Balanced and Unbalanced Antennas, to Balun or Not?

[Kage]

Since this topic seems to be coming up often lately I want to describe a few things here to clear up any misinformation.

First lets get this out of the way..

Balun = Balanced to Unbalanced transformer.
Unun = Unbalanced to Unbalanced transformer (outside the scope of this text).

The below information applies to all frequencies from shortwave up to FM broadcast band...

A balanced antenna like the basic 1/2 wave Dipole is "balanced" because it is what we call center fed.
The more complex explanation goes into antenna theory and how the AC cycle of the RF waveform is radiated relative to phase relationships and so on, but lets keep this simple!

This means that directly in the center of the two elements is where we connect the feed line (coax or twin lead wire) for a balanced antenna.

Unbalanced antennas like the 1/4 wave Ground Plane or whip antennas mounted on metal surfaces (like a car) is fed at the end under the radials or metal roof.

The simplest way to imagine this is that a balanced antenna is literally balanced at the center of the elements. Much like if you took a wooden dowel and tried to balance it on your finger horizontally so that it doesn't tip over and fall.
Unbalanced antennas obviously are fed at one of its ends or under its ground plane and is thus called end fed, or off center fed.

Now here is where things get complicated..

When trying to push an unbalanced line (coax) into a balanced antenna with no Balun to absorb the cycle of current not going out to the balanced system, you get that current traveling down the coax instead. That power has to go somewhere. In the case of a Balun it is absorbed in the coiled field and not improperly radiated, while also helping to push as much signal as possible out to the balanced antenna.
When not using a Balun in this configuration... well you guessed it. Your coax cable becomes part of the antenna and any change in coax length, coax near metal objects, or even grabbing the coax will throw your antenna out of whack and cause high SWR because of reflected signal (your body being the reflector of signal coming from the coax itself!).

To make more confusion...

A J-Pole antenna is not an unbalanced antenna, even though it is fed at the end visually.
A good section of the coaxial cable will radiate unwanted energy potentially screwing up your radiation pattern if you don't use a Balun.
This is because the J-Pole is actually a Dipole in disguise.
The top section of a J-Pole is the 1/2 wave dipole literally connected together disguised as one long tube element. The bottom U matching section is the other 1/4 wave matching stub.
If I remember correctly the way this works is a simple matter of phase matching. The bottom U section doubles as a phase match and a impedance match allowing the signal to be pumped up the dipole 1/2 wave section with phase shifted so that both 1/4 sections of the 1/2 wave pole don't cancel each other out and thus act just like the center fed dipole would, with a bit of trickery.
I know that sounds complicated as hell. It may take a few reads to understand what I am describing. Antenna theory is complex stuff!

A current balun is a simple device. It can be made of 1/2 wavelength of coaxial cable coiled up either on PVC or in an open air loop.
I personally prefer smaller loops like that which can be had around PVC pipe since it creates less stray radiation and is more sturdy construction.
Another way to achieve this in even less space is by using RF choke toroidal cable snaps right below the coax jack.

The usual simple dirt cheap way to make an open air balun for FM broadcast is just coiling 4 to 6 turns of coax about 6" diameter keeping it perfectly coiled so not to have overlaps.
Lower frequencies will require more windings of coax.

It's a simple device to convert a balanced antenna (like a Dipole) to an unbalanced feed line (like Coaxial cable).
If you're brave enough to use twin lead line or ladder line then there is no need for a balun with a balanced antenna since twin lead is naturally a balanced cable.

When using a balanced antenna with unbalanced feed lines YOU MUST USE A BALUN when transmitting.
The reason is simple. If you don't use it, then you will get what is known as Common Mode Current that will flow along your coax. Essentially this means that not only will your antenna be radiating your signal, but so will your coaxial cable!
Obviously this is NOT wanted as it will really screw up your radiation pattern, cause high SWR which can eventually blow up your final transistors, and not radiate your signal nearly as effectively limiting your transmission range.

Different types of antennas have different characteristics on how they want to be fed. The major difference being unbalanced or balanced fed.

Antennas with lower slanted or horizontal radials almost always don't need baluns. The reason is because the vertical element is still technically end fed. The radials simply act like a reflector projecting the signal outwards at an angle towards the horizon, but w/o acting much like a reactive part of the aerial.

There is a lot more antenna theory behind all of this and I could write a book if I were to continue but those sort of books already exist.
Just trying to give a simple antenna theory for dummies guide on this.

Here's a few popular antennas off the top of my head that I would say needs a balun choke or not...

Antenna Balun choke?
----------------------------------------------------
Dipole (Standard 1/2 wave) YES
J-Pole (Standard 3/4 wave) YES
J-Pole (5/8 wave with radials) NO
Super J-Pole (stacked 1/2 wave with phase stub) YES
Ground Plane (1/4 wave) NO
Ground Plane (5/8 wave) NO



Baluns can cause their own set of issues...
Depending on where you mount the Balun it can effect some antennas more than others.
The J-Pole is notorious for this and is in my opinion the #1 reason why most people have issues.
Because of the reactive action of phasing in a J-Pole amongst near objects and even it's own balun, the whole antenna requires careful placement to operate at it's peak performance.
Minimizing how much the balun radiates or interacts with the J-Pole sitting above it can make a few decibels of difference in radiated strength from my experience.
This is why I highly recommend keeping the balun small and DIRECTLY under the bottom of the matching stub.



I'll think of more to add to this later in a part 2 or something.
Anyone else is free to add information or corrections.

[Kage]

Examples of (looped open air) Baluns using coax for FM Broadcast Balanced Antennas:

Open air coiled balun for a J-Pole (I recommend keeping the coiling spaced so not to overlap like in this example)..


Example of a balun coiled around PVC at the base...


Balun for 300ohm closed Dipole antenna being fed with 50 ohm line...
Note this is used often in professional LPFM station setups...

[radioguy]

Kage,

Good info on baluns!

I'd like to mention something along this thread that might simplify matching to antennas like the J-pole. After lots of study concerning verticals, I've come to the conclusion that they are best fed indirectly rather than with a direct connection. I believe it makes for greater ease in matching and less feedline radiation. I'm presently feeding the base of my ground plane via 2 coils consisting of a two-turn tightly coupled link made from solid #14 housewire. The diameter of the coils are about 3 inches, but it isn't critical. The feedline is connected to the secondary coil directly, not to the primary antenna coil.

I feed my GP indirectly with 300 ohm foam TV transmission line, but coax could be used also. I use the balanced line because I don't have to use a balun in the feedline near the antenna. The indirect coupling makes the output of the GP transformer a balanced input for my feedline. I can move around in frequency without making antenna adjustments... only tuner adjustments. My tuner is very low loss and so is the line. The increase in swr doesn't reduce the radiated signal by any perceptible amount as I move across the band, but that's a story for another time. This is about baluns and feedline radiation.

If one uses coax to connect to the secondary coil, I would use a choke balun at the 1/4 wave distance from the antenna loop. This is of course a physical 1/4 wave distance, not a shorter distance corrected for coax dielectric.

If I was going to use a j-pole, I would place a two-turn coil in the "fork" of the j-pole and feed the coil the same way as described above. The coil would inductively couple well to the j-poles low impedance "loop" formed at its base.

I really think the indirect coupling method results in a much cleaner antenna pattern, maintaing the low angle of radiation the vertical is capable of, as well as a better way of minimizing common mode currents over the direct connection method.

Your thoughts?

[Kage]

Nov 20, 2012 19:20:19 GMT -6 radioguy said:

I really think the indirect coupling method results in a much cleaner antenna pattern, maintaing the low angle of radiation the vertical is capable of, as well as a better way of minimizing common mode currents over the direct connection method.

Your thoughts?

I heard about that method somewhere else on the web but can't remember where I read about it.
I have never tried that so I can't really say much on the matter.

Anything to make the J-Pole less touchy is a plus, but I find my setup works almost flawless*, however that was after pulling out my hair and saying a few choice words while tuning it up outside.

Maybe if you get the time you could take some educational pictures of the section on your antenna where you did this and explain how to replicate it?
If so please make a new thread on it. I'm sure lots of others here would be very interested including myself!

* I should note that using an open air balun made with 4.8 feet of RG58 coax (1/2 wavelength of cable) wrapped closely around 1 1/4" PVC pipe and having it directly below the bottom of the J-Pole joint worked best for me so far.

[RFBurns]

One way to minimize the return currents with the loop is to also have a "drip" bridge...ie run the coax from it's connection point away from the J-Pole at a 90* angle about 6 inches, drop back down to your coil another 6 inches down from the drip bridge, and secure the loop to the mast so it won't dance and swing in the wind.

Peace!

[radioguy]

Here's a sketch that will explain indirect feed of these antennas. "Tight coupling" of the air-core windings on the GP is achieved by simply ty-wrapping the two two-turn coils together.
A good source of info is the RSGB (Radio Society of Great Britain). A number of years ago, I came across their equivalent of the ARRL's Radio Amateurs Handbook. It had a lot of info on the best ways of feeding a vertical antenna. If you can get access to their antenna literature, you'll find it worth the time of research.

Attachments:

[Kage]

Great info.
I may have to rig up a slimjim just to test that idea and see how well it works out compared to direct connection when I get the time to play around.

[radioguy]

Kage,

Good! I'd like you to confirm the method on the slim-jim. I really think you'll find the indirect coupling method will save a lot of tune-up grief, among other benefits.

I'll be awaiting your results, but I understand this is the holiday season and your experimental time may be limited.

[Kage]

Dec 3, 2012 17:59:33 GMT -6 radioguy said:

Kage,

Good! I'd like you to confirm the method on the slim-jim. I really think you'll find the indirect coupling method will save a lot of tune-up grief, among other benefits.

I'll be awaiting your results, but I understand this is the holiday season and your experimental time may be limited.

Well it's not so much that my time is limited, it's more a matter of having so many other electronics projects on the back burner ;D

[radioguy]

"Well it's not so much that my time is limited, it's more a matter of having so many other electronics projects on the back burner"

Good for you! it's fun having lots of projects on the burner. You sound like a version of myself 25 or more years ago! ;D Now, my get up and go has mostly got up and went. I still have good ideas but not a lot of energy or time to follow through.

I've scaled back with age and only do one project at a time, but I'll still "play radioguy" till I die.

[retx]

Dear Kage,

And what about for MF?  I wonder whether many actually use "ununs" between the antenna, typically an inv. L type or a simple Vert.

As far as MF and inv. Ls are concerned, some just use an air core var. capac. between the lead in and the tx given the Z of this sort of Marconi which is in the vicinity of 50 Ohm.  I mean, no coil, nothing, just the capacitor which IMHO may not work.

Others use a tuner w/o a coil, others a tuner+coil.

So, would a unbalanced to unbalanced xfrm be irrelevant?

Look forward to reading your comments.  Thanks in advance.

Best 73,
Carlos.  

[Deleted]

Hi,

When it comes to MF, unless you have something with a high radiation resistance, like an Alexanderson array, or an antenna 1/2 wave or more tall (for verticals), or a horizontal less than 1/2 wave in height, you might want to check out impedance transformer un-un's or just work with a straight tuner. These will have the best efficiency with electrically short antennas.

Cheers!

[retx]

May 13, 2013 1:24:45 GMT -6 @cmradio said:

"When it comes to MF, unless you have something with a high radiation resistance, like an Alexanderson array, or an antenna 1/2 wave or more tall (for verticals), or a horizontal less than 1/2 wave in height, you might want to check out impedance transformer un-un's or just work with a straight tuner. These will have the best efficiency with electrically short antennas."

Dear cmradio,

As far my installation is concerned, and for now, I am using a once idle receiving only antenna consisting of a 45 m long inv. that I am still to hang at the very top of the 6 m tower adjacent to the roof instead of just at a point between its middle and the top, and then prepare a convenient lead.

From the drawing I am enclosing in this, I know the lead in is far from ideal, but I wanted to experiment.  The sort of lead is a 50 Ohm cable that has the braid & the inner conductor soldered together, then goes to the house and into the antenna tuner I built.

Any comments or help will, of course, be appreciated.

Best 73,
Carlos.

The overall antenna is like this URL img690.imageshack.us/img690/6655/invlontower.gif:




The antenna tuner, URL img834.imageshack.us/img834/3153/invltunera.gif:

[Deleted]

Oh, a Windom!

Yeah, you can balun feed those.
Some info:
www.packetradio.com/windom.htm

Cheers!

[retx]

cmradio,

I believe the drawing I prepared mislead you: the sloping steel guy wire (the longest of the 3 this towr has [the remaining 2 are anchored on the roof]) is being used merely to enable me getting a vertical lead from the window up.

You may see there's a short wire going from that lead to the 45 m of [horiz.] wire which is acting like a capacitive top, or hat as Engl. speakers call them.

That's why I said the installation is far from the ideal, so it does need improvement, if not for a better tx, then at least to squeeze the max. out of the tx I am using now, a very modest two valve "iTx" set I'm sure you know about.

No, this isn't for "pir. radio", nor any tx I may use in the future will be which is why I'll be happy with modest power. In fact, the RFsource amp. we've talking about elsewhere in this forum has far more power than I'd like to use.

So, finally, what I really must do is to improve the sort of ant. tuner I have after modifying the inv. L, perhaps by placing a coil/inductor on the roof.

Best 73,
Carlos.