5
$\begingroup$

I've been told countless times that an antenna should be 1/4, or 1/2 wavelength. That increasing this size to a higher multiple of 1/4 will work, but it will distort the radiation pattern to include several high gain lobes shooting energy off in useless directions (like straight upwards).

But I also see stuff like this for sale https://www.dxengineering.com/parts/dmn-x300a. How does this antenna work when it's around 1.5x the wavelength for 2m, and about 4.25x the wavelength for 70cm? Is it just some tech I'm not familiar with, or are these just giant useless poles?

$\endgroup$

2 Answers 2

8
$\begingroup$

It's not a single long element, it's a collinear antenna, which is an array of several dipole elements stacked "end to end", connected in such a way that the current on each element has a well-defined phase relationship to each other element. This gives a single strong lobe (with higher gain than a dipole) instead of multiple "useless" lobes. The more elements you stack, the greater the gain gets, within practical limits.

In the case of most antennas that look like that Diamond, the phasing is done by inserting coils in between the elements that delay the signal on its way up, creating a phase jump between the top of one element and the bottom of the next — but there are several different ways of building collinears, including coils between elements, transmission line stubs between elements, elements separately fed with coax "phasing lines" of particular lengths, and the lazy but effective coax collinear which swaps the inner and outer conductors of each element to get a 180° phase shift.

In the simplest case a collinear has maximum gain at 0° elevation, but one interesting aspect is that you can slightly adjust the phase of the higher or lower elements to get an "up-tilt" or "down-tilt" of the pattern. So if you have a repeater on a mountain high above the area it's servicing, you can "aim the pattern down" and get more gain down on the ground. Unlike physically tilting the antenna, this is an omnidirectional effect — the peak gain is below the horizon in every direction.

$\endgroup$
0
$\begingroup$

The picture shows 1.5 wavelength, frozen in a moment in time to show the current directions. Broadly speaking one of the red areas will cancel out against the blue, leaving one red area effectively radiating, so that is like a 1/2 wavelength right there, even though the antenna is 1.5. Secondly what hobbs said, the blue part could be wrapped in a center loading coil or basically it wouldn't radiate and then it's like 2x 1/2 wavelength stacked on top creating an array to produce gain in some direction/angle. Probably the easiest way to think about it is freeze the antenna at a moment in time when currents are at the max and draw it and picture the magnetic field, then do the same when voltages are at the max and repeat. Both fields basically combine into one, that is the pattern you're looking for. Also think about looking at the antenna from different angles, which fields add up, which cancel out. All it takes is for a little bit not to cancel out and you will already have [some] radiation.

antenna picture

$\endgroup$
4
  • $\begingroup$ I think you're on to something in that, yes, it's right that this radiates! But: "one of the red areas and will cancel out against the blue, leaving one red area radating" isn't how electromagnetic waves and antennas work! And consequently, the radiation pattern of a $1.5\lambda$ antenna is not the same as that of a $0.5\lambda$ antenna. So, I don't think we can let this stand as it is, as it is sadly incorrect :( Things will radiate even if equal amounts of currents flow in opposite directions, as you're ignoring the spatial aspects here, which define an antenna! $\endgroup$ Commented Jul 31, 2022 at 11:21
  • $\begingroup$ but, for the $1.5\lambda$ dipole (and only for the $1.5\lambda$ dipole) you're right in result, if not in explanation: The effective area of a $1.5\lambda$ dipole is the same as that of the $0.5\lambda$ dipole. But again, that's not because of regions of current density cancelling each other out simply by existing in the same antenna, that's because they're exactly a wavelength apart, and thus, as you very astutely observed, the generated fields a bit away from the antenna cancel out. $\endgroup$ Commented Jul 31, 2022 at 11:25
  • $\begingroup$ But the fact that it's the fields, not the currents, cancelling out is the important part here, and why antennas of different shapes have different radiation patterns. If we could simplify things to "same-sized regions of opposite current cancel out", none of our antennas would work (because there's no current without an opposing current somewhere else). $\endgroup$ Commented Jul 31, 2022 at 11:26
  • $\begingroup$ agree to disagree @MarcusMüller $\endgroup$
    – pgibbons
    Commented Aug 2, 2022 at 3:17

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .