Jeff asked about adding a 'Balun' to a simple dipole, so I thought to disertate a little once more on the subject...
The purpose of a Balun is to allow the connection of a coaxial feed line ( unbalanced feedline) to a balanced antenna , such as a dipole.
Picture at left is a Circularly polarised turnstile antenna. Although this is for 75ohm coax, similar strusture are possible for 50ohm as well. Significant in this particular design is the fact that the 75ohm phasing cable section serves the function of the Balun as well.
What we are trying to achieve with a Balun, or its electrical equivalent, is to prevent common mode currents from flowing down the coax feedline. These currents are induced when a dipole is simply connected directly to a coax feedline, one element to the inner conductor, and one to the outer. The currents flowing in the dipole elements result in the electromagnetic wave being generated and radiated by the dipole elements. However, the current flowing in the element connected to the coax outer sleeve has to return to the generator ( the transmitter) and so does this by flowing on the outer sleeve of the coax cable. This current induces radiation from the coax outer sleeve, thereby distorting and destructively interfering with the dipole radiation pattern. This current can also return all the way back to the transmitter, and induced energy flows in adjacent cables and looms - In a small UAV with wiring close to everything, this sometimes manifests as servos twitching in sync with the video or datalink transmissions, etc, among other phenomena.
The balun on the left could be used to feed a folded dipole. Such a dipole has a feedpoint impedance of around 300ohms, and this balun exhibits an impedance step-up ratio pf 4:1. SO a 75ohm feedline would result in a good match to the 300ohm dipole, with a unbalanced to balanced trasformation in the deal.
However, there exist a number of alternative 'baluns' that can be used where an impedance transformation is not desired, a sort of 1:1 Balun.
Note that these are not really baluns in the true sense, but actualy perform the same duty by acting as a choke for the RF currents flowing on the coax outer sleeve.
The left image is often referred to as the 'Bazooka' Balun. It is a 1/4 wavelength of tubing, snugly fitted over the coax sleeve insulation, with the bottom end of the tube soldered all the way around to the coax sleeve braid. The top end of the tube is open and insulated from the rest of the antenna. This works by the 1/4 wave section forming a short circuit to the flowing currents at the base, and a high impedance at the top, choking of said currents.
The following are variations of the theme:
Where in the Bazooka Balun a tubular sleeve surrounds the Coax, a single 1/4wave length of conductor can be substituted in the following manner.
The Bazooka balun is preferred and is more efficient.
In order to not distort the antenna radiation patterns ans not cause EMI with other on-board electronics, it is always desirebale to use a Balun type feed for balanced antenna such as dipoles, Turnstiles, etc. The examples shown can be used with 1:1 and 4:1 impedance match for all dipole types.
The Nampilot.
Comments
Iskess, what antenna are you referring to and what is its construction, coax diameter, etc. Can you post a photo and give some info/dimensions? or refer to a website where I can find it?
I do realize that my posts are not DIY recipes; they were never intended to be. As I siad in the first series, it was to point out the pitfalls in antenna stups, so that when someone goes out to purchase the latest 'blue-Diamond' -'Golden-Ray' Zap Antenna, he can at least be wary of its claims and associated Snake Oil.
I have also previously posted photos and short descriptions of the Split Sheath balun Turnstile and the Skew Planar wheel Antenna I use on 2.4GHz for video. As also indicated I achieve 15km line of sight range with perfect video, and 10km range with the A/C in any attitude. Also posted were some photos and descriptions of the 868MHz datalink antenna ( the dual ring/slot antenna) with which I achieve over 20km range, line of sight, with 300milliwats.
I find it difficult to try and post a DIY recipe as I cannot see how these antenna can be made to work well without having the test equipment to test and trim them. On the Turnstile, a difference in length of 0.5mm of the elements changes the antenna from a good one to a mediocre one. If you use only SWR to trim the pictured turnstille, you can achieve a good SWR by trimming both elements, but they will not necessarily each be of the correct length relationship to ensure good circularity - you need a Vector Network Analyser to measure the phase difference between the two to get the lengths correct. And no two that I build are exactly correct from the same recipe are exactly correct - simply a bit more solder at the feedpoint junction between inner and outer tubes affects the tuning and circularity... I also know that you cannot simply build such an antenna from a simulation..
The Skew Planar wheel also has it issues in tuning - It is easy to tune for 1:1 SWR, and element lengths and bend points are not that critical, within 1 or 2 mm.. However, the circularity of the pattern is critiacly dependant on the angles and relationship between the 4 skew dipoles. Flattening the angle by 10 degrees can reduce the circularity by 1/2..
Dipole and simple vertical antenna are easy cookbook candidates without the need for fancy test equipment, but I do not know of a simple way around this..
I have been accused of snobbery and dissertationism by some, I am sorry for that! That is not the intention at all.!
I suppose I am doing something right if my stuff seems to work - how to help others to do the same is my dilemma!
Nampilot
Iskess, yes that is exactly what the Bazooka Balun is. Then length of the tube is important. If you have a very snug fitting tube with no practical gap between its inner diameter and the coax outer insulation, you need to compensate the tubing length by the velocity factor due to the dielectric constant of the sheath.
The Velocity Factor VF equals the reciprocal of the square root of the dielectric constant ( permittivity factor) of the material. Polyprop is around 2.2 to 2.4, Teflon around 2.1, etc. So compute a 1/4wave length and muliply by the VF.
If a tubing diameter is chosen such that there is a gap between the tubing inner and coax sheath of at least 1/2 the coax diameter, then a straight 1/4 wave long tube with no VF correction will work just fine. Also best to use Teflon type coax for this as soldering the tube to the coax outer shield takes some heat and damaging the cable inner insulation at that point will be as bad as no balun at all...
Nampilot
I guess there is no ferrite bead that fits?
Yes, the ferrite and 'wrapped' coax chokes also serve the purpose. I did not mention the 'wrapped' type because is becomes increasing difficult to make a small radius coil from the coax at the microwave frequencies. This coil of coax needs to be as close as possible to the end of the coax, at the antenna location. RG317 is typical of the smallest practical diameter cable used for these applications - about 2.2mm diameter - smaller diameters have even greater losses as GHz frquencies. The coax when wound into a choke coil should not be bent to a radius less than 8 to 10 times the cable diameter to prevent deformation of the inner insulation. That would make a coil of around 20 to 30mm diameter, making it a little unwieldy to mount, etc. At least 10 to 12 turns would be sufficient.
The Ferrite ring choke is effective, but ferrite with the correct permeability must be used. Therein lies a dilemma.
At lower frequencies, 10MHz to 30MHz, high permeability ferrite can be used to good effect, resulting in fewer ferrite rings being required. A dipole at 20MHz would use maybe 30 to 50 such ferrites to be effective, with a permeability of u=2500 ( AMIDON Ferrite type 73). The high u factor implies a high inductance ( so fewer rings) , therefore a high reactance against the current flow - a good 'choking' effect. This high u factor is useless at GHz frequencies, and the permeability has to be reduced to be effective. reducing the u factor also reduces the inductive reactance and so more ferrite rings are needed..AMIDON type 43 (u=850) ferrite rings are good up to 5 or 6 GHz.
A further problem as we go up in frequency is that the ferrite beads need to be a very close fit on the cable. The gap between the ring and the cable should be less than 5% of the cable diameter. A ferrite choke made up from 100 rings, with an inner diameter of 5mm, fitted over a coax cable of 2.2mm outer diameter WILL HAVE NO CHOKE EFFECT AT ALL! So, ferrite ring chokes are not so simple at microwave frequencies I fear. AMIDON ( a large USA Ferrite mnfr) does not have snug fitting sizes for the smaller coax cables typical in these applications. They have a 1.6mm ID, and the next up is 5mm ID...
Bellow are ferrite choked cables for the HF frequencies ( sub 30MHz)
Here is an air cored coax choke, also for HF frequencies.
As Walter Maxwell stated in the book Reflections the I 3 current can do funny things on a coax shield going up a tower using a directional antenna. There are current baluns and voltage baluns. For a 1 to 1 I use ferrite beads or wrap the coax. The lower the Freq the more wraps needed.
Great, as always, thx for teaching is good antenna basics.
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