# Why aren't baluns needed for a monopole to unbalanced coax connection?

I've asked before regarding dipoles connected to unbalanced lines. From what I gathered, the current in the inner shield when reaching the dipole (in transmitting case) can either go into the pole connected to the shield and also the shields outer skin (I'm assuming this dependacy is related to the two impedances), the current that doesn't match the inner current gets radiated.

Now the question I specifically ask is, if this is correct, then why, when using a monopole, does the current in the inner shield not travel entirely down the outer shield, since none can travel to the pole since there is none in a monopole case.

I did a very crude test on a monopole and when I touched the feedline coax it didn't change the SWR at all across the frequencies. I'd guess this is proof the monopole doesn't cause outer shield currents - a very different story to my v dipole.

The antenna that you call a monopole may actually be a coaxial dipole

simulate this circuit – Schematic created using CircuitLab

I apologize for the very crude drawing.

A coaxial dipole has two sections. One section is a single conductor arm. The other section has a central conductor with two coaxial conductors around it. Where the two sections meet, the two outer coaxial conductors are electrically bonded.

One might think of this as a regular dipole where the feedline is snaked through one of the dipole arms.

Current from the transmitter flows through both the inner conductor of the feedline and the outer conductor. When the junction of the two antenna sections is reached, current from the inner conductor continues into the simple conductor arm. Current from what was the coax outer conductor, and is now the "middle conductor" of the coaxial dipole antenna, turns around and flows on the outer coaxial conductor.

The coaxial section of the antenna is typically shorter than the simple conductor section because the phase velocity of the wave is affected by transmission line effects in the coaxial section.

why, when using a monopole, does the current in the inner shield not travel entirely down the outer shield,

If this is indeed your antenna, the answer is simple. The current can only flow so far back down the outer "shield", because it it only extends so far, and is not connected to anything at one end.

If you instead had a system like below, I believe current would flow further down the outside of the outer conductor.

simulate this circuit

• A so-called "monopole" antenna is really a dipole: there's always a counterpoise. In your first diagram it's the sleeve (mostly, if resonant). In your second, it's the coax shield. Commented Jan 21 at 23:51
• @JohnDoty Yes, I agree. I find the term "monopole" to be something of a misnomer. Commented Jan 21 at 23:59
• Thanks for the clear explination, helped alot - only "The current can only flow so far back down the outer "shield", because it it only extends so far, and is not connected to anything at one end" i dont understand why the curent wouldnt just get reflected back and maybe travel on the outer sheild. Or is this related to when you were talking about the phase affects and some interaction with the center peice Commented Jan 25 at 11:27
• @Georgekirby, actually, you are right, some of the current may follow the inner surface of the outer sleeve back up to the antenna, then down the outer surface of the coax. However, when current is traveling in both directions, there are standing waves. A current node of a standing wave is a point where there is minimum current. The end of the outer sleeve is such a current node, and that helps prevent the current from following the path just described. But yes, some of the current may follow that path. Commented Jan 25 at 12:26
• I see, that explains why some people still use baluns on 'monopoles' then, so this current node of 0 does not apply to dipoles then, only to the cases above. Commented Jan 25 at 23:25