as for choice of one of the 2 isolated backstays
to use, your choice should be guided by the frequencies you want to favour:
- the longer one will favour working lower frequencies
- the shorter one will be a bit less efficient on the really low frequencies (2-4 Mhz) but will favour low take-of angle radiation for long range DX.
As a matter of fact, you can ± calculate the upper frequency limit at which low take-off angle is still possible for a given length of antenna
(300/freq in Mhz)*0.625
will give you the Maximum Antenna
Length in metres for alow take-off angle radiation for a given frequency
Example: 17m band is the highest DX frequency you want to use.
(300/18,130)*0,625 = 8.83 metres MAL (not taking into account small effect of velocity factor)
MAL = the total length from the top of the isolated stay until the antenna connector on the ATU (so including the strecth of GTO-wire from the lower end of the isolated stay untill the ATU)
Including velocity factor effect you end up with ± 9.2m real wire length.
But on eg 4.4 Mhz marine
band this antenna wire is only 0.13 wavelength so expect losing efficiency there...but the SGC-230 will tune it.
Say you isolated stay is about 13 m (your typical 43 feet) and you have 1 meter exra of GTO lading to the ATU = 14m total wire length:
Freq = (300/14)*0,625 = 13.4 Mhz.
This means you will already loose low angle take-off radiation for the widely used 20m-HAM band but will be ideal for the 12 Mhz marine
band. 0.625 wavelength is also the antenna length at which you get highest gain concentrated in the low take-off angle lobe.
If you reduce the total antenna length to 13m, the 20m HAM band is spot on the high gain upper limit.
A 13m antenna is still about 0.19 WL on 4.4 Mhz so not so far from a 1/4 wave.
So far this theory ...to lead to the choice of the stay to use in your case.