On Sat, 10 Dec 2005 18:40:43 +1300, "Ross Biggar"
wrote:

I am putting up a second tower , but it will be about 200feet from the shack
and about 70feet high.
What coax is recommended to reduce loss to a minimum,and to feed a multiband
beam with a 2kw amplifier.
Hard line excepted due to cost.

Ross,

Interesting to see use of such a long line. We have been reliably
informed that nobody uses more than 75' or so!

Starting with the question "what is wrong with commonly available
RG213", you would expect a loss around 2.4dB in a 100m run (200' + 70'
+ 30' tails) with an average VSWR of 1.5.

Given that the lowest ambient noise level on 20m is around 20dB above
typical receiver noise floor, the impact of 2.4dB of loss on receive
is insignificant.

On transmit, you will lose about 45% of your power in the line, so
with your 2KW (output?) amplifier, you will still have 1100W arriving
at the antenna. Will that do the job OK?

Is ladder line the panacea? Wireman 554 directly connected to a 50 ohm
load would have a loss of ~1.7dB and a little extra for baluns brings
you close to 2dB, so it is not a whole lot better than RG213. However,
if you used a 9:1 balun at each end, you would expect line loss of
~0.6dB and a little extra for baluns brings you close to 0.8dB. Now
that seems respectable. Problem is that you live in the land of the
long white cloud, and ladder line performance is degraded
significantly when wet, so it might not be acceptable in your
situation when wet.

Lets look at home made open wire line using 2mm copper spaced 150mm
for a 600 ohms line. If you used the same 9:1 balun at each end, you
would expect line loss of ~0.2dB and a little extra for baluns brings
you close to 0.4dB. Now that seems quite good. Anecdotally, such an
air spaced line is not affected significantly by weather / water, but
that will depend on the quality of the insulators and your rigging
methods.

Remember that the open wire solutions above need to be tuned feeders
or you will need an ATU. I suggest that you will need the ATU for
multi band operation, so you should allow another tenth of a dB or so
for ATU loss.

Someone will probably suggest that LDF6-50 (32mm (1.25") hardline)
could achieve 0.3dB loss, but could you afford it, would it be good
value?

Owen
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