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THE UGLY BALUN choke
- Thread starter davev8
- Start date
Use a center fed 1/2 wave antenna for an example. The feed point typically has 'o' volts and max current. At 90 degrees away from that center point the voltage is at maximum and the current is at the minimum. At any point between the center and the ends the 'EI' or power will be the same, so as voltage increases the current decreases.
Now the 'good' part!
That voltage/current relationship is true for a resonant antenna. What happens with a non-resonant antenna? The current either leads of lags the voltage by some amount which is related to the amount of reactance present and it's sign (+/-, inductive or capacitive). So that voltage/current relationship is changed by the number of degrees the current leads or lags the voltage. What's that do? It means that there is less power being developed which in turn means that there's less power present and being radiated. That's also why a resonant (non-reactive) antenna is a nice thing to have.
- 'Doc
Now the 'good' part!
That voltage/current relationship is true for a resonant antenna. What happens with a non-resonant antenna? The current either leads of lags the voltage by some amount which is related to the amount of reactance present and it's sign (+/-, inductive or capacitive). So that voltage/current relationship is changed by the number of degrees the current leads or lags the voltage. What's that do? It means that there is less power being developed which in turn means that there's less power present and being radiated. That's also why a resonant (non-reactive) antenna is a nice thing to have.
- 'Doc
I have a question for you. On a dipole with the coax shield connected to it's exact center and the coax center conductor attached through a series air variable capacitor and gamma match,does the dipole need to be electrically isolated from the boom?
Yep, there is some of the more complicated I was talking about. There is far more to it than just that as well. That being said, the current line of thought is a bit far off topic and imho deserves its own thread if we continue...
The DB
This is way over my head in understanding, and it all seemed to make sense at some point. Now I'm not sure. I figure that is the way bad ideas go.
I was just hoping my center fed 1/2 wave model would help in some better understanding how I see the currents log works, not the technical part but the reference part. I think another thread might be in order too.
I was just hoping my center fed 1/2 wave model would help in some better understanding how I see the currents log works, not the technical part but the reference part. I think another thread might be in order too.
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A few thoughts on the feedline choke. Are we sure that the base of the antenna is always the best position to put such a device? Aside from my post above I don't recall of hearing or seeing any other positioning on such a device.
As the outer shield is acting as part of the antenna it would have current nodes and voltage nodes. Would not positing the choke (which is made up of a coil, which is a current impeding device) on a current node (assuming you could find one) be more effective than anywhere else on the feedline?
If you are in a situation where the the capacitance present in the CMC's and the choke become resonant, wouldn't moving the choke on the feedline present it with a different capacitance value, and thus removing the effects of said resonance?
The DB
As the outer shield is acting as part of the antenna it would have current nodes and voltage nodes. Would not positing the choke (which is made up of a coil, which is a current impeding device) on a current node (assuming you could find one) be more effective than anywhere else on the feedline?
If you are in a situation where the the capacitance present in the CMC's and the choke become resonant, wouldn't moving the choke on the feedline present it with a different capacitance value, and thus removing the effects of said resonance?
The DB
The currents being dealt with are only on the soutside of that coax, the choke won't affect what's happening inside at all (just to make sure that's clear).
The most common position for a choke is as close to the feed point as possible. That limits the 'size' of the feed line being affected by those CMCs, prevents them from using the whole thing as an antenna. That choke acts as a 'closed door', nothing get's past it so there won't be any nodes or nulls along the rest of the feed line. That's the whole idea, nothing getting past that choke, and naturally, there's going to be -some- get past it but nothing like if it wasn't there. Another common point to put a choke is where the coax enters the house. The feed line up to that choke may radiate but there wouldn't be any radiation in the house. If you can do both positions that's just insurance and shouldn't hurt anything. If that/those choke(s) are designed correctly they should affect things beneficially. But there's so much 'fudge' inn that correctly designed choke that it's usually a matter of it reducing the CMCs and possible radiation rather than stopping it completely. Those chokes are frequency dependent. If you change bands then you've probably reduced their effectiveness too.
- 'Doc
The most common position for a choke is as close to the feed point as possible. That limits the 'size' of the feed line being affected by those CMCs, prevents them from using the whole thing as an antenna. That choke acts as a 'closed door', nothing get's past it so there won't be any nodes or nulls along the rest of the feed line. That's the whole idea, nothing getting past that choke, and naturally, there's going to be -some- get past it but nothing like if it wasn't there. Another common point to put a choke is where the coax enters the house. The feed line up to that choke may radiate but there wouldn't be any radiation in the house. If you can do both positions that's just insurance and shouldn't hurt anything. If that/those choke(s) are designed correctly they should affect things beneficially. But there's so much 'fudge' inn that correctly designed choke that it's usually a matter of it reducing the CMCs and possible radiation rather than stopping it completely. Those chokes are frequency dependent. If you change bands then you've probably reduced their effectiveness too.
- 'Doc
I think you missed the DB's point completely, if i'm not mistaken he's suggesting some antennas may well benefit from the choke not being directly at the feed point, another point no one seems two have brought up, are 2 or more chokes spaced at a certain distance,
as the DB suggests the current maxima point, better or worse than one placed at feedpoint?
if i recall W8JI suggested two ground planes when discussing starduster type antennas and how hard taming cmc was, but not multiple chokes in series or split apart,there's as much to be revealed about this as the sigma 4 in my opinion.
there's a lot Steve G3TXQ didn't go into, but i agree with northern 35 to date Steve's article is THE most beneficial on the subject and the other article posted by the O/P from hamuniverse, or ham fucking interplanetary communications or wherever the fuck the nonsense article came from is the most damaging, as Steve proved no one choke will serve to cover much more than a few mhz efficiently.
The only thing that page about ugly baluns offers is a few ideas on construction methods, the one size fits all suggestion is bullshit, ham snake oil from someone who never bothered going to the trouble of actually testing it like Steve did.
Doc your right in the idea for nothing to get past the shield, also right in not everything will be stopped, but you don't appear to grasp why, its because the coil is an inductor tuned to give impedance to particular frequencies (which is why one type can't cover all) , impedance at rf is equivalent to adding resistance at dc, the more you add, the less current can flow, but no matter how much you add unless you put an insulator,even then most insulators will alow a tiny amount of current to flow, in some current will flow, even if its only .000000000000000000000000000000001 amps. the object isn't to stop common mode current, only thing will do that is a properly designed antenna thats balanaced and fed with a balanced feedline to a balanced output on radio, so that perfect 100 % cancellation will occur, the REAL object is to minimise that shield current to levels where it won't cause problems, or to the point where imbalance is almost neutralised.
This is exactly what I was asking about.
I am only concerned with the CMC's on the outside of the shielding here, not the currents flowing between the shielding and the center conductor.
It makes sense to me that the use of a current limiting device to minimize common mode currents would more effective on or near a current node, as opposed to a voltage node (or somewhere in between).
It follows the same principle of a loading coil in a 1/4 wavelength antenna used for shortening purposes, the further away from the current node you get (and thus closer to the voltage node at the tip) the larger the coil has to be to achieve the same amount of shortening.
The goals between the two are different, but being current devices, are their effects (at least to a point) not amplified/attenuated based on their distance from a current node?
The DB
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