Mike,

Due to limited space in the roof - I'm renting so won't risk an external
antenna - my setup is similar to this. I use an OCF dipole (some call it
a Windom but this is incorrect) with a 300 ohm feedline running to a
wallplate in the loungeroom. From there it goes to a 4:1 TV-style balun
and into 75 ohm coax. Works quite well. The dipole is in parallel with
an existing VHF TV antenna, so it should do well at 2 meters too when I
finally decide to play up there.

Not sure about your earthing situation, mine didn't need it. The setup
looks like this. I'm in Australia if you're worried about the weird
orientation:


^
\
North

__________________( )_____________
| 10M H 5M |
| H |
| 4M H 3M |
| H |
| H
H
El-Cheapo H
4:1 Balun H 300 ohm
{X}=========== ribbon feedline
|
| 75 ohm
| Coax
|
[SDR-1000]

Rob




mike wrote:

Given I am currently using 75 ohm coax to feed my random wire, I
believe I could use a 300 ohm to 75 ohm (4:1) TV twin lead to coax
converter and get away with it.

Please correct me if I am wrong, but I think I would attach the random
wire to one terminal of the 300 ohm input. The other would go directly
to earth ground, or via a gas arc tube to ground.

On the other side, I would attach my coax cable.

To give the coax feeder a ground at the antenna end, I would to open
up the converter and solder in a ground wire on the coax sheild
terminal. This would kill off common mode currents.

This would give my antenna feeder system a matching transformer plus
give the antenna a direct path to ground.

Theoretically, I am I missing anything?

mike

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.

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.

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.


"mike" wrote in message
...
Given I am currently using 75 ohm coax to feed my random wire, I
believe I could use a 300 ohm to 75 ohm (4:1) TV twin lead to coax
converter and get away with it.

Please correct me if I am wrong, but I think I would attach the random
wire to one terminal of the 300 ohm input. The other would go directly
to earth ground, or via a gas arc tube to ground.

On the other side, I would attach my coax cable.

To give the coax feeder a ground at the antenna end, I would to open
up the converter and solder in a ground wire on the coax sheild
terminal. This would kill off common mode currents.

This would give my antenna feeder system a matching transformer plus
give the antenna a direct path to ground.

Theoretically, I am I missing anything?

mike

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.

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.

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.


"mike" wrote in message
...
Given I am currently using 75 ohm coax to feed my random wire, I
believe I could use a 300 ohm to 75 ohm (4:1) TV twin lead to coax
converter and get away with it.

Please correct me if I am wrong, but I think I would attach the random
wire to one terminal of the 300 ohm input. The other would go directly
to earth ground, or via a gas arc tube to ground.

On the other side, I would attach my coax cable.

To give the coax feeder a ground at the antenna end, I would to open
up the converter and solder in a ground wire on the coax sheild
terminal. This would kill off common mode currents.

This would give my antenna feeder system a matching transformer plus
give the antenna a direct path to ground.

Theoretically, I am I missing anything?

mike

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

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

On Wed, 24 Sep 2003 15:05:09 GMT, mike wrote:

Given I am currently using 75 ohm coax to feed my random wire, I
believe I could use a 300 ohm to 75 ohm (4:1) TV twin lead to coax
converter and get away with it.

Mike, I used a 1:9 un-bal transformer, Its early and I'm rushing ,
but, 9 turns trifilar IIRC on an T50-2 toroid. the endfed was about
65' long. I fed it with 75R coax with the ground side of the unbal
going to the screen of the coax.


a bit like this (ant -u-u-----u-u--coax--u-u--gnd )
where --u-u-- is a winding of the unbal.

It worked reasonable well with 10watts on 40m and above using an atu,
but had problems with 80m due to RF feedback. A counterpoise from the
unbal gnd would probably have helped this.
gotta go.
atb Mike W
--

On Wed, 24 Sep 2003 15:05:09 GMT, mike wrote:

Given I am currently using 75 ohm coax to feed my random wire, I
believe I could use a 300 ohm to 75 ohm (4:1) TV twin lead to coax
converter and get away with it.

Mike, I used a 1:9 un-bal transformer, Its early and I'm rushing ,
but, 9 turns trifilar IIRC on an T50-2 toroid. the endfed was about
65' long. I fed it with 75R coax with the ground side of the unbal
going to the screen of the coax.


a bit like this (ant -u-u-----u-u--coax--u-u--gnd )
where --u-u-- is a winding of the unbal.

It worked reasonable well with 10watts on 40m and above using an atu,
but had problems with 80m due to RF feedback. A counterpoise from the
unbal gnd would probably have helped this.
gotta go.
atb Mike W
--