Instead of using a pre-made co-phase harness for dual cb antennas; can I use a "T"?

Question

You know, Run 2 short pieces of coax from the antennas into a "T" and then another piece from the "T" to the radio.:)

Answer 1

A BIG FAT NO!!! That will burn up the finals in your radio. Except, You would have to construct it with the proper impedance cables. A co-phase harness IS NOT made up of 50 Ohm coax. It is made up of two 100 Ohm lines joined to come up with a 50 Ohm load at the CB antenna connector. If you use two 50 Ohm lines (one from each mount) and "hook" them together with a "T" and then run a line to the radio, you get one heck of a miss-match because the two 50's become 25!!! and I guarantee you that 25 Ohm load will fry the finals!!! At 27 Megs that's as near a dead short you can get short of just grounding the antennas.

The answer above IS DEAD wrong. Also the length of the coax is utterly immaterial. The days of the ol' 80% factor was solved years ago with the modern day low loss cables.

Answer 2

Sorry, both the above answers are wrong to some degree.

I agree, using a T coupler will not work.

But, the way the coax's combine, you would need 75 ohms, not 100.

Cable length affects the phasing of the 2 antennas and is important. A cable like this is usually called a phasing harness. But changing the frequency will change the necessary cable length of the phasing harness, so they are pretty usless anyway. Without knowing the phasing, you wont know the pattern of the antennas.

I feel a single, correctly connected and installed antenna will do the best job.

Answer 3

Yes you can. You have to make the coax that goes off the T exactly the same length. That way, the phase angle of the signal will be identical at both ends.



I am correcting above to say, above will work reasonably well, but is not 100% correct. By doing above, the impedance at the feed point will be 25 ohms (as pointed out) and the VSWR seen from the radio will be 2:1 which is actually not bad. Most modern day radio will not be damaged by this.

To do this correctly, the harness of each side of the T needs to be 75 ohms (as pointed out), and must be odd multiple of 1/4 wavelength * velocity factor. This will create an impedance transformation section that will end in about 100 ohms.

The length of the right side and left side must be the same in increments of 1/4 wave * velocity factor. (impossible to explain in words here...) but the length is important.

By parallelling this at the T, the impedance will be correctly matched to 50 ohms.

I absolutely agree, this is a futile effort as correctly installed single antenna will work far better, especially in mobile applications.