KF5FUR i am a avid 102 fan, i have ran 102 singles 102 cophased doubles, and it is my prefered antenna! there is not alot of topic on co phasing . my expierence has been on 11 meter band width. and i know theres rules for optimim gain/ ect ect... so beings your question is relaitive to coax length and a coax system of different impedences i will state i have never ran anything other than a t connector directily to my radio , or a combined harness (either made by me , or store bought) all i have expierence is working rg 59 (which in my oppinion has a very small inside single strand conductor).. i have added small patch cords (less than 6 feet) and swr dose not seem to change a whole lot (assuming your running legal 4 watts)..i have never once had swr readings (above 1.3:1) within legal power range (4 watts.
Here is the 3 setups i have co phased with 102" sst whips.
1) small car 8.5' (home made harness rg 59) pl259-2x (8.5') front mount 102s 48" spacing, swr- 1:1 barefoot, 1.3:1 200w. signal was well well worth the added expence.,
and seeming to be (not fact or proven) front to rear directional!
2)suburban Again same setup (8.5) feet rg 59 harness home made . far front mount antenna base at bottom of hood line. Spaced 65" center to center),,,but with a new twist...(i added a coupling and ran another 8 feet of rg 213) .
So radio out 8 feet 213+coupling+home made cophase harness (8.5) feet, total length (18.5').
once again swr near perfect! i will attest that that setup ran a 1.1:1 with 4 watts, and maybe a 1.3:1 200 watts.
again signal was awesome! and that setup rocked !
heres what i would suggest,,.
Due to manufacturing tollerences, ect it is stated there is a velocity factor due to coax/ bla bla bla...
Even if its 100% correct . you will find a variance due to manufacturing tollerances, and sample size.
bottom line (i allways make my final coax cut on a 50 ohm feed line 1 foot longer than 1/4 wavelength of desired resonant frequency) for 11 meters.
so that being said, make your cophase harness 8.5', ,,forget its length.....or make it 12. (whatever feet). or make it 18.( ) feet....and forget its length!
cut the 50 ohm line that feeds this to achieve a total length of 1/2 , 5/8 or 3/4 wave length +/- (velocity factor), at your frequency (?)......_+ 1 foot (if a lower frequency.
^ however me doing all the math time and time again has proven= im wasting my time.......after i check swr way above and way below my desired resonant frequency. I conclude this>
I am wasting my time doing math, and getting exact lengths down to the nats ass.
Play around with a few swr meters, play with some different 50 ohm final patch cord lengths and you will conclude the same thing.
(now to those of you who are more advanced than a swr meter and reflected energy).
yes i know it dose matter and yes i realize if you have fancy tools you can further tune a system, and what not.
But the average guy dose not have thousands of dollars to spend to find the exact resonant freq, and alot of time to spend dialing it.
I will say this much (2 102s on 11 meters booms! royally!) and there is no setup i have found better, despite "optimum gain being spaced a half wave length apart".