Balun for Vertical 1/4-wave - required ?

I am planning a 1/4 wave vertical pole-mounted antenna with sloped counterpoises, fed using a 50ohm coaxial.

Question is - should I use/ need a balun ?
I would think I should use a 1:1 current balun.

I see everywhere that a dipole or other antenna requires a balun (1:4 of course) to minimise feedline issues, but while researching 1/4 wave whips, I do not see a balun being specified ?!?

Is there a reason for this or is it simply an oversight by the posters ?

thanks for insights !

The dipole is symmetric. The currents on your vertical are - by design - not symmetric.

The radials (counterpoises) act like a ground plane - think of this as half a dipole, with ground plane radials as the symmetry plane. So yes, the radials are connected to coax ground without a balun.

volker@muehlhaus said:

The dipole is symmetric. The currents on your vertical are - by design - not symmetric.

The radials (counterpoises) act like a ground plane - think of this as half a dipole, with ground plane radials as the symmetry plane. So yes, the radials are connected to coax ground without a balun.

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Thats my point exactly - with opposite conclusions !

The co-ax shield can be shown to carry unbalanced currents due to skin effect, and hence the need for the RF choke or current balun. The vertical, along with counterpoise, is balanced while the co-ax is not. Hence the need for the balun to ensure a proper current distribution & better radiation pattern.

However it is almost never shown in the schematics which I have seen. While for a dipole the balun is almost always shown ! Well, in good designs anyway.

The co-ax shield can be shown to carry unbalanced currents due to skin effect.

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Sheet currents have nothing to do with skin effect. They are representing common mode wave reflections by the load.

You didn't show your antenna geometry, so wen can't know if it's mimicking a good single ended load. Technical implementation of a monopole antenna with finite ground can be however expected to generate a certain amount of common mode reflections respectively sheet current. In so far it isn't a bad idea to place a toroid or tube core around the coax cable near the antenna.

FvM said:

...
You didn't show your antenna geometry, so wen can't know if it's mimicking a good single ended load. Technical implementation of a monopole antenna with finite ground can be however expected to generate a certain amount of common mode reflections respectively sheet current. In so far it isn't a bad idea to place a toroid or tube core around the coax cable near the antenna.

Click to expand...

Oh, I thought my stating "..1/4-wave vertical ..pole mounted.. with counterpoises.." was sufficient to explain the geometry.

Anyhow you have supported the advantage of using a balun, so I will put one in. Considering the simplicity of the implemenation, why not. At best it will prevent problems, and at worst... there seems no down-side.

Thanks for pointing out the sheet current vs skin current difference !

kripacharya said:

The vertical, along with counterpoise, is balanced while the co-ax is not.

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No.

Simplified, the vertical is a lambda/4 monopole over ground. The coax shield is assumed "GND" and is indeed connected to the ground plane (here: to the radials). Perfect case for asymmetric feed.

In contrast, for the dipole we need the balun to create a virtual ground at mid-potential, so that we can connect the coax shield (ground) there.

That's interesting. In all the literature I have read, it seems to say that a monopole over a ground is exactly like a dipole...

typical quote from one such writeup --

"The quarter-wave monopole antenna is a single-element antenna fed at one end, that behaves as a dipole antenna. It is formed by a conductor lambda/4 in length, fed in the lower end, which is near a conductive surface which works as a reflector (see effect of ground) and is an example of a Marconi antenna. The current in the reflected image has the same direction and phase as the current in the real antenna. The quarter-wave conductor and its image together form a half-wave dipole that radiates only in the upper half of space..."

No doubt this comparison is made with a dipole to facilitate easier understanding. But then, what is the difference of monopole vs. dipole with regard to current & voltage distributions ?

Also, since the monopole is much more unbalanced by nature of its design, surely this will result in higher likelihood of unbalanced currents flowing in the co-axials shield and hence result in unwanted radiation from the shield as well ?

I would think this would compel one to definitely use a current balun for the monopole, even more so than for the dipole ! Your point about a "virtual ground" isn't really clear.

it seems to say that a monopole over a ground is exactly like a dipole...

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No, like a half dipole. Respectively it's correctly fed by an asymmetrical coax cable.

You don't need a balun for an ideal monopole and you can well operate a finite ground plane real monopole without it. But you'll get a certain amount of sheet current due to the imperfect ground plane, that's why I suggested a common mode filter. But it's no balun, it's only performing some residual balancing and stops the cable from radiating.

kripacharya said:

That's interesting. In all the literature I have read, it seems to say that a monopole over a ground is exactly like a dipole...

Click to expand...

Half a dipole, with the ground plane = symmetry plane.

The same applies to the feed - think of a symmetric feed (opposite polarity) for the dipole, and a single ended feed (hot wire + GND) for the monopole.

kripacharya said:

Also, since the monopole is much more unbalanced by nature of its design, surely this will result in higher likelihood of unbalanced currents flowing in the co-axials shield and hence result in unwanted radiation from the shield as well ?

Click to expand...

I think you are mislead by looking at the current. Current in the antenna wires is always opposite polarity, that doesn't help us to understand the situation.

Look at the voltage instead, with earth ground as the reference. The coax shield is at earth ground potential and the center conductor is at Vfeed. For a symmetric feedline, we have wires at +Vfeed/2 and -Vfeed/2 symmetric against earth GND.