...the angle between the elements of each seperate dipole i guess is 60 degrees (6 elements equally spaced around a circle).
There normally would be relatively little interaction with the elements that far apart.
The dipole lengths were taken from an online calculator that automatically reduced the length by 5% for the 45deg angles, with just a little added to allow for adjustments.
That is one possible problem: there are many factors that affect the resonant frequency of
a given length antenna: the wire diameter, insulation type, wire angle, ground characteristics,
how the ends are attached to the insulators, etc. That's why most of us cut the antenna wire
long to start with, then trim it to get the SWR dip centered in the band.
Sadly, since this is on a fibreglass telescopic mast, theres no way i can use the analyzer at the feedpoint, and theres a 10m run of RG-58 between. However, i would have thought that if i was close with the initial measurements, ie the dipole was close to resonance where needed, the effect of the coax would be minimal? Perhaps its that effect im seeing?
The effect of the coax depends on what measurements you are taking. The frequency at
which the SWR is lowest stays the same when you add coax, so that is a perfectly reliable
measurement that can be done through a long length. (Well, not perfectly
you don't have an effective balun, as the outside of the coax is part of the antenna, and
changing that changes the antenna.)
The measured antenna impedance varies in a predictable manner, making circles around
the Smith Chart. The range of impedances is smaller the closer the match, but the frequency
at which X = 0 varies in a very complex manner (as it depends on the variation of the
antenna feedpoint over frequency.) Tuning an antenna for X = 0 at the end of the feedline
is NOT a reliable method of adjusting the antenna, unless you correct for the feedline length.
If i can reasonably rule out interactions between the dipoles in this configuration, then i can happily lay the blame on the coax.
I don't think either is to blame. Well, I suppose it is possible that you got some 75 ohm
coax by mistake, but so far I haven't seen any indications that there is a problem with
the coax, and I would expect minimal interaction among the wires with 60 degree spacing.
Im going to recheck the lengths on this one, in case i have done something silly with the cutting. I can also test the 17m dipole on its own.
The first approach is to sweep the SWR analyzer through the bands and record the frequency
of minimum SWR for each band - it might not be inside the band edges. Use that frequency
to determine whether to shorten or lengthen the wires. When you get all the minimum SWR
points where you want them, the antenna is resonant (or close enough).
That doesn't mean that the SWR will be low - those are two different factors. The resonance
is set by adjusting the wire length. The SWR at resonance is determined by the feedpoint
resistance: making the wires flatter will increase this, reducing the angle will lower it. Once
you get the antenna resonant in the band, you can measure the impedance at the end of the
coax and use the VK1OD calculator as described previously to determine the actual resistance
at the feedpoint: it might be 28 ohms or 90 ohms, both of which would give an SWR of 1.8 : 1.
Knowing that, you can try adjusting the vertical angle of the wires to reduce the SWR.
But the first step is to plot the SWR vs. frequency across each band of interest: you want
the frequencies of minimum SWR to be inside (or close to) the bands of operation. That is
the most reliable way to adjust your antennas - and it is very common that they need some
adjustment once installed, rather than just relying on traditional cutting formulas (which are
only good as an estimate anyway.)