RG-59 for antenna lead?

[clhamilton]

I need to replace a failed antenna lead for a car mounted portable UHF (5W, 450 - 512 MHz) handheld transceiver. The antenna is 1/4 wave on an NMO mount. The failed cable was cheap RG-58, with a run of about 8 feet.

I don't have any RG-58 on hand but I do have some high quality 1505 RG-59. I was wondering if the 1505 would work well, even though it is 75 Ohm vs the 50 Ohm of the RG-58.

Any advice would be appreciated.

Thanks,
Chris

 

[mrweather]

It'll work fine. You're not transmitting with it so the impedance mismatch won't be an issue.

 

[kruser]

It'll work fine. You're not transmitting with it so the impedance mismatch won't be an issue.

It looks like the OP is transmitting with it as he listed a 5 watt output in his post plus he used the word "transceiver" which sounds like a transmitter to me.

My answer is it will work in an emergency but you could cause damage to the radio if you are transmitting. I would get a hunk of the correct 50 ohm coax. Using 75 ohm may not only hurt your radio but it will also not be as good on transmit as more of the power will be reflected back down the cable instead of going out the antenna.

 

[clhamilton]

thanks Kruser. Indeed, I will be transmitting. I'm off to get some RG-58 for a proper impedance match.

 

[zz0468]

The mismatch caused by transmitting into 75 ohm coax is not high enough to be noticed, in most cases, and certainly not high enough to cause any damage. Depending on the antenna (a dipole, for instance) 75 ohm coax is actually a better match than 50 ohm cable.

 

[kb2vxa]

Now there's a man who knows his coax. So far nobody mentioned it so maybe I'd better tell you that 59 is slightly larger diameter than 58 so it won't fit the same connectors. I have no idea what if anything is at the antenna end but it's a safe guess a PL-259 goes on the other. There's a simple fix, just use the larger diameter reducer ferrule.

 

[clhamilton]

The antenna end of the cable solders in to a thru-hole NMO mount. The NMO mount is a common roof mounts for vehicle antennas for 1/4 wave, etc. The transceiver end will have either BNC or SMA. I have nice Kings brand BNCs for RG59, and I imagine I can find an SMA or at least a ferule for the RG59. I'm tempted to try the RG59 first since I already have the cable, connectors, and crimp dies, but I can easily get some RG58 and borrow the tooling from a friend for that size too.

Thanks.

Now there's a man who knows his coax. So far nobody mentioned it so maybe I'd better tell you that 59 is slightly larger diameter than 58 so it won't fit the same connectors. I have no idea what if anything is at the antenna end but it's a safe guess a PL-259 goes on the other. There's a simple fix, just use the larger diameter reducer ferrule.

 

[kruser]

The antenna end of the cable solders in to a thru-hole NMO mount. The NMO mount is a common roof mounts for vehicle antennas for 1/4 wave, etc. The transceiver end will have either BNC or SMA. I have nice Kings brand BNCs for RG59, and I imagine I can find an SMA or at least a ferule for the RG59. I'm tempted to try the RG59 first since I already have the cable, connectors, and crimp dies, but I can easily get some RG58 and borrow the tooling from a friend for that size too.

Thanks.

That is something I should have mentioned is you may run into troubles with the coax fittings as Warren pointed out but it sounds like you have the correct fittings and tools so that is a big plus. I know some of the cheap RG59 used aluminum conductors for the shield so you could not solder it and that was a problem for me when I messed with it. But I think you mentioned that you had the good stuff so hopefully it has a copper shield or something you can solder. I still run across the old aluminum shielded coax at work from and old antenna distribution system years ago.
As far as using it to transmit, I guess it is ok as others have stated. I've always been using the fact that I have a radio designed for a 50 ohm impedance and an antenna designed at 50 ohms so why use anything other than 50 ohm coax. But, I now agree that you will most likely be fine as I really doubt you are transmitting that much!

 

[mrweather]

Guess I should learn to read.

For short term work, RG59 will do. But I wouldn't use it exclusively. The impedance mismatch isn't that big of a deal but why force the issue if you don't have to.

As far as PL connectors go, the reducer that's used for RG8X cable will work with RG59 because both cables are the same diameter.