The first thing is to ask how long the cables are.
There are some lengths just unfortunate for the
frequency in use. Anyways - work through this.
When you say "from the amp to the transmitter",
is that from the RF power amplifier to the transmitting
radiators (antennas)? Do they come from a power
splitter, or are there 8 "amps"?
Realise
that the "screen" or "shield" braid on a coaxial
cable is misnamed. It is actually one half of an
unbalanced transmission line structure. If the cable
sees anything other than a resistive load equal
to its characteristic impedance, you will have a
mismatch at the end of the cable, and standing waves
in it. If this happens, then antenna currents will
run on the OUTSIDE of the braid. They will radiate
freely, and couple strongly to the rest of the cables
in the bunch. The radiation pattern is likely to
be wierd.
2.85 MHz has a wavelength of 105.26
metres. The phase velocity of the wave in most cable
is about 66% x velocity of light. In low-loss foam
dielectric cable, it gets faster, and in spiral
supported conductor types, it can get to more than
90%. Whatever, it lets you know cable lengths that
can be troublesome if you have a significant VSWR,
these being quarter and half wavelengths.
The way out of this is to power them each, and
check the VSWR. Seek to match the loads on the cables
to get it to 1:1. By now your troubles have gone
away. You can allow the cables to operate with some
standing wave in them by preventing RF currents
from flowing on the outside braid, by placing high
permeability ferrite toroids along them at intervals.
If the cables are thin enough, you can coil them
up with say 8 to 12 turns about the size of a dinner
plate, taped up, to make an in-line inductance that
will stop the outer braid current.
Finally,
the approach to the antennas is critical. The one
place where there IS supposed to be a standing wave
is on the antenna conductors themselves, and unless
they are fed properly, such that their near field
is not using any part of the arriving cable outers
as their counterpoise, they will make troublesome
currents on the cables. With a wavelength as long
as 105 metres, I would be surprised if you could
avoid it unless you took care to keep the radiating
bits to themselves.
I know am guessing a
lot, because your scenario sounds unusual. I cannot
think what it is for? Yes you can use triax with
the outer grounded, or put them in conduits, and
try not to turn the conduits into antenns, or you
can bury them, etc. You don't have to do any of
that if you get them matched to a VSWR of less than
1.2 to 1. If you are running kilowatts, then you
need better than 1.05 to 1.
