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
Hi Iskess,
you are correct in all cases! It will work as the balun for your dipole. In your second case, you will simply change from a sleeve dipole to a 'normal' two element dipole. And yes, it will be no better or worse than the 'rubber ducky'. all these are simply dipole, all with similar gains and radiation patterns.
The Nampilot
It's already the correct size and shape for the band.
Alternatively, could I remove it from the sleeved dipole, turn it around backward, slide it back on to the rubber ducky, solder at the bottom, and solder another wire the same length as the active element where the sleeve used to be soldered.
Now I have a dipole with a balun.
Or would this be just as good as the rubber ducky?
Thanks
Iskess, yes it is. The sleeve of the sleeve dipole forms the balun AND the 2nd dipole element, and is connected to the coax braid at the point where the inner coax wire exits the core.
Nampilot
@Martin - Collins used to make good mechanical filters...for SSB anyway..
Nampilot
Hi Jonathon,
Not an oversight actually..The coaxial sleeve dipole is merely the dual of the 'conventional' Bazooka Balun dual element dipole.
There is absolutely no difference in bandwidth between the two types. In both cases the elements lengths are as 'critical' and in both cases the requirements re velocity factor apply. In the sleeve dipole the top element is normally simple the coax center conductor extended. This is also normally left with its insulation intact, which means its length must be compensated for by the insulation's permittivity. The same reasoning applies ( as in my earlier comments) re the Balun tube , its lenght and its diameter in relation to the coax outer insulation. So, the 'tuning' of the two types are equally simple, or complex, depending on your take, and performance is the same. Richard Johnson's Antenna Engineering Handbook gives a good story on these types regarding bandwidth.
The Sleeve dipole bandwidth can indeed be increased, by changing the length relationship of the balun part to the center conductor radiator, ---OUTSIDE OF THE OPTIMUM---. Bandwidth increases of up to 20% are possible. The price you pay is that the radiation pattern changes, and sub lobes begin to appear. Since these changes cannot easily be measured or quantified by the DIY'er it does in fact make this antenna more difficult to construct effectively. Hence my ignoring it...
Location in a small UAV is also sometimes not easy, since the coax feedline really needs to extend in line with antenna for at least 1.5 to 2 wavelengths, to prevent distortion of the radiation pattern. For a linearly polarised antenna in a UAV, vertical polarization is normally preferred, which makes locating the sleeve dipole awkward due to the combined coax and antenna length. Note the for the convention two element dipole a similar rule applies ; the coax should extend at right angles for 1.5 to 2 wavelengths from the antenna..
And finally, I am sure most folk have used a sleeve dipole at some point - those litle 'rubber-duck' style antenna on 2.4GHz, etc, are just that:
The Nampilot..
I can't believe you went into all that detail and didn't mention that the sleeved dipole is its own balun, and a heck of a lot more wideband and easier to tune than anything you've diagrammed here.
I use torroids today to choke emi and rfi as close to the GPS electronics as possible. Sometimes you will have them on ESC pwm leads from manufacturers as well. This helps isolate interference as a filter. Adding a capacitance can further filter out noise on power lines if needed. Ring noise capacitors on boards are used for this purpose. Line lengths and traces can become resonate at 1/4 wavelength and some other multiples. The idea is to detune the leads to not it to grab say a 900 MHz video xmission and feed it to the GPS electronics. An antenna is a tuned circuit resonating for a designed frequency.
Great, very useful and interesting.
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