some links to info on baluns .

[BOOTY MONSTER]

http://www.k0bg.com/common.html
http://www.hamuniverse.com/balun.html
http://www.hamuniverse.com/testingswr.html
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=13273
http://www.dxzone.com/catalog/Antenn...d_Lines/Choke/

hopefully these help some .

[MoJo]

many tanks....

[Mr. 557]

Hey, I have never had any experience with Baluns. Can anyone explain to me what their applications are for exactly, i.e., can they be used in conjuntion with Gamma matches or do they need to be applied alone and how to go about calculation the size application in laymans' terms?

[MoJo]

Thats what I was getting at too when I asked booty to start this thread...

[BOOTY MONSTER]

this liknk will explain what common mode currents are and what to do about them .
http://www.k0bg.com/common.html

this link explains how to build a bauln .
http://www.hamuniverse.com/balun.html

i can cut and paste them here if yall dont want to click on the links . im too lazy to do all that typing when these links say it better than i can with A LOT LESS TYPING !! :lol::lol:

[BOOTY MONSTER]

this is cut and pasted from the following link
http://www.k0bg.com/common.html

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Common Mode Currents

Last Modified: Mon, Sun, April 19, 2009

Contents: Basics; Inadequate Ground Planes; What To Do; What If You Don't?; Closing Thoughts;

Basics

Common mode currents can flow in all sorts of systems, not just antenna feed lines. Since we're dealing primarily with mobile HF antennas which are fed with coaxial cable, we'll limit our discussion to them.

In an ideal world, RF flows down the outer surface of the center conductor of the coax cable cable, and returns on inner surface of the coax shield. When there is an imbalance in the antenna (for what ever reason), current will flow on the outside of the coax shield. This may not seem possible, but it's important to remember, unlike DC, RF current doesn't flow through the conductors, it flows on the surface of the conductors. The current which flows on the outer surface of the shield is called common mode current. In other words, it is the unbalanced current not returned within the coaxial cable.

This leads to a very important question. If the current isn't returned in the cable, where does it go? The answer is, it radiates! In fact, the amount of radiation from the coax cable is directly proportional to the common-mode current on that cable. This leads to another question. What causes common mode currents? Well, hopefully this article will answer that question sufficiently.

Inadequate Ground Planes

It is important to remember that a vertical antenna is one half of a dipole. That's why they're sometimes referred to as a monopole. The missing half is replaced by a ground plane typically made up of radials. Unfortunately, we can't use radials to provide a ground plane in a mobile scenario, so we rely on the mass of the vehicle, and the capacitive coupling the vehicle has to the surface under it. From an RF standpoint, the resulting ground plane is wholly inadequate, and as a result the ground plane losses are high.

Common text sources state that mobile HF ground plane losses average between 2 and 10 ohms (10 through 80 meters). The truth is, the average is closer to 5 to 20 ohms (respectively), and may indeed be higher. The more ground plane loss, the higher the common mode currents, and the more severe the RFI problems. It should be evident that minimizing ground plane losses are important, both from an efficiency standpoint, as well as curbing both ingress and egress RFI. Bonding is one way to do this. Proper antenna mounting is another. There is a little more technical information on ground planes here.

What To Do

Coax ChokeThe best way to control common mode currents is with a choke. You can use the same type of choke that you use for the motor control leads. That is, mix 31 split beads, and preferably the 3/4 ID units. They can be purchased from DX Engineering and others. These will allow 6 turns of RG58, and 5 turns of RG8X (as shown in photo). Note that the coax is not tightly wound around the choke. In this case, the diameter is about 3 inches. Any tighter, and the core could migrate and cause a short.

The choke should be installed as close to the base of the antenna as possible. The last place to install them is inside the vehicle. After all, we want the RF on the outside, not the inside of the vehicle.

How many beads you'll need is not a cut and dried scenario, but there are factors which need to be considered. Short, stubby antennas will always exhibit more common mode current problems than full-sized antennas. The main reason is, the amount of RF flowing in the portion of the antenna below the coil is concentrated is a much smaller physical area, than it is in a larger antenna.

Those nifty appearing, clamp type mounts seemingly are the rage, but the fact remains, they too add to the level of common mode current. Remember, the further away from the ground plane the base of the antenna is mounted, the greater the common mode currents.

There is one more major consideration. If the impedance of the choke(s) is too small, and the level of common mode current is high, it is possible to over heat the choke which could destroy the ferrite material. There is a hidden factor here. If indeed the level is high enough to destroy a choke, you best revisit your mounting technique.

What If You Don't

Certainly there will be RFI issues especially if you're using an automatic antenna controller. And, the resulting RFI might be doing funny things to the electronics. However, the biggest factor is what we're doing to our receive capabilities. If common mode current can flow out as a radiated signal, then other signals (common mode noise) can get in the same way!

What really matters is the S+N/N ratio (signal plus noise to noise) said antenna system can induce in our receivers front end. While receiver dynamics are important, common mode noise exacerbates the problem. Fact is, in a mobile scenario, the coax cable makes a better noise antenna than it does a signal antenna! It is, after all, inside the vehicle where most of the electrical noise is generated in the first place.

This brings up one more point. If you wish to minimize both common mode currents, and common mode noise, the requisite chokes must be mounted outside the vehicle! Remember this; Any part of the coax or control cable before the choke, it part of the antenna, and should be kept outside the vehicle.

Closing Thoughts

What ever RF is dissipated in the requisite choke(s), and as ground plane loss, goes up in heat, and is not radiated. Quantifying the amount of loss in any given installation isn't all that difficult. At the bottom of this page on efficiency is one methodology, but that only tells you what your approximate efficiency is. It doesn't tell you how good it could be if the antenna in question was better mounted, or how much better a full-sized antenna would be. Until you go through the exercise yourself, you'll have to believe me when I say, the difference can be as much as 25 to 30 dB.

In my Antennas, Commercial article, is a section on stubby antennas. I mention them again here, because they have become the leading cause of common mode current RFI. They're popular for a number of reasons, not the least of which is their ease of mounting. The hidden factor here is, the usual mounting methodology greatly increases common mode currents. If you're using one of these stubby antennas, it behooves you to read the article.

Home

[BOOTY MONSTER]

this is also cut and pasted but from a different source
http://www.hamuniverse.com/balun.html

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Build an All Band HF Air Core
1:1 Choke Balun
THE "UGLY BALUN"


A balun's purpose is to allow connecting a balanced, (e.g., a dipole or driven element) to an unbalanced line such as coax which is not balanced, thus the name, Balun. The 1:1 choke "balun" is not actually a balun. It's function is to help eliminate rf currents from flowing on the outside of coaxial cable using the principle of choke action.

In transmitting antennas, this is accomplished by presenting a high impedance (resistance), to RF currents flowing outside the coax shield. This forces currents in each side of a driven elements to be equal. This is especially important in beam antennas because it prevents distortion of the beam's pattern caused by unequal currents in the driver(s). In a simple dipole, the balun (choke), assures that the dipole, and not the feed line, is doing the radiating!

When you connect center fed antennas, like dipoles, V's, triangles, yagis, rhombics, loops and so on, to coaxial cable, unless care is taken, it is not difficult to end up with feeder radiation. Not only can the loss in power be quite significant, but the radiation characteristics of the antenna system will also be seriously compromised.
In laymen's terms, it won't be what you are expecting from the pattern of your antenna.
As the feedline becomes part of the antenna, currents can flow from the line into the mains and on TV cables, metal masts and yagi booms, causing a variety of EMI problems that can be very difficult to trace. Frequently these problems are simply due to unbalance - and the solution is the humble air choke.
If an antenna system is fed at center with a parallel conductor line (provided that correct installation procedures are followed) balance will be maintained, USING A BALUN, with currents in equal and opposite phase canceling each other out.

When the connection is to a coaxial cable, WITHOUT A BALUN, this cannot occur because currents flowing inside the cable from the connection to the inner conductor are separated from those flowing on the outside from the connection to the shield, and the result is unbalance causing feeder radiation. However, if the two electrical circuit elements (antenna and coaxial cable) are coupled using a balan, balance will be maintained.
Enter.....The Ugly Balun!.....

http://www.hamuniverse.com/uglybalun2.GIF

An Inexpensive, High-Performance, Ugly 50 ohm Balun
"Building a no-grief 1.8MHz to 30MHz 50ohm-balun is easy.!"

"No costly ferrite-cores are needed, just a short length of 3 to 5 inch size plastic pipe, about 25 feet of 50ohm coax plus some nylon cable ties.

Solid-dielectric coax is best for this application because foam-dielectric has a tendency to allow a change in the conductor to conductor spacing over a period of time if it is bent into a tight circle. This can eventually result in voltage breakdown of the internal insulation.

The required length of the plastic pipe depends on the diameter and length of the coax used and the diameter of the pipe. For RG-213/U coax, about one foot of 5 inch size pipe is needed for a 1.8MHz to 30MHz balun. For 3.5MHz to 30MHz coverage, about 18 to 21 feet of coax is needed. This length of coax is also adequate for most applications on 1.8MHz.

18 to 21 feet should cover all of 160 through 10 meters.
The number of turns is not critical because the inductance depends more on the length of the wire (coax) than on the number of turns, which will vary depending on the diameter of the plastic pipe that is used.

The coax is single-layer close-wound on the plastic pipe.

The first and last turns of the coax are secured to the plastic pipe with nylon cable ties passed through small holes drilled in the plastic pipe.

The coil winding must not be placed against a conductor.
The name of this simple but effective device is a choke balun.
NOTE: Some people build choke-baluns, without a plastic coil-form, by scramble-winding the coax into a coil and taping it together. The problem with scramble-winding is that the first and last turns of the coax may touch each other. This creates two complications. The distributed-capacitance of the balun is increased and the RF-lossy vinyl jacket of the coax is subjected to a high RF-voltage. The single-layer winding on the plastic coil-form construction method solves these problems since it divides the RF-voltage and capacitance evenly across each turn of the balun"....AG6K

Credit for this article goes to AG6K, Rick Measures and was edited from a Pre-copy version of another article titled "A BALANCED - BALANCED ANTENNA TUNER" published in QST,February, 1990.

[Crusher]

2 dencent books I picked up from the arrl website are, transmission line transformers and the other is, baluns and ununs. These books explain a lot and for those that are interested, even goes into some depth about splitters and combiners for amplifiers. Talks about ferrite and iron core. Mixes, size how to wrap and of the such.