On Thu, 25 Sep 2003 17:45:56 -0400, "Zombie Wolf"
wrote:
Ladder line will offer little benefit is this kind of setup. Once you ground
one side of it, it isnt even ladder line anymore, but is a single wire
hookup to the long wire. Running the odd side of the ladder line to ground
thru a gas arc makes little sense, since if you are gong to ground the wire
in that side of the line, you may as well ground it directly and just get it
over with. It is the LIVE side that would need to go thru the gas arc, if
you were going to actually implement this setup.
Oddly, when using an ohmmeter on both of my 300 ohm to 75 ohm
transformers, I find continuity between all 4 connections.
There is continuity between both 300 ohm terminals and the center
conductor as well as the sheild. This is normal and expected I am
told. Both the antenna and the coax sheild recieve their ground (at
the moment) from one of the 300 ohm terminals. The other terminal goes
to the antenna. Given this continuity I didnt see a point to giving
the coax sheild another ground as it already has one via this common
ground.
Thanks for the correction. Your idea makes more sense. I would want to
directly ground the antenna, as I have done in practice. The arc tube
would go between the transformer winding and the center conductor?
Its energy gets transfered to the coax center conductor via the
transformer and the shield of the coax is grounded at the transformer
end.
The actual impedance of the long wire will vary with both the length of the
wire, and the frequency it is being operated on. Effforts at "matching" this
setup thru a "transformer" would assume its impedance will remain more or
less constant , which , as I just got done saying, is not the case. You will
get no "transformer" activity here with one side of the ladder line grounded
anyway.
In another thread I had questioned the rational of ladder line. Here,
I am using 75 ohm coax. The sheild of the coax is grounded at both
ends to reduce common mode currents. However given the above
statements, effectively the center conductor has a path to ground at
the transformer end, (although that path takes it throught the toroid
windings first).
The reasoning behind your statements concerning "common-mode noise" and
coaxial cable escape me. There really is no such thing as "common-mode
noise" on a coax cable. either the shield is grounded , or it is not, in
which case , the shield does not shield the inner conductor, which is where
the radio takes the signal off the coax. It is the center conducter that
picks up noise and feeds it to the radio. This is usually due to either the
shield of the coax not being grounded or the chassis of the rig not being
grounded (which usually accomplishes the shield ground anyway on most rigs).
This lets the noise into the coax so that it can get onto the center
conducter in the first place. You most certainly would not use things like
gas arc supressors in these grounds, since you want a constant connection to
ground on these, rather than one that only operates when a high voltage
spike appears. Gas arc and gap type supressors are used on parts of the
system that are NOT normally directly grounded, like the center conductor of
the coax. If a lightning spike appears on the center conductor, then the gas
arc or gap will "activate" and feed it to ground, which is the plan here.
The rest of the time, the gas arc or gap type supressor is electrically
open.
See above statements for clarification. My sheild is directly grounded
at my antenna tuner. The antenna and coax sheilds share a common
ground via the transformer.
mike
|