDR receiver to 50ohm systems.
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