Long wire transformer idea
 

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

In article , mike
writes:

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?

You could simple attach the 300-to-75 Ohm matching thing to the long
wire and use the coax...provided that the Z-changing-transformer can
handle HF. It won't be good if it's just a TV accessory; those seldom go
down to 30 MHz.

You might check the "300 Ohm" side of the transformer to see if it
actually has a DC path across it (an Ohmmeter will show that). Such an
accessory doesn't always have a DC path there. If it ain't there, no sense
in using it for your application.

The long-wire's impedance is going to range all over the place on HF so
it won't make much difference to a 75 Ohm coaxial cable feed. Most
frequencies will be way off in impedance matching regardless of the ultimate
impedance of the receiver.

Keeping the coax feedline offers some protection from household QRM
that is between receiver and antenna.

Len Anderson
retired (from regular hours) electronic engineer person

In article , mike
writes:

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?

You could simple attach the 300-to-75 Ohm matching thing to the long
wire and use the coax...provided that the Z-changing-transformer can
handle HF. It won't be good if it's just a TV accessory; those seldom go
down to 30 MHz.

You might check the "300 Ohm" side of the transformer to see if it
actually has a DC path across it (an Ohmmeter will show that). Such an
accessory doesn't always have a DC path there. If it ain't there, no sense
in using it for your application.

The long-wire's impedance is going to range all over the place on HF so
it won't make much difference to a 75 Ohm coaxial cable feed. Most
frequencies will be way off in impedance matching regardless of the ultimate
impedance of the receiver.

Keeping the coax feedline offers some protection from household QRM
that is between receiver and antenna.

Len Anderson
retired (from regular hours) electronic engineer person

Even if the thing acted as a transformer at HF, I doubt that it would handle
much TX power ...

Carl - wk3c

"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

Even if the thing acted as a transformer at HF, I doubt that it would handle
much TX power ...

Carl - wk3c

"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 24 Sep 2003 22:04:06 GMT, (Avery Fineman)
wrote:


You could simple attach the 300-to-75 Ohm matching thing to the long
wire and use the coax...provided that the Z-changing-transformer can
handle HF. It won't be good if it's just a TV accessory; those seldom go
down to 30 MHz.

You might check the "300 Ohm" side of the transformer to see if it
actually has a DC path across it (an Ohmmeter will show that). Such an
accessory doesn't always have a DC path there. If it ain't there, no sense
in using it for your application.

The long-wire's impedance is going to range all over the place on HF so
it won't make much difference to a 75 Ohm coaxial cable feed. Most
frequencies will be way off in impedance matching regardless of the ultimate
impedance of the receiver.

Keeping the coax feedline offers some protection from household QRM
that is between receiver and antenna.

Len Anderson
retired (from regular hours) electronic engineer person

It does have a DC path so I went ahead and gave it a whirl, plus added
another 60 feet or so to my antenna, which probably didnt help.

Seems to be working OK.

I found a cheap source of T68-2 toroids so I going to pick up 5 or 6
and wind up several giving me an assortment of matching posts like the
ICE transformer. This would allow me to create an internal inductor
for my tuner as well.

mike

On 24 Sep 2003 22:04:06 GMT, (Avery Fineman)
wrote:


You could simple attach the 300-to-75 Ohm matching thing to the long
wire and use the coax...provided that the Z-changing-transformer can
handle HF. It won't be good if it's just a TV accessory; those seldom go
down to 30 MHz.

You might check the "300 Ohm" side of the transformer to see if it
actually has a DC path across it (an Ohmmeter will show that). Such an
accessory doesn't always have a DC path there. If it ain't there, no sense
in using it for your application.

The long-wire's impedance is going to range all over the place on HF so
it won't make much difference to a 75 Ohm coaxial cable feed. Most
frequencies will be way off in impedance matching regardless of the ultimate
impedance of the receiver.

Keeping the coax feedline offers some protection from household QRM
that is between receiver and antenna.

Len Anderson
retired (from regular hours) electronic engineer person

It does have a DC path so I went ahead and gave it a whirl, plus added
another 60 feet or so to my antenna, which probably didnt help.

Seems to be working OK.

I found a cheap source of T68-2 toroids so I going to pick up 5 or 6
and wind up several giving me an assortment of matching posts like the
ICE transformer. This would allow me to create an internal inductor
for my tuner as well.

mike

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 20 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

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 20 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

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