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co-phaze coax and antenna tuning

longhaireddwb

longhaireddwb

W9WDX Amateur Radio Club Member
Oct 8, 2008
555
30
38
Colorado Springs, CO
My buddy has got an older peterbult and wanted me to fix his coax and tune antennas. Both of the pl259s at the antennas had the coax ripped out of them. I cut the coax back about four inches to find good clean wire and braide. Put new pl259s on them. I cut the same amount of coax off both sides.

I then used my antenna anylizer and a single piece of coax that ive made going with the math and velocity factor of the coax to get the most acurate readings. After tunning both antennas to read 1.2:1 i then checked swr in the truck at the radio connection and im seeing a 1.8:1.


Any idea why this is? Did i do wrong by cutting the co-phaze coax? Should i tune antennas by putting anylizer at the feedpoint and cut antennas the same on both sides at the same time then takecanotger reading and continue?

Any comments are welcome.
 

Rule #1-It is called a phasing cable not co-phasing. Just because it has two antennas does not mean there is a "co" required. Lesson over.

Rule #2- NEVER alter the length of a phasing cable.
 
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I understand now that shortening the coax is not a good idea but he was not going to replace his coax being it is a company truck.
I dont remember what the impeadance was, sorry.

With the pl259s missing this was better than nothing as it will work now but he might not get out as well as a single coax and tunrd antenna.
So im guessing that tuning at the feedpoint would be best as it will take into concideration of the shortened coax and make it as good as possible?
 
In his case I expect it was an impedance matching cable that happened to be 90 degrees long. Changing its length altered the required 90 degree electrical length needed to create the transformer.

It is quite common to change the length of phasing cables to alter antenna patterns.



Captain Kilowatt said:
Rule #1-It is called a phasing cable not co-phasing. Just because it has two antennas does not mean there is a "co" required. Lesson over.

Rule #2- NEVER alter the length of a phasing cable.
Click to expand...
 
vo1ks said:
In his case I expect it was an impedance matching cable that happened to be 90 degrees long. Changing its length altered the required 90 degree electrical length needed to create the transformer.

It is quite common to change the length of phasing cables to alter antenna patterns.
Click to expand...


Well yes, but I was being case specific in this case. This case did indeed involve impedance matching and not so much concerned about pattern.
 
longhaireddwb said:
I understand now that shortening the coax is not a good idea but he was not going to replace his coax being it is a company truck.
I dont remember what the impeadance was, sorry.

With the pl259s missing this was better than nothing as it will work now but he might not get out as well as a single coax and tunrd antenna.
So im guessing that tuning at the feedpoint would be best as it will take into concideration of the shortened coax and make it as good as possible?
Click to expand...



the 1.8:1 swr is approaching the swr of running only one antenna, i.e. like you only have one antenna and a dummy load at the end of the other 75 ohm cable, but I'm not sure how the pattern is affected. You can run ok but for a cb type accepted radio it is most likely affecting efficiency especially with only 4 watts.

Have you checked the mirror mounts and grounds to make sure those are there and good if at all?

You might try adding a 3 foot extension 50 ohm cable with a barrel connector behind the radio(in the 50 ohm portion of coax) and see what that does for you.

Just thinking outside the box here. Best of luck enjoy the hobby.
 
Last edited:
If both sides of the phasing line were cut the same amount, and it appears this was the case, then the pattern would not be affected at all since the phase is the same as before however the transformation ratio would be affected and thus the impedance.
 
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vo1ks said:
Garth - Pull out your Smith chart! Let's see some real numbers here...
Click to expand...


I burned it the day I left MBS Radio. :laugh: Mr. Smith and I never did quite get along. :cry: Enlighten me. You can use this one. :D


smith_chart.jpg
 
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I have to say that you guys are getting a bit over my head on this. Remember this is only a CB radio, no amp and if a person gets out two miles hes happy. Any more than that and hes really happy.

I think the next time I get a chance I'll tune the antennas at the feed-point to see if I can bring the total SWR down some more. Lower is better. And if it wont drop any then I will put a small 50 ohm jumper to the radio before the PHASING coax and see what happens. Never hurts to try as I got some laying around.

I will check to see what the impedance is before I change anything just to let you all know.
This post is very interesting. Please continue. Its full of good info that some folks don't know about. :whistle:
 
easy solutation = get a 18 feet of rg58 50ohm coax and run just 1 antenna
can still keep the 2nd antenna for looks ,but dont use any coax too it .
co-phase antennas are alot of work with little to no reward.best of luck
to ya
 

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