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Author Topic: Balun for a dipole  (Read 2872 times)
MERSAULT
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Posts: 21




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« on: January 16, 2013, 09:25:12 AM »

Hello,

I'm trying to make a half wave dipole for 500 MHz.
I have an unbalanced output, so I need to balance it. For this I'm trying two options.
The first one is to use a RF transformer like the ADT1.5-1 from minicircuits (http://www.minicircuits.com/pdfs/ADT1.5-1.pdf) and as a plus an impedance matching can be done (from ~73 Ohms of dipole to 50 Ohms of my unbalanced input due to the impedance ratio). I connected the transformer as it follows.
1.- Primary dot = RF signal
2.- Secondary Center Tap = RF GND
3.- Primary = RF GND
4.- Secondary dot = a quarter wave monopole
5.- Unconnected
6.- Secondary = the other quarter wave monopole

What I saw in a VNA it was a min peak in a frequency near to 1.2 GHz (about -10 db) and nothing more, so, what can be wrong? I've seen this configuration (named flux coupled) in many sites so I wonder why I can't get good results.
Do I need a ground plane? (actually, I don't have one, onle the RF GND or shield of the coax cable)
Do I need external passive elements? (capacitors, inductances, resistors)
Is this type of balun a power splitter?.

My second option is to use a quarter-wave length coax cable from the output to the antenna. This will allow balance the output but no impedance matching (is a 1:1 balun). About this my question is, the quarter-wave length is velocity factor corrected or not?

My last question is more or less general, How can I check if an output is balanced or not?, i Mean, if I implement one of the solutions listed above how can I see if it works correctly?.

Many thanks in advance.

Kind regards!.
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KK5J
Member

Posts: 76




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« Reply #1 on: January 16, 2013, 11:10:01 AM »

Not my cup of tea but... might try leaving seconday center tap open. Not ground.
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KB5UBI
Member

Posts: 97




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« Reply #2 on: January 16, 2013, 11:23:34 AM »

Make a 4:1 coax balun from a 1/2WL piece of coax and find the 200 ohm feed point of the dipole.
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WB6BYU
Member

Posts: 13026




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« Reply #3 on: January 16, 2013, 12:17:14 PM »

I assume that the two monopoles are pointing in opposite directions to make the dipole.
If they are pointing the same direction to form a parallel transmission line stub, then your
VNA results might make sense. 

The connection to the transformer appears to be correct, though as KK5J noted you could
try leaving the center tap on the secondary open to see if that changes things.  But if
you don't see anything changing around 500 MHz on the VNA then I'd suggest
something major is wrong:  a shorted connection, or the wrong length elements (total
length of the dipole should be about 11" or 285mm.)

You might start by connecting the dipole directly to the coax without a balun
to see what result you get on the VNA:  it might not be perfect, but you should see
some indication of near resonance.


There are many other types of balun that are practical at UHF.  The one with a second
length of coax parallel to the first, with the shields connected together 1/4 wave back
from the feedpoint, is common:  the second conductor does NOT need to be coax; it
can be any conductor about the same diameter.    I've also seen this built using brass
or copper tubing, which is stiff enough to support the dipole elements attached to
the ends.  To do this, build a "U" or shorted stub 1/4 wave long and attach the dipole
elements at right angles on each end, pointing in opposite directions.  Then the
coax can run up inside one of the tubes to the feedpoint.  (There the shield connects
to the end of the tube with the coax inside, and the center conductor reaches over
and connects to the other side of the stub and dipole.)  Do not apply the velocity
factor corrections to the coax:  that applies inside the coax, but the velocity factor
of the quarter wave stub in this case depends on the OUTSIDE of the coax.  By using
some air spacing the velocity factor will be close enough to 1.0 that you don't need
to make any correction for it.

Another way to make a balun is to use a quarter wave of brass or copper tubing with
the antenna end open and the far end shorted to the coax shield:  that also forms an
open (coaxial) quarter wave stub that looks like a high impedance to common mode
currents.


Other approaches might work depending on your application:

The first 70cm antenna that I worked with used a conventional half wave coaxial
balun (4 : 1 voltage balun) with a delta match to a continuous driven element:  the
tap points on the antenna were adjusted to get a good match.

A half wave coax balun can also be used with a folded dipole.

G0KSC has a page on balun methods and construction here: 
http://www.g0ksc.co.uk/creatingabalun.html

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AC2EU
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Posts: 338


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« Reply #4 on: January 16, 2013, 03:24:13 PM »

The transformer specs says it has an insertion loss of 3db!
I'd take my chances with some good coax for that frequency and forget the transformer( you didn't mention what you were using)
In fact, if you are not using much power, maybe rg59 or rg6 (75 ohms)
a Quarter wave length will mess with you . Half wave multiples are better, the impedance at the shack end is close to what is presented at the antenna node. Yes, the length is velocity factor corrected.
You can also use a coax calculator to figure out a length that will be around 50 ohms at the shack end for the frequency of interest.
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WB2WIK
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Posts: 20543




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« Reply #5 on: January 16, 2013, 03:31:08 PM »

I'd want to see a photograph of the dipole and where/how the balun is connected and the transmission line run from that.

The transformer selected doesn't show 3 dB loss in the spec, but it does show about 1.3 dB loss on the graph.

No dip at all near 500 MHz for a 1/2-wave dipole on the VNA indicates something is very wrong.
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KK5J
Member

Posts: 76




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« Reply #6 on: January 16, 2013, 06:04:24 PM »

Might chk to see if the device is reversed...
« Last Edit: January 16, 2013, 06:50:11 PM by KK5J » Logged
MERSAULT
Member

Posts: 21




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« Reply #7 on: January 17, 2013, 11:36:02 AM »

Hello,

Thanks for the answers!

The transformer specs says it has an insertion loss of 3db!
I'd take my chances with some good coax for that frequency and forget the transformer( you didn't mention what you were using)
In fact, if you are not using much power, maybe rg59 or rg6 (75 ohms)
a Quarter wave length will mess with you . Half wave multiples are better, the impedance at the shack end is close to what is presented at the antenna node. Yes, the length is velocity factor corrected.
You can also use a coax calculator to figure out a length that will be around 50 ohms at the shack end for the frequency of interest.


So you say a half wavelength velocity factor corrected 75 Ohm cable directly from my 50 unbalanced output to the feed point of the dipole?

I'd want to see a photograph of the dipole and where/how the balun is connected and the transmission line run from that.

The transformer selected doesn't show 3 dB loss in the spec, but it does show about 1.3 dB loss on the graph.

No dip at all near 500 MHz for a 1/2-wave dipole on the VNA indicates something is very wrong.

Here you have the photos !
This is the PCB layout for the transformer: http://i50.tinypic.com/23u7jnk.jpg
This is how I soldered the transformer: http://i50.tinypic.com/f3back.jpg
This is the final result: http://i50.tinypic.com/2my4t49.jpg

Using dipole antenna calculator I obtained a length of 14.3 cm for each monopole. I used RG-174 cable with a VF of 0.66. The length of the cable is half wavelength corrected, about 18.9 cm.

What I saw in the VNA was this:

Log magnitude graph: http://i48.tinypic.com/67nmg7.jpg
SWR: http://i45.tinypic.com/nc02zt.jpg

Might chk to see if the device is reversed...

I check every connection using a multimeter. Everything is OK in the PCB and in the transformer. I checked before I soldered all and also everything was fine.

Maybe I'm missing a little detail.
What I want, at last, is to have a balanced output for obtaining the dipole radiation pattern. Impedance mismatch from 75 to 50 Ohms is not that bad, so, I could live with that.

Thank you again.

Kind regards.
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WB6BYU
Member

Posts: 13026




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« Reply #8 on: January 17, 2013, 12:21:34 PM »

Thank you!  It makes life much easier when projects are well documented.

In this case, look at the SWR curve between 400 and 600 MHz:  the SWR drops down
to about 2 : 1, with a small peak in the middle of the range.  On one hand, that would
indicate that the antenna is connected and working as intended (more or less.)  There
are two things to note:

1) the SWR is about what you would expect if the transformer were installed backwards,
though I don't think that is the problem in this case.

2) the small peak in the middle makes me think that there are actually two dips:  one
due to the antenna and one due to the feedline.  The antenna might be the lower
frequency one, due to capacitive coupling between the antenna element and the copper
foil on the circuit board, or possibly other metal in the field. 

Was the antenna suspended in the air when you took the measurements?  The feedpoint
impedance of a dipole varies with height above ground, and could very well be 37 ohms
(giving an SWR of 2 : 1 relative to 75 ohms) if the antenna were too close to a metal
surface or other conductor.

Is the ground side of the primary insulated from the copper foil?  Is there copper foil on the
back of the circuit board?  (Either of those could affect the match.)


As a side point:
From the photo it looks like you are using flat strips for the dipole elements:  that will affect
the resonant length slightly.  Basically the resonant length depends on the diameter of a
round conductor, and W9CF has some papers and a calculator to determine the effective
diameter of rectangular conductors: 
http://fermi.la.asu.edu/w9cf/equiv/index.html



Quote from: AC2EU

...a Quarter wave length will mess with you . Half wave multiples are better


No, you are confusing the issue.  The feedline length makes no difference
in this case, unless the balun is ineffective so that the coax is part of the antenna.
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AC2EU
Member

Posts: 338


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« Reply #9 on: January 17, 2013, 12:51:25 PM »

Quote
So you say a half wavelength velocity factor corrected 75 Ohm cable directly from my 50 unbalanced output to the feed point of the dipole?

If you use 75 ohm cable the match is about 1:1, so any reasonable length is ok. Matching with 75 ohms at the antenna is your best option.

Also there is way too much copper around that antenna That's probably a big part of the problem. Use a perf board with no copper and the coax should be at a right angle to the dipole elements.
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MERSAULT
Member

Posts: 21




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« Reply #10 on: January 18, 2013, 05:01:40 AM »


Was the antenna suspended in the air when you took the measurements? 

Is the ground side of the primary insulated from the copper foil?  Is there copper foil on the
back of the circuit board?  (Either of those could affect the match.)

Yes, the antenna was suspended in the air about 60 cm from the floor.
The balun was put in a one sided PCB. The ground is insulated from the copper foil.


If you use 75 ohm cable the match is about 1:1, so any reasonable length is ok. Matching with 75 ohms at the antenna is your best option.

Also there is way too much copper around that antenna That's probably a big part of the problem. Use a perf board with no copper and the coax should be at a right angle to the dipole elements.

"reasonable length" stands for any multiple of quarter wavelength? (rememeber that I want to balance the output).
I removed the cooper around the antenna and the results were the same.

Using the VNA I did an experiment. I started to cut the monopoles (pieces of same length at both sides) and see what happened with the log magnitude graph. When I had monopoles of about 8.4 cm I obtained an almost perfect antenna for 975 MHz. ( ~ -30 dB, 51 Ohms, SWR ~ 1.02) and it was the only min peak in the entire range (from 1 MHz to 3 GHz), then I cut pieces of 16.8 cm to obtain, in theory, an antenna for ~ 500 MHz but I got the same poor results as before.
I really don't know what's going on.

I will try to use a cylindrical pieces of cables as monoples and see what happens.

Also, I'm still wondering how I can check if an output is balanced or not.

Thank you.

Best regards.
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G8HQP
Member

Posts: 120




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« Reply #11 on: January 18, 2013, 08:21:55 AM »

A crude test is to touch the cable, or move your hand along it. If this affects the results then the balun is not working too well.

As others have said, you need to get rid of all the copper near the feedpoint. This adds capacitive centre-loading so will tend to move the resonance upward in frequency and raise the impedance.
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MERSAULT
Member

Posts: 21




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« Reply #12 on: January 18, 2013, 10:56:41 AM »

A crude test is to touch the cable, or move your hand along it. If this affects the results then the balun is not working too well.

As others have said, you need to get rid of all the copper near the feedpoint. This adds capacitive centre-loading so will tend to move the resonance upward in frequency and raise the impedance.

I touched the cable and the graph varied a little. If I move the cable near to the conecction point to the PCB the graph varies a lot.ñ
I removed as much cooper as I can obtaining the following board http://i49.tinypic.com/2a4xs93.jpg. Nothing changed.

Regards.
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WB6BYU
Member

Posts: 13026




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« Reply #13 on: January 18, 2013, 11:17:40 AM »

When you shortened the dipole, did you see the point of low return loss currently
near 500MHz move up the band?  Did the 1GHz dip move? 

I think the response around 500 MHz is due to your dipole, and the one near 1 GHz
may be due to the coax length or some other resonance.  If this is the case, then
the lower dip should have moved upwards as you trimmed the antenna but the
higher frequency one stayed the same.  (There also might be other resonances
at work.)  The 500 MHz resonance doesn't dip to a perfect match due to some
other reason:  if you are seeing some variation in behavior as you touch the coax,
that would suggest poor decoupling, which will give a less-than-perfect antenna
and may be the cause of the mismatch.

The ~14cm element lengths should be close enough for this purpose, since you
are sweeping over a wide frequency range.

You might try removing the connection between the transformer center tap and
the ground side of the incoming feedline to see if that makes a difference.

One way to check for balance is to jumper the ground side of the coax to each side
of the antenna in turn:  if the antenna is properly balanced, the effect should be the
same regardless of which side you connect it to.  A slightly less extreme test that
you can run just the balun by itself is to connect three equal resistors in series
across the output and connect the jumpers to one or the other side of the middle
resistor.

If adding a ferrite bead choke around the coax at the feedpoint makes any difference
in your plot, that would also indicate imbalance.  One thing you could try would be
to thread a bead over the RG-174 and slide it up and down the cable:  if the position
doesn't make any difference, you probably have good decoupling.

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