Co-Axial cable impedance for VHF

[catalac08]

What is the correct impedance co-axial cable for a boat VHF?

 

[William_H]

50 ohm is the correct impedance for the cable. (perhaps sometimes called 52 ohm same thing)
TV antennae cable 75 ohms. good luck olewill

 

[catalac08]

Thanks william
keith

 

[Dockhead]

50 ohms, and this is extremely important. 75 ohm cable will not work.

 

[William_H]

50 ohms, and this is extremely important. 75 ohm cable will not work.

Please don't be so adament. that it won't work. Actual characteristic impedance of cable really only shows as losses increase. ie at higher frequencies and longer cable runs.
So I would suggest that with a VHF set perhaps over 2 or 3 metres then losses would be quite small and probably insignificant between 75 ohm and 50 ohm cable. However over 10 metres or more losses with correct impedance but chaeper cable can mean you lose 1/4 or more of your power. (Still often not noticed) but losses migh be a lot greater using 75 ohm cable in a 50 ohm system. olewill

 

[Salty John]

As said, impedance is 50 ohm. Other considerations would be corrosion resistance, uv resistance and using the right size for the length of run.

Here's a bit of info:

http://www.saltyjohntheblog.com/2013/06/marine-coaxial-cable.html

 

[CreakyDecks]

There are two things to think about. The first is impedance mismatch which would happen at the ends where the 75ohm cable meets the 50ohm equipment. This is not related to cable length. A mismatch reflects power back to the transmitter (or the antenna). The impedance of the cable is decided by the ratio of the outer and inner diameters of the plastic dielectric and what that plastic is, so you can have thin 50ohm cables and fat 50ohm cables. You can't tell what the impedance is by looking at the outside of the cable unless it is labelled, which it often is.
The second thing to think about are cable losses due to a lossy dielectric. This isn't a reflection it is actually power loss in the cable itself, warming it up. Fat expensive cable is generally less lossy but obviously less convenient. You can't bend it as tightly without risking changing the impedance, which would then mean you'd wasted your money.

 

[mcalan]

I have just tested my VHF cables resistance and found it to be 2.1 Ohms within the inner conductor and 3.3 Ohms within the outer is this acceptable or does it need changing .?

Many thanks

 

[oldvarnish]

I have just tested my VHF cables resistance and found it to be 2.1 Ohms within the inner conductor and 3.3 Ohms within the outer is this acceptable or does it need changing .?

Many thanks

Would that be DC resistance you're measuring?

 

[mcalan]

Yes I think so I just set my meter to ohms and took the readings I'm not an experts sorry!

 

[AntarcticPilot]

Yes I think so I just set my meter to ohms and took the readings I'm not an experts sorry!

A standard multimeter won't measure the impedance (which is what the 50 Ohms refers to). You need equipment measuring at the intended frequency, which is not a DIY thing for most of us (including me!)