What is a balanced antenna?

[Marconi]

This question has come up in a discussion, and I need a good definition and a description of exactly what a balanced antenna is. I am interested only in 11 meter base antennas in this regard, among them include the Imax, A99, I10k, Wolf .64, coilys new base, the V5/8, Penatrator, Astroplane, Starduster, 1/2 wave dipole, a folded dipole, a gamma fed yagi beam, a yagi with a hairpin feeder using an insulated driven element, a quad beam, and a quarter wave ground plane.

What makes one antenna a balanced antenna and another not a balanced antenna?

 

[Captain Kilowatt]

Hi Marconi.Well I would say that the simplist way to tell a balanced antenna is that both sides of the feedline "see" the same thing and neither side of the antenna is connected to ground.Of the examples you mentioned the balanced antennas would be the half wave dipole,the folded dipole,the yagi with a hairpin match,and the quad.Neither side of the antenna is connected to ground except in the case of using coax as a feeder where the shield is the ground connection but this upsets the balanced condition of the antenna somewhat.Any antenna that is "worked against ground" is NOT a balanced antenna such as the Imax,A99,or any groundplane type antenna.That would also apply to gamma fed yagis as the center of the element is grounded and the actual feedpoint is somewhat off center.Parasitic elements may be grounded at the center as on a regular yagi and the antenna is still balanced as we are only concerned with the feedpoint.

 

[Marconi]

Thanks QRN, could not have had a better explaination. Makes total sense the way you explain it.

The question now is how does the design of the Starduster fit into this? I always considered it a form of a 1/2 wave dipole as in the old sleeved dipole, but since it has a support that is grounded, I guess it makes it a 1/4 wave ground plane that is not balanced, right? Folied again.

QRN, do I have to go back in my old notebooks and changes all those misstatements?

Thanks,

Eddie

 

[Captain Kilowatt]

OK let me think about the Starduster............. Was it not just a 1/4 wave vertical with the radials sloped downward at an angle? That would make it an unbalanced antenna. I think I see where you are going with this because if it were just a single 1/4 wave vertical with a single verticle "radial" it would be a dipole and would be balanced but if it were a normal groundplane it would be unbalanced.Now what it is if the radials are sloped downward,still an unbalanced antenna.Now make the top of it three (or four ?) elements that angle out from the middle and do not ground the connector then it would be balanced.Have I confused you yet? I have done so to myself I think. :?

 

[Marconi]

Well if I'm not mistaken Antenna Specialists made the SD and called it the M-400. At some point they also made the M-800. It looked similar to the SD'r, but as you describe it had additional things sticking out all over the place, sorta like you describe, but not really. The M-800 was real and it sure was ugly too. Maybe that is why we never have seen many.

Thanks for you assistance again.

 

[W5LZ]

A ballanced antenna is one that has equal currents flowing in 'both' of it's halves. The shape of the antenna doesn't make much differnece (if any). That means that a groundplane can also be a ballanced antenna if both halves (vertical element and radial system) have equal currents flowing in them. The number of radials makes no significant difference (as long as there's at least one of them!).
How an antenna is fed can make a difference (direct feed, matching system, etc.). But, if done correctly, no significant difference, if the currents are 'equal' (and opposite, BTW). The imdeance matching system, gamma, beta, whatever kind of match, all have properties that can disturb the 'balance'. The 'trick' is to minimize those 'disturbabces', keep the ballance.
Another point of imballance is how the thing is fed, what kind of feed line. A coaxial feed line is not a ballanced feed line, so, the 'ballance' between the two has to be maintined. There are two basic ways of going about that, change to a ballanced feed line, or provide some kind of 'interface' between the antenna and coax. The most common type of interface is a balun. Another means is by using a 'choke'. A 'choke' works because of the way coax conducts stuff, center conductor and ~insides~ of the braid. The choke keeps/reduces the currents flowing on the ~outside~ of the braid. That's not the easiest idea to understand, but true none the less (the inside of braid conductor and outside of braid conductor thingy). Another way of looking at it is that it doesn't really mater what ~actually~ happens as much as what ~appears~ to happen. The 'biggie' about an unballanced radiator is the 'side affects', not necessarily the actual radiation. Thats from the practical point of view.
That help any?
- 'Doc

PS - After rereading this it looks like this isn't one of my 'good spelling' days. Corrected a couple, and got tired of it. Since there really isn't any 'new' words in this, I'll bet you can figure out which are mispelled.

 

[Marconi]

Well 'Doc. If an antenna is naturally unbalanced because one side, usually the ground side, is typically attached to the earth in some way, can we really make it balanced by controlling the currents to make them equal in the radiator and the Ground plane, excepting that the phase is opposite.

Don't we just have a balanced system, but still have in unbalanced antenna?

You right this is cornfusing. It is like a saying I often quote.

"They give us good advice with a bunch of pretty words and then they take it all away with a few more at the very end." Lawyer talk.

 

[W5LZ]

Marconi,
Depends sort of on how you do the 'grounding'. If a 1/4 wave length is used as the conductor to ground, then the RF really doesn't 'see' that connection to ground, right? So, electrically it may be grounded but RF wise it isn't. Frequency dependent.

Terms can be misleading. 'Ground' used as in a groundplane doesn't mean grounded like in something run to dirt. Just means the 'other half' of the antenna. If you want a 'safety' grounded antenna, then use 'RF's characteristic of not 'seeing' a conductor that's the 'wrong' length. Groundplane antennas (and most other 'two-sided' antennas) don't have to be grounded (dirt type) to work as an antenna, just for safety reasons.
- 'Doc

PS - "1/4 wave" used above really means 'odd multiples' of a 1/4 wave, just to keep things sort of straight.

 

[Marconi]

I agree with that completely and it always makes me laugh a bit when I hear someone advise to ground, ground, ground, that base antenna as though that will make the difference in whether you get out a 100 miles rather than just 5 miles.

What I'm considering here is again, after many years, of trying to understand what we once did with our old Starduster antennas, when we added a 9' jumper into the mast pipe of the antenna, and then hose clamped a barrel connector to the mast 9' below the feed point. In reading I find this describes a bazooka balun, pretty much to the T. I may have already discussed this to some degree here in the forum in another thread.

The only difference I see between what we did back then from that described of this bazooka setup, wss in our not making sure the top of the sleeve was insulated from the ground-side of the antenna. Of course the way a Starduster is built, with a sleeve enclosing the feed line, it makes them very similar to the way the bazooka balun has been described to me. Excepting of course from the way it is insulated at the top near the feed point.

All I know is the trick we used worked to prevent FR from showing up under those old Starduster antennas. Admittedly the only thing I had back then, that told me what was going on, and whether the fix stopped the common mode currents, was an old Field Strength meter I had from the 80's. Maybe that was enoug cause it sure did work.

 

[Guest]

Isn't the Starduster more of a center-fed dipole than a ground plane antenna?

 

[Marconi]

Well that is what I thought for years. That is also the reason for this question. However with these explanations it appears to me that it is a 1/4 wave ground plane and they are not balanced.

The determining factor is the fact that the hub is at ground potential as are the GP elements, that makes it grounded and that makes it unbalanced. If the antenna were insulated from the mast then it would be a 1/2 wave dipole.