4
$\begingroup$

Consider a biquad antenna used for receiving, not for sending. They are typically built with each side having a length of $\lambda / 4$.

If a biquad antenna was built with a side length of $\lambda / 2$ or even $\lambda$, would this give better results (for receiving)? If yes, how much better?

$\endgroup$
  • 2
    $\begingroup$ I'd guess no. With dipoles, making them larger than a half-wave significantly changes the radiation pattern in a way that is rarely useful. See antenna-theory.com for an illustration. I can't find a lot of good information on biquads, but I suspect it's similar. $\endgroup$ – Phil Frost - W8II Jul 4 '14 at 12:57
3
$\begingroup$

You need some theory to answer this - it's not entirely simple.

Consider that each quad loop really consists of 2 dipoles, with the ends connected together (at the corners of the quad). So, each quad loop really acts as two dipoles located parallel to each other. (The reflected loop, in a similar way, consists of two of these 'pseudo-dipoles').

So, if you then consider what would happen if you would extend the dipole to double its length: Instead of one dipole (which has low impedance at its center), you'd have two dipoles, which have a very high impedance at the center - almost impossible to feed (thousands of Ohms). But it would still be possible.

What's not so interesting is that a large part of the field irradiated by the dipoles would cancel each other (because they're too close), so you would probably 'feel' a slight improvement, but not nearly what you'd expect from such an effort.

Edit: Modified the text slightly to - hopefully - make it less confusing thanks for the remark, Phil). Making use of the edit, do note that if you'd extend the sides to 3/4$\lambda$ you'd actually get 3 dipoles and low impedance. But again, the gain won't be what you'd expect, as, again, parts will cancel. Moreover, if you'd think about using the biquad as a feed for a parabola, you'd be covering a much larger part of the useful area.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.