You gave repeatedly stated that the concept of balancing cant apply to an RF dipole, which I agree with, and yet you persist in saying that you need a balun to properly match it to an unbalanced coax, which by your definitions, is also a relatively meaningless term, since at RF the actual voltages with respect to ground at the far end are fairly indterminate.
You are just parroting two statements, probably culled from simplistic text books, without actually thinking what they actually mean when put together.
If you cant see the logical contradictions in your arguments, I cant really have anything more to say.
I never said any such thing! If you can read that into anything I did write, it's no wonder you've got yourself into a twist.
No, you still don't understand...
Funny, I'd been thinking of saying exactly those same things to you.
For everyone else, the bottom line is that Andy Wade was exactly right in saying: " If you connect a coax feeder to a symmetrical aerial (such as the centre-fed dipole of a typical TV aerial) without using any form of balun then the outer of the feeder becomes part of the aerial. If you're transmitting, the outer of the coax will be 'RF-hot' and will radiate, quite possibly causing EMC problems to low-level parts of the transmitting equipment as well as safety concerns if high RF power is involved. When receiving, unwanted signals picked up on this hot feeder will find their way into the receiver, however perfect the screening of the coax itself."
Just in case that was a serious comment, heres teh serious asnser is I see it.
There are two basic forms of antenna that present a purely resistive impedance the basic quarter wave, which presents at 75 ohms, and the half wave that presents at typically 300 ohms.
Its so happens also that a parallel pair of wires, or a coaxial cable, represents over a broad range of frequencies, a non reactive transmission line.
It also happens that a coaxial cable at 300 ohms characterisic impedance is almst impossible to arrange. The materials available simply don't allow it, so most coax is around the 50-75ohm range, and therefore suit cable as feeders for low impedance quarter wave antennae.
If you use a half wave style antenna, you need to either couple it to
300ohm transmission line - two strands of copper at the edges of a flat ribbon of insulator - or use an impedance matching transformer, which will probably erroneously be called a balun, to connect it to a 75ohm coaxial cable.
i suspect that the whole thing springs from people referring to 300ohm line as 'balanced' and coax as 'unbalanced' at which point a 'balun' is declared to be the thing that connects them. Neither of the two transmission lines are balanced or unbalanced, and the function of the blob of ferrite and wires that connects them is actually to perform an impedance match.
Note that in either case, the system is neither balanced nor unbalanced electrically, as it has no real reference voltage about which it may be said to be balanced. You can earth one side or in fact the other, of any of the systems without affecting operation.
Balance operation only really has meaning when its - typically - transmitting between two systems which share a common third path. It is in effect a way of eliminating 'ground loops' since the signal current does not share the ground parts of the circuit, and hence the possibility of ground current interfering with the signal simply does not exist.
At RF th concept is almost meaningless: wire inductance over any length is so high as to render the concept of a common ground of low impedance almost meaningless. If you want an RF grund, the best you can do is a big sheet of conductor, and if you need to move signals about, a piece of wire or a track is a very bad way to do it. All RF circuitry that takes signal over anything but the smallest distances (less than 1/8th wavelength typically) is done with a transmission line of some sort or another. Even if that's simply a copper track on one side of a board with a ground plane on the other.
Balun merely means balanced to unbalamced: it has absolutely no real meaning at RF. Other than as an impedance matching device, one side of which has one limb connected to a local ground plane.
Or just an impedance matching device, or possibly a way to DC isolate two circuits.
If an antenna has a transformer on it, its because its built to a different impedance to the line its designed to feed, or because someone decided that it would be a way to con people that its somehow better.
Obviously a 300ohm antenna feeding 75 ohm coax, with no impedance matching transformer, is going to be an appalling mismatch, and the whole coax will indeed act as an antenna, but thats not because its unbalanced, its because its not been matched impedance wise, and is no longer acting as a transmission line should.
You are completely wrong, and are now introducing red herrings.
Oh dear, since you don't want to give up, I'll have one more go.
Think in terms of symmetry rather than balance, and think about where RF current is able to flow rather than voltages to some arbitrary reference point. The aim of a feeder is connect an antenna structure to a receiver or transmitter as transparently as possible; in particular we don't want the feeder to become part of the aerial and hence to radiate or receive. The effects of such a 'hot' feeder are almost always undesirable (which is where we came in).
Unfortunately the feeder, made of metal wires, doesn't know this and given half a chance it will become part of the aerial. From the receiving viewpoint the feeder, being 'metal in the sky,' _will_ receive signals and interference and we must try to arrange things so that these unwanted signals don't reach the receiver's input. IOW what we want to prevent is common-mode to differential-mode coupling in the feed arrangements. The wanted signals are clearly in differential mode (currents in opposite directions on the legs of a 2-wire feeder) whereas the interference received on the feeder itself is in common-mode.
Now consider a dipole antenna, mounted in the clear and centre-fed with a symmetrical 2-wire feeder (300 ohm ribbon, or similar). We'll transmit from a balanced or floating source. This is a perfectly symmetrical arrangement and each leg of the feeder will carry equal and opposite currents and deliver them to the two halves of the dipole. This is just what we want; there's no common-mode current on the feeder and no far-field radiation from it, assuming the wire spacing is small compared to the signal wavelength. So far, so good I hope. By reciprocity what applies for transmission also applies in reverse for reception, and common-mode interference received on the feeder doesn't get in to the receiver.
Now replace the symmetrical feeder with a coax cable and connect it to the dipole terminals with no balun. This arrangement is not symmetrical
- one wire of the feeder is wrapped inside the other.
We need to digress for a moment and think about differential- and common-mode currents for a coaxial cable. Diff-mode is fairly obvious - equal & opposite currents on the inner and outer conductors. Provided the frequency is high enough (higher than a few hundred kHz for the sort of coax we're talking about) the skin effect ensures that the current in the outer conductor flows on its inside surface. There's negligible current on the outer surface and hence no external field - and that's the great advantage of coax. Any common-mode RF current, OTOH, has to flow entirely on the outer surface of the outer conductor and leads to radiation. It's this fact, that any common-mode current flows in one of the two conductors only, that makes coax 'unbalanced.'
Going back to the centre-fed antenna, the dipole leg connected to the centre conductor sees only the differential-mode current, whereas the other leg sees the opposite diff-mode current /plus/ any common mode current. We now have a situation where the current in the two halves of the dipole can be unequal, with the difference flowing in common-mode on the outside of the feeder. Just how the current divides between the driven element connection and the outer of the feeder will depend on the relative impedances and radiation resistances looking into the element and the feeder's common mode path. Since the latter is likely to be several wavelengths long and routed past all manner of obstacles, this will generally be unpredictable.
Another way of looking at the situation is to separate, conceptually, the two feeder modes - IOW we have a dipole fed by a nice symmetrical feeder (diff-mode) but it has a random long length of wire (the outer of the coax) connected to one leg. Looked at that way, the balun-less feed suddenly seems quite horrendous (which it is).
Tonight's further reading is an excellent introduction to the subject of baluns:
Dont bother. Look up the Wiki and you will see that apart from issue with powerful transmitting antennae, where circulating currents in the coax outer make it significant, the balun is an impedance matching item solely.
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"When a dipole is used both to *transmit* and to receive, the characteristics of the feedline become much more important. Specifically, the antenna must be balanced with the feedline. Failure to do this causes the feedline, in addition to the antenna itself, to radiate. RF can be induced into other electronic equipment near the radiating feedline, causing RF interference. Furthermore, the antenna is not as efficient as it could be because it is radiating closer to the ground and its radiation (and reception) pattern may be distorted asymmetrically. At higher frequencies, where the length of the dipole becomes significantly shorter than the diameter of the feeder coax, this becomes a more significant problem. One solution to this problem is to use a balun"
Note that this applies SOLELY to high power use as a transmitting antenna. IT HAS NOTHING TO DO WITH A RECEIVING ANTENNA.
A little knowledge, it appears, in your case, is a dangerous thing.
WE are taking TV reception aerials, Not kilowatt transmitters.
In principle a coaxial cable doesn't act as an antenna (receiving or transmitting) simply because of mismatch. Mismatch /per se/ doesn't cause current to flow on the outer surface of the outer conductor.
OTOH mismatch increases the attenuation in the cable and increases the peak current and voltage at points along the feeder in accordance with the standing wave pattern, effectively reducing its power rating. Also the presence of reflections may cause deterioration of a signal in the time domain - ghosting, intersymbol interference, etc., particularly on long feeders.
In reality imperfect screening ('leakage') leading to feeder pick-up or radiation may be worsened by the presence of a standing wave but this is usually in the realm of second order effects compared to the consequences of omitting a balun where one is required.
Dear O dear!, a Balun can be an impedance match as well as a balanced to unbalanced converter!...
Thats not terribly well written..as is quite a bit on wikipedia..
With all due respect there is no difference in an aerial system you use to transmit and to receive there is nothing that sets them apart in theory at all.
In practice there is in that you can get way with impedance mismatches in say FM or TV aerials, all you do is loose some signal, set up reflections, alter the polar pattern etc.
You don't really want to do this in transmission as RF power is expensive and you simply don't want to waste it, nor do you want to have currents about the place in the wrong places;)..
With all respect, learned old sage, I think you demean Mr Wade who's spent years in TV and Radio aerial design and as well as running companies that make TV and distribution equipment!..
As above the theory is the same..
Suggest you read up the article thats been attached to the previous post. Look up the difference betwixt Marconi quarter waves, open and folded dipoles, differing types of Baluns and matching systems etc, etc..
It is the point as subsequent posts will show. The only inaccuracy in that statement is where you are driving a Marconi quarter wave vertical where it is an unbalanced system!...
As I said, a little knowledge is a dangerous thing.
I guess you are the sort of person who runs a 20hp electric motor off bell wire..cos a piece of wire is a piece of wire, irrespective of the current its carrying..
we are not talking imepdanace mismatches. W are talking about 'balancing' a dipole on a caoax feed 'top reduce interference'
And the result of THOSE *IS* actually to end up with the coax acting as an antenna.
Well so did I, once. And its not necessary to have full understanding of anything to be commercially successful either.
It is not. The currents in te cable are a few hundreds of orders greater.
The wiki article specifically uses the application of baluns to transmitters *only* in respect of 'balancing' the dipole and isolating it from the feeder.
Baluns as impedance matchers, which shouldn't be called baluns, I have no problem with. Any half wave system that is feeding 75 ohm coax needs one.
Baluns as a means of isolating a *receiving* antenna of 75ohms impedance from a cable of the same, are completely useless in a *receiving* environment, and do sod all to reduce interference. Their use in a transmitting environment is to reduce the fractional power *off* the cable..a pretty small fraction, but measurable.
On Thu, 09 Oct 2008 12:06:20 +0100 someone who may be The Natural Philosopher wrote this:-
A better analogy would be saying that wiring up a motor is no different to wiring up an alternator. That is the case, as demonstrated by the motors of trams and an increasing number of trains being used as alternators in regenerative braking. There are some differences to do with how the stator magnetic fields are produced (I am not talking about permanent magnet motors), but these are ancillary and minor as they are in radio/television systems.
Whether transmitting or receiving the aerial is part of a system and the theory and operation are identical. If they were not identical it is difficult to see how a signal could be transmitted between the two.
Of course television transmitting aerials don't generally look like television receiving aerials. There are several good reasons for this, which can be simplified to the fact that the transmissions are generally in many directions while the reception is (generally) in one direction.
An exception to this are some of the smaller relays. For example Aberbeeg receives signals on a log periodic aerial and transmits them on log periodic aerials.
Well not identical, but "reciprocal": whatever affects the transmit situation will affect receiving in a similar manner.
There can be a communication problem here because RF engineers tend to take this "reciprocity principle" for granted. Even when thinking about receiving aerials, they often switch to talking about the transmitting situation because it is easier to describe. That is perfectly valid between professionals, but it can leave sceptical outsiders such as TNP believing they aren't keeping to the subject; when actually they are.
What TNP also hasn't grasped is that the outside surface of the entire length of coax can act as a long-wire aerial. This is completely independent of its normal function of "piping" signals down the inside [1]. In this long-wire mode, the coax can pick up interference from whatever sources it happens to run close to - notably of course the mains wiring, which is directly connected to many generators of impulsive and other noise.
All of these interference signals are picked up on the outside surface of the coax, but they normally cannot get in through the shield [1]. If the shielding is good they can only get inside the coax at the top, where the TV aerial is connected. But then they compete with the TV signal itself, and Andy explained why this can be a particular problem for digital signals.
The aim of the balun on a TV aerial is simply to block the entry point at the top of the coax, for interference signals that have been picked up by the coax in its unwanted long-wire mode.
[1] Google for "skin effect", which makes the inside and outside of the coax shield behave as two completely separate conductors.
You obviously have little idea about best audio practice. That is having a completely separate ground from other non audio equipment - which in this case would be your motor.
Besides, most decent mics are balanced and have no earth connection...
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