Quad vs. Yagi (part 2)

 

In my previous post I was discussing about absolute Quad vs. Yagi performance figures. I think it is clear from the analysis that Quad does not beat a three element yagi when installed over real ground. Now.. why does quad is claimed to work so much better? Oftentimes I hear claims that Quad is more tolerant to construction errors. So lets have a look…

Quad Behavior, Construction errors

For reference the original design in free space below.

Impact of boom Length

Boom + 30cm	Boom +60cm

Boom -30cm	Boom -60cm

Lengthening  the boom has very little impact, but making the boom too short in such tightly coupled design starts disrupting the front/back. The gain figures remain fairly constant and within 0,2dB range.

Element length errors.

This quad design is based on HRS model, where the radiator is fed with ladder line and tuner is required. So I keep the radiator same and modify the resonant reflectors.

Reflector Loop -25cm	Reflector Loop -50cm

Reflector loop +25cm	Reflector Loop +50cm

So the reflector loop measurements significantly more critical than the boom length. The used 25cm measurement error sound fairly large, but trust me, it is very easy to make if you do not for example use wrong velocity factor when you design the antenna.

Yagi Behavior, construction errors

Again, the original design in free space for reference.

Boom Length Construction Errors

Director Placement

Director +30cm	Director +60cm

Director +30 Reflector -30	Director +60, Reflector -60

As expected when the length of the boom is increased the gain increases.

Lets next make the boom shorter and see simulate what happens.

Director -30cm	Director -60cm

Director –30cm, Reflector +30cm	Director –60, Reflector +60

I would call this a very robust design. In normal construction tolerance the placement in the boom has only few dB difference. So as long as reasonable common sense is used in construction tolerance yagi is no worse than quad.

Antenna Tolerance To Element Length Errors

Lets first start shortening the elements.

Director -20	Director -40

Director –20, Reflector -20	Director –40, Reflector -40

And now lets make elements a bit longer

Director +20	director +40

As expected lengthening the director turns it quickly a reflector disrupting the pattern completely. It depends completely on the yagi design how critical this measurement is. In this design the tolerance is reasonably small, so you better learn to measure or at least fail on the short side.

Reflector +20	Reflector +40

Unlike the director the reflector is reasonably tolerant to impact of shortening the element.

Summary of Construction Error Tolerance

The myth: “Quad is more tolerant to construction errors than Yagi”.

I would conclude based on these two antennas compared side by side that there is no truth behind the claims for quad being more tolerant to errors than yagi.

Of course by making yagi director too long it is reasonably easy to disrupt the pattern completely, and even reverse the beam, but that is an extreme case. Both designs are reasonably tolerant and if you can measure with accuracy of 5cm and you have the material velocity factors, including the material thickness taken into account when designing the antennas, the measurements are not that critical. Accuracy of few centimeters is well in the limits of tolerance and capability of average builder.

I think the main reason for people considering quad being more tolerant is the number of bad yagi recipes out there and builders using just wrong materials. Good yagi design is also tolerant to tapering and material thickness variation. I checked this yagi model with –50% and +100% material thickness with little impact to pattern.

Part 3 and final conclusions still to follow