2m 5-element yagi with staggered driven elements

Question

I have seen sketches of 3-element 2m yagi antennas in which the driven dipole elements are staggered i.e., one element is positioned slightly in front of the other along the length of the (plastic) horizontal beam. these elements also "overlap" slightly, as they protrude through the beam pipe for an inch or so and the coax is connected directly to the elements at the ends of these protrusions.

I've been told that the staggered elements raise the feedpoint impedance to furnish a correct impedance match to RG-58/U coax without requiring a balun transformer. Is this a true claim? And do the elements need to be driven at their overlapping ends? Or does this overlap matter at all? In short...

Where can I find detailed design rules for staggered dipoles like this? I would like to try this design in a 5-element Yagi.

In the image linked below, you can see the "stagger" (one element is in front of the other) and the "overlap" (the elements cross over the horizontal beam midpoint).

Answer 1

Thanks for bringing this technique to our attention. I applied the concept to a simple half-wave dipole to gauge the impact.

Spacing the halves of a 1-inch diameter, 40m dipole $.001\lambda$ apart (about 3.3-inches), overlapping them by 36 inches and feeding them with a segment that joined the staggered dipole halves at the midpoint of the antenna, the feedpoint impedance increased from $72\Omega$ to $92\Omega$ with no change in the resonant frequency. Note that each wire comprised a large number of identical-length segments to avoid possible computation errors. Reducing the element diameters to 1/2-inch reduced the impedance to $82\Omega$, again with no change in the resonant frequency. This could be used to raise the feedpoint impedance of the driven element of a parasitic array, which is ordinarily lowered by inducing currents in the parasitic elements.

I don't know of any closed-form expression to calculate this behavior in pursuit of a working design. Since the resonant frequency wasn't affected by the staggered feed, it doesn't appear that the principle of a gamma match is applicable. It might be possible to build a traditional yagi design with the staggered driven element and simply trim the overlap until you reach your impedance target.

A balun will still be required to prevent unbalance currents from traveling down the outside of the coax shield.

Answer 2

The staggering in the drawing is small, 0.7", and the overlap is not specified. This makes me think it's done for mechanical reasons.

The feedpoint of a dipole or yagi is difficult to support mechanically - both halves need to be insulated but we'll supported. One neat solution to this is to stagger them somehow, so they can both pass through the boom and be supported there.

I've built broomstick-and-wire-and-glue yagis and staggered the driven elements by 10 mm or so, just to provide mechanical strength. Both halves of the driven element can go through the boom. Then I can solder the coax on one side and its strong and neat.

Yagis can be designed for various feedpoint impedances. I can't simulate your one now, but it might well be designed for 50 Ohms. If you're planning to feed your yagi directly with coax, this is what you'd do. 15 to 25 Ohms is more common, and might give slightly better performance, but a yagi designed for 50 will work fine and is nice and simple to feed. Impedance is mostly affected by reflector spacing and length.

There will be no significant impedance transformation from the feed staggering and overlap in your drawing.

With a direct coax connection you still need a balun. This could be a small coil of coax, or a 1/4 wave sleeve on the coax.

Answer 3

It is a design for a 3el yagi. the feed overlaps because the "wires" are metal rods thru the beam which allows for adjustments. there is a 4el design out there using this method