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CB Coax Help!

T

thebyus

NAXJA Forum User
Location
Albuquerque, NM
So I installed a CB a couple weeks ago. Took the XJ out with some friends and it worked great. Turned out that the mount for the antenna needed some reinforcing and needed to be through-bolted, rather than just screwed in to the sheet metal; so the whole antenna assembly came apart, through-bolted the mount, reassembled the antenna assembly (added a spring) and figured I'd retune because of the added spring height.

Lo and behold, SWR is WAY in the red on ALL channels. Diagnose it as a short in the system somewhere. Threw the meter on it and the center conductor had no continuity. To make a long story short, the PL-259 all but fell off in my hand because the solder on the center conductor had broken during disassembly and reassembly of the antenna assembly.

:flamemad:

So here is the question. Can I simply lop off 3 inches of my 18' coax, solder on a new PL-259 and be good to go, or do I really need to re-run an entirely new 18' of coax in order to fix this?
 
Last edited:
Just cut off about 3" or so and resolder connector, the 18' length of coax is part of the 1/4 WL of the antenna. You will be able to set your VSWR once the connecter is reconnected.
 
Cut off as little as possible. Why not just resolder it?

Tim, Hillbilly, your both right, but for different antennas.

For an antenna which presents 50 ohms at the feedpoint, like a 1/4 wave, Tim is correct, the feedline length is irrelevant.

However, as Hillbilly pointed out, for an antenna which is not 50 Ohms at the feedpoint, like a 5/8 wave stick, the antenna manufacture is using the feedline as a matching network, the length of the feedline is very important to the antenna tuning.

So you need to know what you're working with before deciding what length of feedline you actually need.

In the old days before fiberglass sticks were popular, Antenna manufactures made antennas that all presented 50 Ohms to the Coax. There are still a few out there (Larsen, K40 Baseload), but it seems it is easier to make an antenna that is nothing more than wire covering a fiberglass stick covered by heat shrink than design a decent antenna system that works well.

-Ron
 
WB9YZU said:
Cut off as little as possible. Why not just resolder it?

Tim, Hillbilly, your both right, but for different antennas.

For an antenna which presents 50 ohms at the feedpoint, like a 1/4 wave, Tim is correct, the feedline length is irrelevant.

However, as Hillbilly pointed out, for an antenna which is not 50 Ohms at the feedpoint, like a 5/8 wave stick, the antenna manufacture is using the feedline as a matching network, the length of the feedline is very important to the antenna tuning.

So you need to know what you're working with before deciding what length of feedline you actually need.

In the old days before fiberglass sticks were popular, Antenna manufactures made antennas that all presented 50 Ohms to the Coax. There are still a few out there (Larsen, K40 Baseload), but it seems it is easier to make an antenna that is nothing more than wire covering a fiberglass stick covered by heat shrink than design a decent antenna system that works well.

-Ron
Click to expand...

Oh so correct Ron, I didn't go into detail guess I should have not knowing his antenna just wanted to error on the safe side.

73,
Fred/N0AZZ
 
I have used stick antennas for many years, including various length FireStik 5/8 wave antennas, and never experienced any coax length issues.
 
Tim_MN said:
I have used stick antennas for many years, including various length FireStik 5/8 wave antennas, and never experienced any coax length issues.
Click to expand...

Then you have been randomly lucky. On Firesticks website they actually tell you to use a certain length based on your installation.
firestick said:
Q Is the length of the coax cable important?
A We find that it is very important ... especially with high performance top-loaded antennas. Your safest bet is to use 18 feet (5.5 meters) coaxial leads on all of your CB installations.
Click to expand...

If you look at a Francis 5/8 wave stick antenna, they get even more specific. Their recommendations are 12 ft or 18 ft depending on the ground plane.

-Ron
 
Last edited:
I run a 102" whip on the back of my XJ and left the full length of coax just for the length to the front. Also have a Yaesu FT-8900 FM Quad Band FM in it.

In my ZJ have 440-160m rigs.

Ron do you run any mobile amateur rigs? I looked you up and see your an E Class also. I'm a DX'er mostly QRP to QRO and use my XJ to reach some SOTA locations as I'm handicapped to an extent. Love the hobby and 4 wheeling seem to go together.
 
WB9YZU said:
Cut off as little as possible. Why not just resolder it?

Tim, Hillbilly, your both right, but for different antennas.

For an antenna which presents 50 ohms at the feedpoint, like a 1/4 wave, Tim is correct, the feedline length is irrelevant.

However, as Hillbilly pointed out, for an antenna which is not 50 Ohms at the feedpoint, like a 5/8 wave stick, the antenna manufacture is using the feedline as a matching network, the length of the feedline is very important to the antenna tuning.

So you need to know what you're working with before deciding what length of feedline you actually need.

In the old days before fiberglass sticks were popular, Antenna manufactures made antennas that all presented 50 Ohms to the Coax. There are still a few out there (Larsen, K40 Baseload), but it seems it is easier to make an antenna that is nothing more than wire covering a fiberglass stick covered by heat shrink than design a decent antenna system that works well.

-Ron
Click to expand...

Being and EE RF engineer, it is nice to see someone else who understands this stuff. To many people just spouting old wives tails for me.
 
OK then Tom, with a basic 50-52 ohms fiberglass stik antenna is coax length really that critical ? If the antenna is 5/8 wave like a Firestik, does that create a need for a specific length of coax ?

I have an MFJ antenna analyzer, and none of my stik antenna installs have had coax length issues. Am I lucky, or does it not matter as much as some would have us think ?
 
Thanks guys!

I ended up trimming about an inch total from the original center conduit, including after soldering it on. I plugged it in, and SWR is AWESOME! Not sure if it is just a better soldering connection, or the addition of the almost 4" spring, but SWR is down in the the 1.2 range, and about 1 on channel 19. A little fine tuning on the antenna tip and I'll be golden!
 
If the SWR was excessive on all channels, there usually is a coax cable issue or antenna stud assembly issue, either of which could cause a short. Improper or poorly done assembly/soldering of the PL-259 screw on cable ends can also be the source of a short of the center conductor to the braid.

Adding a spring changes the physical length of the antenna, and depending on the previous condition of the antenna system (too long/just right/too short), the length change can make the SWR better or worse.
 
thebyus said:
Thanks guys!

I ended up trimming about an inch total from the original center conduit, including after soldering it on. I plugged it in, and SWR is AWESOME! Not sure if it is just a better soldering connection, or the addition of the almost 4" spring, but SWR is down in the the 1.2 range, and about 1 on channel 19. A little fine tuning on the antenna tip and I'll be golden!
Click to expand...

I'd leave it and call it good. Most antenna systems are not wide banded enough to cover the entire band under 1.2:1

HillbillyXJ-PM

-Ron
 
WB9YZU said:
I'd leave it and call it good. Most antenna systems are not wide banded enough to cover the entire band under 1.2:1

HillbillyXJ-PM

-Ron
Click to expand...

That right very few, I wish I had antennas that would "tune" to that low of a VSWR. I'm glad everything worked out right for you sounds like your solder job was good also.

I use a 102" whip antenna and if on the trail in a lot of places have to fold it down to a clip on my roof rack to keep it out of the trees and such. I can't transmit when folded down but the receive is still very good for trail use anyway. As Ron and I spoke about off line we do have HT's as well in our Jeeps as they have become very cheap to buy anymore.

Here is a photo of my antenna folded down to the rack antenna bracket/hold down. My rear bumper came with an antenna mount built on to it along with a Hi-Lift mount nice touchs for no additional costs.

 
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