Coax cable impedences

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BlueSkyNick

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In need of recabling the antennas for GPS (old Simrad), Navtex, and VHF for AIS.

They are all mounted on a pole on the back, and the old cables have been chafed in the past due to twisty cable run.

Are they all going to be 50-ohm, so I can use the same cable and BNC's? Or might there be a mixture in which case I will have to match the right cable for each device?
 
They are all likely to be 50 ohm. It would be unusual to use anything else. However, some GPS have the receiver incorporated with the antenna, and the output from the "white box" is a multicore cable which goes to the GPS display and which carries power and NMEA signals; you will soon see whether the GPS cable is coax or not by looking at the termination on the back of the receiver/display.

You may want to consider using RG223 which is a lower loss and better isolated coax than RG58 (but costs a little more), but in practical terms unless you are running 100m of the stuff you won't notice the difference.

Alan.
 
I am not sure, but if 50 Ohms should be wrong, I dont think your equipment will blow, just the working will be erratic or impaired. If it works it will be ok, what can you lose?

Or try google the manufacturers installation instructions.

Good luck

Ongolo
 
thanks all, just as I thought.

alan - the GPS is definitely coax only because its been severed enough for me to see ..... ie I cut it by mistake !! /forums/images/graemlins/shocked.gif

ongolo - not worried about blowing the kit, but you can lose quite a lot of signal strength through impedence mismatches, hence my question. It makes a difference between receiving an AIS or Navtex message or not, which could of course have consequences.
 
When I asked Garmin Tech Dept. about this, for a GPS 50, about 10 years ago, I was told it was 75ohm. The external aerial that was sold separately for this set has a cable marked RG59U, which, AFAIK, is 75 ohm
 
Just re-checked, definitely RG59/U. They don't make them like they used to ! ( Incidentally, the external aerial alone was well over £100 in those days )
 
Get the best "correct" cable you can fit through the holes! That will probably be the limiting factor - rather than cost.
At the risk of a huge post, this may help:
***************************hopefully_these_splats_will_stretch_the_forum_to_suit_the_original_table,*************************sorry**********************
Most coaxial cables have a characteristic impedance of either 50, 52, 75, or 93 Ω. The RF industry uses standard type-names for coaxial cables. Thanks to television, RG-6 is the most commonly-used coaxial cable, and the majority of connections outside Europe are by F connectors.A series of standard types of coaxial cable were specified for military uses, in the form "RG-#" or "RG-#/U" (RG from Radio Grade, /U indicates multiple uses). They go back to World War II and were listed in MIL-HDBK-216 published in 1962. These designations are now obsolete. The current military standard is MIL-SPEC MIL-C-17. MIL-C-17 numbers, such as "M17/75-RG214," are given for military cables and manufacturer's catalog numbers for civilian applications. However, the RG-series designations were so common for generations that they are still used, although critical users should be aware that since the handbook is withdrawn there is no standard to guarantee the electrical and physical characteristics of a cable described as "RG-# type". The RG designators are mostly used to identify compatible connectors that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables.
Table of RG standards:
type approx. imped. [Ω] core dielectric overall diameter braid velocity factor comments
type [in] [mm] [in] [mm]
RG-6/U 75 1.0 mm PE 0.185 4.7 0.332 8.4 double low loss at high frequency for cable television, satellite television and cable modems

RG-6/UQ 75 PE 0.298 7.62 quad This is "quad shield RG-6". It has four layers of shielding, regular RG-6 only has one or two
RG-8/U 50 2.17 mm PE 0.285 7.2 0.405 10.3 used for thick Ethernet (10base5) and amateur radio

RG-9/U 51 PE 0.420 10.7
RG-11/U 75 1.63 mm PE 0.285 7.2 0.412 10.5 0.66 Used for long drops and underground
RG-58/U 50 0.9 mm PE 0.116 2.9 0.195 5.0 single 0.66 used for radiocommunication and amateur radio, thin Ethernet (10base2) and NIM electronics. Common.
RG-59/U 75 0.81 mm PE 0.146 3.7 0.242 6.1 single 0.66 used to carry baseband video in closed-circuit television, previously used for cable television
RG-62/U 92 PE 0.242 6.1 single 0.84 used for ARCNET

RG-62A 93 ASP 0.242 6.1 single used for NIM electronics

RG-174/U 50 0.48 mm PE 0.100 2.5 0.100 2.55 single [1] Common for wifi pigtails, more flexible but higher loss than RG58.
RG-178/U 50 0.079 2.0 single
RG-179/U 75 0.094 2.4 single
RG-213/U 50 7×0.0296 in Cu PE 0.285 7.2 0.405 10.3 single 0.66 for radiocommunication and amateur radio, EMC test antenna cables. Typically lower loss than RG58. Common.
RG-214 50 0.406 10.8
RG-223 50 0.203 5.4
RG-316/U 50 7×0.0067 in PTFE 0.060 1.5 0.102 2.6 single

Sorry, this was a 'word' document that has died in the transporter.(but you can still see the cable descriptor on the left, then the impedance) Call for Scottie.
 
I recently fitted a NASA Navtex. The supplied aerial cable was (according to the handbook) 75 ohm. It was far too long so I had to cut it. I rang NASA and was told that it was just like ordinary TV cable and that I could use TV aerial connectors. I used some from Maplin but there is definitely some degradation in the signal. It works but more messages are corrupted. Is it the connectors? If so, is there a better alternative?
 
A properly made, matching impedance connector ought to introduce no more than 3dB attenuation, and shouldn't introduce the sort of problem you are describing. I would suspect the connector. Check that both the conductor and the screen are properly connected, and that the screen connects to earth. 90% of all electronic problems are connectors. The other 10% are caused by connectors.
 
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