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.
You seem to be implying that what I said is factually incorrect because I'm not displaying an attitude you approve of.
That particular logical fallacy is called ad hominem, an attack which is not in keeping with the Ham Spirit, either.
Most of the Elmers I know are humble individuals who answer questions directly in a way that novices can understand.
As long as we're hurling pejoratives:
You, sir, are not an Elmer.
(Before you write it, yes, I know that is not me. I don't claim to be.)
Wow, not much Ham Spirit left is you is there....I think this is the point where you are trolling and can only be ignored.
DO have Fun!
I don't think that document says what you think it says. It says that a poorly-sourced 11mm OD ferrite bead can be resonant at up to ~3 GHz. It is not resonant at higher frequencies than that. Also, larger ferrites and ferrites with higher "conductivity" (I think they meant "permissivity") will be resonant at lower frequencies. For any given ferrite, there will be some frequency above which resistance becomes higher than reactance, and it will no longer resonate, which is exactly what I said.
If you would like a reference on that, here is a white paper detailing the phenomenon using rnodern components and testing methods: http://www.digikey.com/classic/documentredirector.aspx?doc=http://w...
The fact that some ferrites resonate at different frequencies than others has nothing to do with proving or disproving what I said, and I'm struggling to understand why you think bringing it up proves anything.
I asked you if you did comparisons, and you threw a bunch of words at that, but I did not see any comparison, just badmouthing other builders and a lot of chest-thumping for yourself, but no data, no proof. 15 km at 900 MHz is nothing, especially considering that you don't mention power levels. If you just search YouTube you will find documented FPV flights at 50-100 km. You accomplished something totally mundane.
A true expert knows his entire field and can explain it, not just the things he personally wants to advocate. You wrote an entire article on baluns and never mentioned once why you need a balun. Balanced" vs. "unbalanced": did you ever explain what that actually means? Could you explain what that means to a novice, other than to repeat the rule?
You shouldn't have left the discussion of the sleeved dipole or chokes to the comments, you should have noted up front that those are the most-common baluns in use, even if they're something you don't personally prefer. And to just write them off as variants of the bazooka, then when called on it, change your story to say they're too inferior to the bazooka to even mention? Either its the same thing, or it's not. Changing your argument to fit the rhetorical needs of the moment is intellectually dishonest.
TL;DR: I don't think you're writing to be understood by your audience.
Jonathan, in case of difficulties in finding the text 'Analysis of Ferrite beads....."
here is just a short extract from that text, indicating issues with resonance.
Ferrite beads used for the supression of electromagnetic interference (EMI) were analysed using the finite difference time domain (FDTD) method. The presence of the bead results in reflected current, cable radiation and resonant characteristics which significantly affect the ability to suppress currents uniformly over a wide frequency range. A typical low conductivity ferrite bead (σ = 1 µS/m, ɛr = 1, μr = 100), with length L = 21 mm and outer diameter D = 11 mm is shown to be a poor isolator at 1.03, 1.71, 2.37 and 2.93 GHz due to this resonance phenomenon. The effective suppression of EMI depends on the properties and dimensions of the ferrite bead in addition to the frequency of operation.....
Joe ZS6JGN / V51NC
Well, I am not sure we need to confuse everyone even more, but lets tie some things up...
"Have you compared the performance of your designs to common commercially-available RC components?"
I would say, yes. If you just read some of the many comments by folk using the Ibcrazy Skew planar wheels, the 'crosshair' antenna, etc, you may have leaned how little improvement they have found from whatever else they may have been using. That is of course a generalisation, there are folk who find good results, I am sure. However, the designs I presented ( the Short backfire CP turnstile, the 'omni' CP turnstile, the 868MHz Jpole co-linear, ,etc) have been tested, patterns measured and performance in flight verified out to 15km line of sight. These systems are now in use at Etosha National Park (x2) here in Namibia, and next month two systems will be deployed in the Kruger Park in the RSA. I could not provide systems with shaky signal / antenna performance so some real effort was put into this to meet the guarantee requirements set by the clients. And I have measured pattern and performance data, a costly excercise, which I have yet to see for ANY of the off-the-shelf RC antenna ( apart from the sleeve dipole duckies..) That has been my point all along, trying to get the user to ask more questions about the antenn they may be considering purchasing - Folk end up buying on blind faith and sleight of word, with no measured facts or figures to back the sale up, no guidelines for use, etc.
"Gisela and/or Joe said that ferrites become less effective at higher frequencies. This is the opposite of the truth. Ferrites become more effective at higher frequencies. What ferrites don't do at higher frequencies is resonate.
At higher frequencies ferrites stop resonating and ....".
I fear that is not correct...They can and DO indeed resonate. The length to diameter ratio of the bead/ring/tube or whatever becomes critical, and the actual material becomes very critical. Sizes of ferrite typically used on the small diameter microwave cables , when chosen with 'ease', show resonances in the 1GHz, 2GHz, 3GHz, and 5GHz ranges. Where a tubular 1/4 wave balun may present 20db of attenuation, the ferrite version can be reduced to less than 5dB if chosen poorly - and how is the unwary DIY user going to know? If you wish to debate this we can rather take it offline, but take a look here before we do that - I can provide another 3 good references on this subject - Ferrites are the poor cousin to the 1/4wave balun at microwave frequencies.
Google this: ( if you are registered on any IEEE site you can download it)
Analysis of ferrite beads for RF isolation on straight wire conductors, by
S.A. Saario , D.V. Thiel ,S.G. O'Keefe, Jun W. Lu
This provides good insight into the exact issues - see mainly pages 85 to 114
I would not like this to become a tussle as to who knows more - the folk on the forum will only suffer from that approach - if we can each teach and learn, then it is worth while. I have posted my background and credentials in previous antenna posts and am reasonably schooled on the subject, having designed many antenna systems for both civilian and military craft, as well as for SunSat..but that does not mean I am to old to learn!
I just re-read the choke comment, and I realized that Gisela and/or Joe were talking about both air core and ferrite chokes for some reason, so I was a little off on that, but I need to clear up something else about that.
Gisela and/or Joe said that ferrites become less effective at higher frequencies. This is the opposite of the truth. Ferrites become more effective at higher frequencies. What ferrites don't do at higher frequencies is resonate.
At lower frequencies, a ferrite can be a tuned part of the entire antenna system, actually contributing to resonance, for example by making a small loop antenna resonate at the frequency of a much larger antenna, or inductively pinching off a section of antenna at higher frequencies to effectively shorten it.
At higher frequencies ferrites stop resonating and become like resistors, dissipating energy as heat. They are still effective at blocking unbalanced/common mode RF current, but they no longer reflect all of that power back into the antenna. This really isn't a problem if the ferrite is right at the feed point. If the current sees a 50-ohm antenna and a 250-ohm ferrite, most of that power is going into the antenna.
I guess you could argue that a perfectly-constructed bazooka is more efficient at GHz frequencies, but to claim that ferrites do nothing at all is just wrong. I would argue that it's nearly impossible to construct a perfect bazooka for GHz frequencies.
Also, to clear up a few disconnects:
A ferrite choke has a ferrite core. Ferrite is an iron-oxide ceramic material. The choke that Gisela and/or Joe showed in a previous comment was an air-core choke. Air core chokes are too big to be useful in RC. Ferrite chokes are sometimes called "beads" because they're the size of beads, and are used all the time in RC.
The bazooka balun is nothing like a choke. A choke is an inductive magnetic device; the bazooka is an entirely electrical device. A choke resists changes in current; a bazooka feeds back an out-of-phase version of whatever voltage it's fed. A choke completely ignores voltage; the bazooka completely ignores current.
The bazooka has nothing to do with a sleeved dipole, either. The sleeve in a sleeved dipole is a radiative element; a bazooka is designed to be a neutral element that ideally does not interact with the antenna's radiation pattern at all. The sleeved dipole is connected at the feedpoint; the bazooka is connected 1/4 wavelength away from the feedpoint.
Bandwidth is directly proportional to the surface area of an antenna; that's literally a law of nature. The relationship between material, surface area, cross-sectional area, length, frequency, and bandwith for an antenna element is well known. Any tech can do the math to match the tubular element of a sleeved dipole to its solid-core element, and for the non-techs there are online calculators for every antenna imaginable.
When you connect two tuned elements in series (in this case a dipole and a bazooka balun) the bandwidth of the entire system will be narrower than it would be for each element individually. Not a problem for old-style CW, terrible for multi-channel video or frequency-hopping telemetry.
Not all "rubber ducky" antennas are dipoles.
"Insulation permissivity?" On the outer insulation of a coaxial cable, affecting the lower element of a sleeved dipole. Are you kidding me?
With all due respect to Richard Johnson, there wasn't a lot of 5.8 GHz stuff out there in 1984, and nothing he ever made had to fit in my 1-kg plane or survive falling from said plane.
Is any of this advice based on actual comparative tests using materials and frequencies common to RC? Have you compared the performance of your designs to common commercially-available RC components?
May we have some advice actually influenced by firsthand knowledge of our application, please? Do I really need to know the best way to feed a folded dipole? I've never even seen a folded dipole in this hobby.
Since Gisela and Joe seem to have glossed over some things, I thought I would explain a few RF concepts:
Here are some ways of dealing with this, in order of efficiency:
- Jonathan KG7FIV
Those two things are not mutually exclusive.
Most "rubber ducky" antennas that come with LRS, telemetry systems, and hand-held radios are less-efficient normal-mode helical 1/4-wave monopoles, in order to tune them to frequencies much lower than an antenna that small would normally hear. At higher frequencies wavelengths get shorter, so it is possible for rubber ducky antennas for those frequencies to have much more efficient antenna designs packed into the same space.
Hence, many 2.4 and 5.8 GHz antennas are center-fed sleeved dipoles (although not all; many 2.4 GHz receivers, for example, are just monopoles.)