Why should the dipole antenna be fed balanced ? 1

[StoppTidigare]

Hi all what happens if one feeds the dipole antenna
unbalanced. Does it shange the radiation pattern ? Input impedance ?
Regards,
StoppTidigare

 

[iroquoix]

The direct connection of a dipole to coaxial cable is not a dramatic mistake in many cases but may cause rf flowing along the coaxial outer braid in some cases which is radiated partially and may cause interference to other systems. The diagram is disturbed as well in most cases. Feeding a balanced load such as a dipole should be done balanced as well. A simple common mode choke which is built by winding the coaxial cable on a ferrite toroid (NO iron powder) or just as an air coil at he point where the cable is connected to the dipole will bring what is needed here: decoupling between the unbalanced source and the balanced load. Ideally the coaxial feed line is routed perpendicular relative to the dipole, otherwise it causes deformation of the diagramm as well as a "deformed" load impedance of the dipole. The common mode choke which is a basic 1:1 BalUn transmission line transformer needs a series inductivity that its reactive resistance is at least four times (up to 10 times) the cables impedance at the lowest used frequency. W2DU built very nice and slinky BalUns with ferrite beads.

 

[iroquoix]

The direct connection of a dipole to coaxial cable is not a dramatic mistake in many cases but may cause rf flowing along the coaxial outer braid in some cases which is radiated partially and may cause interference to other systems. The diagram is disturbed as well in most cases. Feeding a balanced load such as a dipole should be done balanced as well. A simple common mode choke which is built by winding the coaxial cable on a ferrite toroid (NO iron powder) or just as an air coil at the point where the cable is connected to the dipole will bring what is needed here: decoupling between the unbalanced source and the balanced load. Ideally the coaxial feed line is routed 90 degrees relative to the dipole, otherwise it causes deformation of the diagramm as well as a "deformed" load impedance of the dipole. The common mode choke which is a basic 1:1 BalUn transmission line transformer needs a series inductivity that its reactive resistance is at least four times (up to 10 times) the cables impedance at the lowest used frequency. W2DU built very nice and slinky BalUns with ferrite beads.

 

[StoppTidigare]

>The direct connection of a dipole to coaxial cable is not >a dramatic mistake in many cases but may cause rf flowing >along the coaxial outer braid in some cases which is >radiated partially...

I should be able to use a coax in an balanced mode, if I use the balun at the transmitter end: transmitter->balun-> coax->dipole antenna. It shouldn't be necessary to have transmitter->coax->balun->antenna.

Why should I have more or less RF-radiation from the coax if I the coax is unbalanced or balanced ?

When feeding the dipole antenna, one doesn't use the earth for making the antenna longer I think..

So If one feeds a dipole antenna in an unbalanced way,you actually have a lambda/4 antenna, which according to what I've picked up should be fed unbalanced !!...

>The common mode choke which is a basic 1:1 BalUn >transmission line transformer needs a series inductivity >that its reactive resistance is at least four times (up >to 10 times) the cables impedance at the lowest used >frequency.
The input impedance of a dipolecantenna can be evaluted analytically. If you look at page 617, Field and wave electromagnetics, 2 ed: David K.Cheng. it says that the input impedance of half-wave dipole is 73.1 Ohms.

And thats the reason why the 75 Ohm coax was created..
Regards,
StoppTidigare

 

[iroquoix]

A balanced load wants to be fed by a balanced source, the dipole (balanced load) wants to be fed by a balanced line which has the feed point impedance of the dipole at its end.

Otherwise there is a current flowing on the outside of the coaxial shielding braid which is radiated and causes a deformed radiation pattern. Several hams know this but accept it because it is not too worse in their application.

The 1:1 balun between the transmitter and the feedline does not change the coaxial cable to a balanced line but a 1:1 balun between the feed line and antenna serves with a good portion of decoupling which prevents a current flowing down the coax' braid.

If you do not like the balun in the middle of the dipole because of it weight, which causes sag of the antenna, you may try balanced line. These are low loss lines in most cases (what you can buy). You may calculate the correct length to gain the right impedance at the transmitters end where you can use a balun now or you can use a universal balanced (!) antenna tuner to match it to the transmitters impedance.

By the way, please let us use a more polite style of conversation. Jack Stone who is the master of the AntenneX website said something about the right cadence in his forums: "Let's keep it warm and friendly"
I think that this is a very good recommendation.

With greetings from Germany

Andreé Knott
DD3LY

 

[StoppTidigare]

Dear Andreé, I would like to attach a pdf-document
from Dr. Howard Jones author of "High Speed Digital design-handbook of black magic"
See

http://www.sigcon.com

for a webpage with lots of interesting information on matching.

What you'll see is regardless of the potential of the return conductor, you'll always have a current in that lead.
This actually follows from kirchoffs current law, I think.
In a given crossection of the circuit, you'll have at any instant of time equal amount of current flowing into it and an equal amount flowing out of it from the other lead.
So regardless if the coax is balanced or not, one always
have a return current, and if I have a return current, and no shielding I'll detect radiation.
Please tell me how to attach the file, and we'll continue this interesting discussion! My email is :
StoppTidigare2@hotmail.com
Kindest regards,
StoppTidigare

 

[jbartos]

Suggestion: A proper tunner might be added to have the balanced dipole antenna to achieve the maximum transmission of the signal. Visit

http://www.njqrp.org/Rainbow/rb_an4.html

etc. for more info

 

[jbartos]

Small encore: Visit

http://www.wulfden.org/NVQS/nvis.shtml

for:
The reason is that unbalanced antennas frequently create interference problems with telephones and other communications and electronic equipment. These problems are substantially reduced or eliminated with a balanced antenna system.