Coax selection

[NJGumball]

Hi, I was wondering on a co-phased setup I have been running around with for a while. I have a proper 75 ohm RG59 co-phase harness from the radio matted to two RG58 50 ohm coax for my NMO mounts. I was just testing the setup and the SWR is high. Did some research and found that the RG59/RG58 combo I am running may not be correct. Do I need RG59 all the way from radio to antenna? The antennas are 4.5ft apart.

 

[LtDoc]

No, the coax doesn't -have to be- all the way from both antennas to the radio, but depending on distances between all of those things it can be. Basically what you are doing is using that 75 ohm coax as am impedance matching transformer to provide an impedance of close to 50 ohms when both antennas are joined. That's called 'conjugate' matching, BTW. If those antennas are quite a ways from the antenna, then 'normal' 50 ohm coax from the join, or 'T', to the radio works just dandy.
It's not a very realistic idea to expect a "perfect" impedance match (SWR) with a phasing network like this. If it's something near 1.5:1 at the radio you're probably about as close as is practical. Why? 'Cuz the impedance of the antennas, and the feed lines are very seldom exactly what they are purported to be. If you can't do the measuring to determine those impedances then it's always just a guess. Sometimes you get lucky though...
- 'Doc

 

[NJGumball]

The system is not to far apart. I should have mentioned that it is mounted on my car. I have two nmo style mounts that have normal amount of length of RG58 coax 8ft or so? I am only questioning the setup as my swr's are about 2.1-2.5 CH: 1, 19,40. I am trying to nail down why exactly the system's SWRs are so high.