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Author Topic: Do you have to use coax for an "Ugly Balun"? Would zipline work?  (Read 652 times)

AE5KG

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I am currently planning a center insulator for a fan-dipole.
3 1/2" pvc Like this.


When I am connecting the different legs of the dipole together inside, could I make the center pvc piece longer, come out the pipe below the last dipole, wrap 20' of zip line around the pvc, re-enter the pvc and then solder on a SO-239 on the bottom, incorporating a 1:1 current balun into the center insulator?
« Last Edit: March 09, 2021, 10:02:11 AM by AE5KG »
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WA6BJH

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #1 on: March 09, 2021, 12:39:47 PM »

I don’t think it would work very well.  Zip line impedance isn’t 50 ohm. 
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AE5KG

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #2 on: March 09, 2021, 12:42:56 PM »

I don’t think it would work very well.  Zip line impedance isn’t 50 ohm.

You know, I am stupid.  I didn't even think of that. 
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WB6BYU

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #3 on: March 09, 2021, 01:18:44 PM »

You certainly could do that.

Standard zip cord tends to run around 110 ohms or so.  (Speaker
cable may be closer to 140 ohms.)

If the winding is short enough, then the impedance transformation
isn’t that much:  I ran into exactly this same issue using twisted pair
for a balun.  The effect was to add a bit of reactance, which usually
could be handled by a minor change in element length.

Coax is more convenient, of course, as there is no impedance shift.

But you can’t keep the length to a minimum if you are making
an air-core balun:  in my case, I was wrapping it around a pair
of large ferrite beads.  With an air core “ugly” balun, not only
is it only effective (such as it is) over a relatively narrow range
of frequencies, but it takes significantly more length.  By that
point, the impedance transformation affects both R and X, and
it will be a lot more difficult to get a good match.

AA4PB

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #4 on: March 09, 2021, 01:58:46 PM »

If you wind both parallel conductors of zip cord it seems to me that you would be adding inductance to both lines. With coax you are only adding inductance to the outer (shield) conductor in order to keep the shield from radiating (or receiving) RF. You don't want inductance added to the center conductor.

A balun is used to convert unbalanced coax to a balanced line but zip cord is already balanced.
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Bob  AA4PB
Garrisonville, VA

W9IQ

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #5 on: March 09, 2021, 04:29:22 PM »

Don't goof around. Ugly baluns yield ugly results.

Make a proper common mode choke. Start with an FT240-31 ferrite toroid and an RG-400 jumper cable. 10 minutes of work and you have an effective, high resistance, common mode choke that actually works.

- Glenn W9IQ

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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

K0IZ

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #6 on: March 10, 2021, 07:35:23 AM »

I would be inclined to use RG401 or 402, making 8 or 10 turns, 6" diameter.  Either coax will handle full legal power due to Teflon insulation, and relatively light weight compared with bigger coax.  You don't want the center of the dipole dragging on the ground....
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W9IQ

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #7 on: March 10, 2021, 07:54:25 AM »

Perhaps a bit of background on common mode chokes (baluns) would help the discussion.

The "ugly" balun creates only reactance - no resistance. This inductive or capacitive reactance is in series with the reactance of the outside shield of the balance of the feedline. If that happens to be the opposite sign, it will reduce or cancel out the reactance that the ugly balun created. The result is that the effectiveness of the ugly balun is in question. It can make the common mode current worse than with no balun at all.

A ferrite based balun also creates an inductive or capacitive reactance (generally much more than an ugly balun). But its primary goal is to create resistance - something an ugly balun cannot do. This resistance is not affected by the reactance of the balance of the feedline. Thus you can count on consistent effectiveness of this type of balun. The common mode current will always be less with this type of balun.

- Glenn W9IQ
« Last Edit: March 10, 2021, 08:12:52 AM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

WD4HXG

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #8 on: April 17, 2021, 04:45:13 PM »

"Would zipline work?"

The question is open ended as it depends on what your definition of "work" is?

Yes zip line will work, but in all probability its characteristic impedance will not
be optimal. Remember you want the transmission line impedance to be the
geometric mean value of the two impedances being matched. So if you have
a 200 Ohm load and 50 Ohm source then the would want 50 * 200 = 10000
and take the square root of 10000 which will provide 100 ohms. While the
additional loss may be only an extra 0.5 dB , that 0.5 dB shows up as 10
watts of rf being converted to heat when running 100 watts.

You need to measure the zip line impedance as the conductors are
close and the formula for calculating the impedance falls apart when the conductors
are close together. Calculated values can get really weird when the "S (spacing value)
is less than two times the diameter of the conductors. Throw in the insulation is made
of who knows what, probably some PVC variant,  unless you measure the velocity
factor of the zipline, you are taking a stab in the dark at what is a quarter wavelength.
You really want to use about an electrical 1/4 wavelength of line for maximum bandwidth
and not knowing the Vp (velocity factor) can easily result in a line length that is more
than 30% longer then the electrical length. So you can easily wind up with the point of
least loss in the balun being much lower in frequency than what you planned.

In my experience you get lucky about one time in a hundred to one thousand
when trying to empirically use something like zip line in an application the
vendor never envisioned you would use it for. Yeah, I have read the articles
about ops using it as transmission line for portable QRP and getting 250 miles
per milliwatt, but those articles were written when propagation was much better.
PVC is not a horrid insulation but it is not my first choice.

When you add up the likely higher losses, high chance of using the wrong length,
hoping the impedance is right, you really have a snowball's chance in Hades of
"working" being an acceptable form of expectations in most cases for a balun. Of
course you can characterize the zip cord attributes and then calculate the results
to get a decent idea of if it will meet your definition of working.

73

Chuck WD4HXG
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G4AON

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #9 on: April 18, 2021, 12:56:13 AM »

As Glenn pointed out, air cored chokes are poor compared to ferrite ones. Check the chart by Steve Hunt (sadly SK), the chokes using 8 turns of coax on FT240-31, or -43, give a nice wide range.
http://karinya.net/g3txq/chokes/

73 Dave
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KD6VXI

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #10 on: April 23, 2021, 05:37:51 PM »

As Glenn pointed out, air cored chokes are poor compared to ferrite ones. Check the chart by Steve Hunt (sadly SK), the chokes using 8 turns of coax on FT240-31, or -43, give a nice wide range.
http://karinya.net/g3txq/chokes/

73 Dave

A lot has changed since Steve did his measurements.....  And a lot of Steve's stuff was based on the work of Jim brown.

Jim has also had his work published in the New ARRL manuals.  K9YC.

His
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WA2ISE

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #11 on: May 07, 2021, 12:18:08 PM »

Zipline insulation is usually lossy for RF. 
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KF5LJW

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #12 on: May 08, 2021, 12:21:52 PM »

A balun is used to convert unbalanced coax to a balanced line but zip cord is already balanced.
That would be true if you used a Balun, but I have never seen a ham use a Balun before. A lot of hams think they have Baluns, but they are nothing more than a in-line Unun CMC,
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KM1H

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #13 on: May 08, 2021, 12:28:46 PM »

Quote
That would be true if you used a Balun, but I have never seen a ham use a Balun before.

A rather disturbing sentence
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WA2ISE

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Re: Do you have to use coax for an "Ugly Balun"? Would zipline work?
« Reply #14 on: May 18, 2021, 12:13:13 PM »

One twisted pair from an ethernet cable is 100 ohms balanced.  Though it's probably too thin to handle much power. 
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