Balun for PI network antenna tuner
 

Another ham gave me a homebrew PI network tuner he used to tuning
single wire feed antennas. I'm planning to build a multiband antenna
and feed it with twinlead or ladder line since it's less lossy than
coax.

I'd like to make or buy a balun for this tuner. I don't plan to run
more than 100 watts, and will probably start experimenting with QRP
operating as well.

Any guidance is appreciated.

Red

Red,

Well, a balun at the antenna side of the tuner is what most people would do.
Unless you use a link coupled or double balanced tuner, there really isn't much
else of an alternative. 1:1, 2:1, 4:1, 9:1 or what? Well, you're going to see
quite large impedance changes based upon what antenna you use and what bands
you operate. You'll likely get a good match on some bands and not so good on
other bands. I'd recommend a 4:1 balun as a compromise.

And don't forget those HF swap nets.

Best of luck and 73s,

Evan, K9SQG

You will need a 1:1 choke balun between the tuner and the balanced feedline
to the antenna.

Obtain an HF-quality ferrite ring, about 1.75 inches in diameter.
Permittivity around 200-400.

Wind on 8 or 10 turns of 18 or 20 gauge, stranded, flexible,
figure-of-eight, twin speaker cable.

This should cover the range 1.8 to 30 MHz.
----
Reg, G4FGQ

Reg,
I'm afraid your "figure-of-eight" lost me. What's
that?
'Doc

Here in the states we would call it lamp or Zip cord.


--
John Passaneau, W3JXP
Penn State University



"'Doc" wrote in message ...


Reg,
I'm afraid your "figure-of-eight" lost me. What's
that?
'Doc

Figure-of-eight . . . .

Two circles stuck together. Outline of plastic insulation surrounding a
pair of close-spaced wires.

Zip-cord ?
---
Reg



--
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........
"'Doc" wrote in message ...


Reg,
I'm afraid your "figure-of-eight" lost me. What's
that?
'Doc

Is lamp or zip cord twisted with a fairly short lay?

The stuff I have in mind for baluns lays flat. Colourless. It may be sold as
speaker cable.

It has lower copper loss then coax, easier waterproofed and constructed.
----
Reg

===========================

Here in the states we would call it lamp or Zip cord.

Reg Edwards wrote:

Is lamp or zip cord twisted with a fairly short lay?

The stuff I have in mind for baluns lays flat. Colourless. It may be sold as
speaker cable.

It has lower copper loss then coax, easier waterproofed and constructed.

Reg, is that superior to coax wound on the same toroid?
I use RG-400 teflon coax for my baluns.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg,
Okay, now I understand what you're talking about.
For a while there, I thought you may have been talkng
to Mr. Moebus...?
'Doc

Reg, is that superior to coax wound on the same toroid?
I use RG-400 teflon coax for my baluns.
--
73, Cecil http://www.qsl.net/w5dxp

============================

Just compare the coax inner conductor diameter with the 18-gauge wire of the
zip-cord. Conductor resistance is inversely proportional to wire diameter.

Furthermore, the transition from 50-ohm to open-wire line is less abrupt.
In theory, the impedance of the line wound on the transformer should be
intermediate between coax Zo and open-wire Zo.

On the other hand, the difference in loss due to either effect is hardly
detectable. The length of line involned is too small to make much
difference. I shouldn't bother changing the design.

I can't imagine how the coax got in there in the first place.
---
Reg, G4FGQ

In a 1:1 current balun, you would want the Z of the line in balun to be the
same as the transmission line i.e. 50 ohms not some Z in the middle. If it
is different it will do an impedance transformation of the load Z based on
the Z of the line in the balun, its length and frequency. In a 4:1 current
balun the Z should be about 100 ohms. I don't know if zip cord or what ever
you call it makes a 50 ohm open wire line. I think the spacing is to large.
I know that #14 enameled wire close spaced does make a 50 ohm line. Anyway
the difference in loss between #18 wire and RG-400 due to IR drop in the
length of wire in a balun is too small to worry about.


John Passaneau, W3JXP
Penn State University



"Reg Edwards" wrote in message
...
Reg, is that superior to coax wound on the same toroid?
I use RG-400 teflon coax for my baluns.
--
73, Cecil http://www.qsl.net/w5dxp

============================

Just compare the coax inner conductor diameter with the 18-gauge wire of
the
zip-cord. Conductor resistance is inversely proportional to wire
diameter.

Furthermore, the transition from 50-ohm to open-wire line is less abrupt.
In theory, the impedance of the line wound on the transformer should be
intermediate between coax Zo and open-wire Zo.

On the other hand, the difference in loss due to either effect is hardly
detectable. The length of line involned is too small to make much
difference. I shouldn't bother changing the design.

I can't imagine how the coax got in there in the first place.
---
Reg, G4FGQ

Anyway
the difference in loss between #18 wire and RG-400 due to IR drop in the
length of wire in a balun is too small to worry about.

John Passaneau, W3JXP
Penn State University

=========================

Just like I said !

But a 1:1 choke balun has an indeterminate transformation ratio.

On Thu, 22 Jan 2004 10:43:15 -0600, Cecil Moore
wrote:

I use RG-400 teflon coax for my baluns.

I've done that too, however, the other day I was made aware of
teflon's problems with cold flow. You might want to look into that.

Danny, K6MHE

On Thu, 22 Jan 2004 15:30:52 +0000 (UTC), "Reg Edwards"
wrote:

Figure-of-eight . . . .

Two circles stuck together. Outline of plastic insulation surrounding a
pair of close-spaced wires.

Zip-cord ?
---
Reg

Apparently because when you separate the two at the thin part
at one end, you can just zip it apart easily as far as needed.

A 1:1 choke balun made out of coax "chokes" off current flowing on the
outside of shield, it has no effect on what is happing inside the coax
between the inside of the shield and the center conductor. If you made a
choke balun wound on a toroid or with beads with 2 foot of 75 ohm coax and
used it on an antenna with a Z of 50 ohms at 28Mhz. That 2 foot of 75 ohm
coax would transform the 50 ohms to 59.4+j21.3 ohms at the output end of
your balun. If this is a problem it's up to the user to decide but I
wouldn't like it. The same effect would happen if the balun was made with a
parallel conductor line be it "zip cord" or enameled wire. If the Z of the
line in the choke balun doesn't match the Z of the feed line it will cause
an impedance transformation that will with vary with frequency. Of course if
one is only using the balun at 7Mhz or lower it will not be as noticeable as
it is at 28MHz.

--
John Passaneau, W3JXP
Penn State University


"Reg Edwards" wrote in message
...

Anyway
the difference in loss between #18 wire and RG-400 due to IR drop in the
length of wire in a balun is too small to worry about.

John Passaneau, W3JXP
Penn State University

=========================

Just like I said !

But a 1:1 choke balun has an indeterminate transformation ratio.

When connecting a 50-ohm circuit to a 600-ohm line via a choke balun it
makes no difference what the impedance of the short length of line forming
the choke might be.

For practical purposes the severe mismatch is due entirely to the difference
between 50 and 600 ohms.

One must avoid suffering from delusions of accuracy.
---
Reg

That's true but sometime we use current baluns in 50 ohm systems and one
should be aware of what happens to the Z when the line in the balun is some
random impedance.
I've done some tests on various types of baluns looking at balanced to
unbalanced conversion verses frequency and losses and I like 3 to 4"
diameter toroids with a mu of 100 to 200 wound with Teflon coax. Most of my
antennas are close to 50 ohm and I don't need any unwanted impedance
transformations.


--
John Passaneau W3JXP
State College Pa

This mail is a natural product. The slight variations in spelling and
grammar enhance its individual character and beauty and in no way are to
be considered flaws or defects.


"Reg Edwards" wrote in message
...
When connecting a 50-ohm circuit to a 600-ohm line via a choke balun it
makes no difference what the impedance of the short length of line
forming
the choke might be.

For practical purposes the severe mismatch is due entirely to the
difference
between 50 and 600 ohms.

One must avoid suffering from delusions of accuracy.
---
Reg

wrote in message
...
On Thu, 22 Jan 2004 15:30:52 +0000 (UTC), "Reg Edwards"
wrote:

Figure-of-eight . . . .

Two circles stuck together. Outline of plastic insulation surrounding a
pair of close-spaced wires.

Zip-cord ?
---
Reg

Apparently because when you separate the two at the thin part
at one end, you can just zip it apart easily as far as needed.

========================

There are two varieties. One lays down flat on a flat surface. The other
has a twist, a lay, in it.

Only the first should be used for baluns.

If either is described as 'zip-cord' then there is ambiguity. Which is why I
described describe the cord used for baluns as 'figure of eight'. But I
suppose an ambiguity still remains.
---
Reg.