Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

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Airwind
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Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by Airwind » Fri Apr 26, 2019 11:23 am

Hey, I was wondering about a few things on this topic... Suppose I wanted to wire up a DIY 433 MHz yagi with a folded dipole driven element. Technically speaking, what elements are needed to "complete" the antenna and for its good operation?

Looking at the docs and designs I came up with this list:
  • An impedance matching network (1:4, LC, hairpin/beta, etc)
  • A balanced to unbalanced converter
  • A common mode choke
  • Anything else I missed?
Impedance matching
I read online that a folded dipole impedance in clear space is about 280 to 300 ohm, but when put as a driven element in a yagi, the impedance drops to near 200 ohm. This is why there is normally a 1:4 impedance transformer present to lower the impedance to 50 ohm and match it with the feedline. I'm not so convinced about this, as I read the impedance is actually a complex number, so some measurements and adjustments with a VNA are in order. To tune an antenna, the resistive (real) part of the impedance should be as close to 50 ohm as possible while the imaginary part should be close to 0, so some capacitive/inductive elements may be required i.e. a LC network. A set of series/shunt inductors/capacitors can be added at feedpoint to finetune the impedance, and a good resource I found is this interactive online smith chart:
https://www.will-kelsey.com/smith_chart/

Balun
This is where I'm confused most. What exactly is a balun? Because different people have different representation for this keyword. In regards to a yagi, it is said that a 1:4 balun is needed, but is it really a balun or is it just a 1-to-4 (or 4-to-1) impedance transformer? Which devices do actually perform balanced-to-unbalanced tasks?

Common mode supressor
When connecting an antenna to a coax feedline, an electrical connection is made to the cable outer shield. In an ideal situation the currents in the inner coax conductor and the outer sheld are equal and opposite and the cable doesn't radiate because the effects cancel each other out. But quite often there will be some residual currents present on the outer shield and this will cause the feedline to radiate and become part of the antenna. This is undesired because it often worsens the antenna efficiency, so normally a certain type of devices (1:1 current baluns or common mode chokes) are installed to minimize this effect. I've seen some ferrite core windings being used, but those are mostly useful for lower frequencies i.e. HF; they're not so good for UHF. For my antenna I thought about using abazooka/sleeve balun, but here we are again calling this device a balun. Is it really a balun or just a common mode choke?

Thanks for any comments...

snn47
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Re: Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by snn47 » Fri Apr 26, 2019 4:30 pm

BAL UN or
BAL(anced/ UN(balanced) is needed when you want to connect an unbalanced/asymetrical cable/antenna (e.g.coax) to a balanced/symetrical cable/antenna e.g. a halfwave-dipole.

Not mentioned in the name is that BaIun can also provide impedance matching, eg. 1/4 to connect a folded full Lambda Dipole of ~240 Ohm to 60 Ohm coax.

UN UN or UNbalacend/UNbalacend or provides just impedance matching when the impedance differs.

BALUN and impedance matching can be done with ferrite single ring or double double ring (two wholes) which can provide a high bandwidth, while coax-cable or stripline design are quitenarrow using printed circuit boards.

A Yagi is one design to provide directional antenna pattern and gain and consists of amin. of the driven 1/2 or folded Lambda antenna, a director and a reflector, but can consist of 13 or more additional elements. The bandwidth is compared toe.g. logarithmic periodic antenna designs with about one octave smaller but can reach higher gains.

I hope that helps a bit to understand this complex material.

HighSNR
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Re: Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by HighSNR » Fri Apr 26, 2019 7:50 pm

The impedance should be approximately real as the length has been tuned to make it resonant(XC = XL). If possible it is always better to tune an antenna this way rather than rely on a matching network of L's and C's to shit it round the smith chart. Sometimes (particularly with small antenna) this is not possible.

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Airwind
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Re: Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by Airwind » Sun Apr 28, 2019 10:22 am

snn47 wrote:
Fri Apr 26, 2019 4:30 pm
BAL UN or
BAL(anced/ UN(balanced) is needed when you want to connect an unbalanced/asymetrical cable/antenna (e.g.coax) to a balanced/symetrical cable/antenna e.g. a halfwave-dipole.
EDIT: After reading some more material online it seems like I was wrong about this one... Like I said, people will often call a number of things a balun, hence part of the confusion. Apparently a balanced or unbalanced line refers to the voltage on the fedline, not the current. So when talking about a 1:1 current balun we're wrong to call it that because it doesn't really do any impedance/voltage transformations - it just reduces the common-mode current, so it should more appropriately be called a common-mode choke. If currents are equal and opposite, the feedline doesn't radiate.

This answers my question about only using a common-mode choke as the balun, which is not enough. A yagi apparently needs a common mode choke and an additional 4:1 voltage balun (i.e. a 1/2 lambda loop) that does some impedance transformation and converts a balanced feedpoint to unbalanced feedline. Not all baluns are impedance transformers and not all impedance transformers are baluns.

Now I will try to adjust the impedance to 50 ohms real via the VNA, but might use a LC (i.e. adjustable cap or inductor) network for finetuning.
HighSNR wrote:
Fri Apr 26, 2019 7:50 pm
The impedance should be approximately real as the length has been tuned to make it resonant(XC = XL). If possible it is always better to tune an antenna this way rather than rely on a matching network of L's and C's to shit it round the smith chart. Sometimes (particularly with small antenna) this is not possible.
Oh? What is the difference between adjusting a length and applying a matching LC network?

I mean theoretically speaking, matching an antenna means making it resonant at particular frequency. To achieve this one has to "modify" the antenna in a way to eliminate the imaginary part of its impedance (reactance) and make its real part (resistance) as close as possible to that of the feedline i.e. 50 ohm. Since inductance and capacitance are always present, they cannot be eliminated, but they can be adjusted in a way to cancel each other out.

So basically all antenna parts have their own capacitance/inductance and they behave like capacitors and inductors accordingly. By adjusting the length and distances of certain elements one can imagine they're basically changing the values of existing capacitors and inductors in a given LC network.

So why is adjusting lengths better? Is it just out of principle or is there some technical reason to it?

snn47
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Re: Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by snn47 » Mon Apr 29, 2019 7:54 pm

Get a copy of the ARRL or RSGB antenna handbook and you have the Basics in a simple form.

If you use a transmission line that has the same impedance as the antenna and is symertical in case of dipoles you don't need any BALUN or UNUN.

The same is true for asymetrical antenna designs like ground plane of sleeve or coaxial antenna that can be connected directly to coax which have the same impedance.

For wide band operation a BALUN or UNUN with and without impedance transformation use a ferrite core.

For narrowband use in principle almost any transmission line, asymterical and symetrical can be used for impedance matching or providing a BALUN.

Other factors are what power handling capacity is needed or at which frequency they are to work.

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Airwind
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Re: Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by Airwind » Sat May 04, 2019 11:07 am

snn47 wrote:
Mon Apr 29, 2019 7:54 pm
Get a copy of the ARRL or RSGB antenna handbook and you have the Basics in a simple form.
Here's a snippet from the ARRL handbook 2017, chapter 21.2.3: Baluns
A balun can be constructed in a number of ways: the simplest being to coil several turns of coaxial cable at the antenna feed point. This creates inductance on the outer surface of the cable (the inner surface of the shield and the center conductor are unaffected) and the resulting reactance opposes RF current flow. There are other methods, such as the use of ferrite beads and cores, that are discussed in the Transmission Lines chapter.
Sigh... You see, this is why I get confused. They're talking about baluns, but what they really mean are common-mode current chokes. Baluns are really voltage transformers.
snn47 wrote:
Mon Apr 29, 2019 7:54 pm
If you use a transmission line that has the same impedance as the antenna and is symertical in case of dipoles you don't need any BALUN or UNUN.
I'm using a coax (unbalanced) to feed a yagi with a folded dipole driven element (balanced), so a balun is needed (and a common-mode choke out of good practice). The RX/TX and the coax feedline are 50 ohms, so the antenna needs to be tuned to match that.


Anyway, I'm attaching a picture to show my progress on the antennas. I'm not sure, if the different diameters of the tubes used in the driven element will have any noticeable effect on the SWR considering they are going to be geometrically symmetrical, but we'll see.
Attachments
yagis.jpg
DIY yagis
yagis.jpg (106.34 KiB) Viewed 1716 times

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Airwind
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Re: Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by Airwind » Fri May 10, 2019 6:54 pm

I finally understand what a balun is!
So I'm going to write this down in case some other confused soul comes along and tries to learn the same stuff. Anyway, what is a balun you ask? Well as it turns out I had the correct answer right underneath my nose all the time, but I was too confused with all the terminology that I somehow ignored it the whole time.

Definition of Balun according to Wikipedia (and a lot of other people):
"A balun is an electrical device that converts between a balanced signal and an unbalanced signal."

That's all there is to it. It may or may not also be an impedance transformer, but a balun really is just a device, a converter between balanced and unbalanced signals or feedlines. Finally the next big thing to understand are balanced and unbalanced feedlines - what are they exactly? Most people will probably imagine that feedlines like twisted pair, ladder line, twin lead, etc. are always balanced lines while a coaxial cable would always be an unbalanced line, and they are mostly correct. But the real answer again lies in the definition of balanced and unbalanced lines.

Definition of Balanced and Unbalanced according to Wikipedia:
"A balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. An unbalanced line is a transmission line (often coaxial cable) whose conductors have unequal impedances with respect to ground."

That's the thing. I.e. for a properly connected ladder line both conductors have equal impedances since the wires are identical (diameter, material, spacing, etc) and both have the equal impedance in reference to ground. Since this is true it is therefore a balanced line according to the definition. A coax is frequently used for unbalanced lines since its shield is normally grounded. So the center conductor will have some given impedance to ground, but the shield's impedance to ground will be zero because it well... it is the ground. Since the impedances to ground are unequal, this is therefore an unbalanced line.

A balun is a device that converts between the two, but may also perform other things like impedance transformation. Not all baluns are impedance transformers and not all impedance transformers are baluns. For example, consider the following image. See that one contact of the coil on the unbalanced side is grounded (so the impedances on both contacts will be unequal in reference to ground) while both contacts on the balanced side are floating. There is no electrical contact between the two coils. Although the center tap may be grounded in certain setups, the impedances of both outputs to ground are still equal.
Image

For a Yagi antenna a lot of people are recommending the 4:1 coax balun. This device is apparently two things in one. First it is a 4-to-1 impedance transformer (I read some theory behind this how the coax loop delays the signal so it reaches the other end out-of-phase) and second it is a balun. The balun function is apparently visible because the feedpoints have no connection to the ground (coax shield) Both are connected to the central conductor and they therefore have equal impedances in reference to ground - therefore a balanced signal.
Image

There is a lot more to this topic, but I'll leave it at that for now.
Also, current baluns = common mode chokes (not really baluns).

P-40 Warhawk
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Re: Antenna tuning, impedance matching, baluns, common mode currents, my head exploding...

Post by P-40 Warhawk » Tue Aug 20, 2019 3:01 pm

Gentlemen .
This why people bang their their heads on the desk . Their is a engineering term called K.I.S.S. it's a anachronism for keep it simple stupid . All the person would like to know is what kind of impedance matching device for a given antenna should be used . Now I would think he is talking about receiving only . So I would think the choice would be between impedance matching balun and isolating matching transformer. I am like this guy . With everything that is out their about impedance matching devices I find myself sitting in the corner babbling .

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