I just saw this post. My Sirio PL 5000 w/145 mag mount is about 78" tall. I did change the coax to 18' of rg-8x. It is a little tall but it really get out very well.
Why change coax to 18 ft of rg-8x? especially as the mag 145 pl is supplied with not only high quality mil spec rg58 but also a halfwave impedance repeater of it at 27.5 mhz. Starting already to reveal what these antennas were designed for, not legal cb, but highest performance in illegal band between US/UK legal bands around 27.500 mhz.
funnily enough US cb band ends at 27.405 and UK band starts at 27.60125, as you can see, 27.5 lies almost plum in the centre, there's madness in their method.
18 ft is a myth for rg58 and an even bigger myth for rg-8x which will no doubt have velocity factor in the .79-.85 range. theres absolutely no reasoning behind your choice of length,apart from cb myth.
if you want to replace the original coax, why not use a halfwave multiple like sirio do, ever wonder why they give you exactly 3.6m on an mag145pl?
rg58 has velocity factor of approx .66, so 300/27.500/2 x .66 = 3.6m, tells you they expect it to be used between 27 and 28 mhz,centred on 27.500
if it didn't do anything why would a company like sirio go to the bother of exact cutting and not just stick random lengths on?
assuming your velocity factor is .84 then for your coax thats gonny be 300/27/2 x.84m =4.66m or x 3.28 to get it in feet for you non colonialists,lol, = 15.30 feet multiples or 15 ft 3 1/2 inches, a 30 ft 7 inch lead will show exact same swr.
or
300/27.5/2 x .84 = 4.58m or x 3.28 = 15.02 feet, 15ft is close enough.
it doesn't need to be odd wavelengths either, that only applies to odd 1/4 wave multiple impedance inverters,primarily used for cophasing antennas or matching antennas.
you can change the 27 to 27.5 if you only use between 27 and 28 mhz or 27.185 if you only use 40 ch.
27.000 centres it for anywhere between 26 and 28 mhz but is only adviseable on the bigger wider banded sirios like hp/megawatt 4000 and performer 5000.
it won't change swr at antenna,only tuning antenna will achieve that, but it will give more accurate readings as every halfwavelength in coax the impedance repeats itself, minus minute cable losses, especially at such short lengths.
the further you go from that centre frequency the less accurate your readings will be as reactance starts to play a part.
Sirio have taken into account most people measure swr in a remote location, ie shack or away from antenna, not where it should be measured, right at feedpoint of antenna,therefore giving you the most accurate readings a remote location will allow for.it will always be slightly higher even if its not noticeable on a pretty innacurate cb swr meter at the feedpoint as there is no return loss to reduce readings.
Sirio don't fuck about,they have thousands of pounds/dollars worth of anechoic chamber for accurate testing of scaled down models and military grade modelling software,again costing thousands,they have improved upon even avanti's designs for performance albeit not for strength to cut costs and retail price, these guys know what they are doing.
18 feet is nothing more than snake oil,i've supplied the reason, the mathematics (for different cables and centre frequencies) and the extent Sirio go to perfect things, its time this 18 foot myth <can not stand it>was put to bed once and for all.
