It is practically impossible to match coax cable to an end-fed antenna for
various bands, without traps or impedance matching tricks at the end of the
coax. The outer insulation and even the core insulation of coax is normally
not high voltage proof, so don't rely on it, certainly not near an "earthed"
wire.
Special marine antenna cable can be bought but is expensive. I used
multicore green earthing wire and spacers. These can be made of 5cm pieces
of white conduit pipe (UV stabilised) with two holes on each side to fix a
tag to the feeder wire and the backstay. Wrapping around both, fixes it
well. Perhaps shrink tubing would even make it more "professional". The wire
has been there for more than 5 years and the plastic has not deteriorated.
The inside wires were getting black, so soldering on both ends is necessary.
I am now going to replace it by special multicore UV stable HV cable, neatly
tied to the backstay.
It is still the question if spacers are electrically better than tying a
cable close to the backstay. The coupling between the feeder and the rest of
the backstay makes the antenna anyway into an a-symmetrical off-centre fed
thingie that may radiate well on one frequency but miserable on another. The
tuner will make the whole system resonant but that does not guarantee good
radiation or prevent RFI. Sometimes a dummy load would perform the same way.
With spacers, a 600 ohm feeder could be created (at least for some length)
to keep stray radiation at lower levels but it must be symmetrically fed and
commercial tuners don't do that.
Probably the best is trying it out, as there is not much calculation that
can be done. Thick marine antenna cable tied to the backstay makes the
system at least wind and foolproof. And pray for no RFI into the GPS and
mobile phone antennas.
----
Kris
VK4CPG s/v Marin Hedon