0
$\begingroup$

Everyone knows that a 1/2 λ dipole has on the elements a standing wave the voltage and current of which are a bit less than 90° out of phase with each other, and the departure of this phase difference away from exactly 90° is the non-reactive component of the standing wave which results in radiation.

My understanding is that the out of phase component of the standing wave is reactive circulating energy never to leave the antenna due to the antenna being a resonant system, and the in phase component of the standing wave results in real work being done which produces radiation.

At the tips of the antenna elements, for an ideal dipole, voltage and current must be precisely 90° out of phase, how can it be any other way ? and apparently this phase difference between voltage and current changes along the length of the elements as a function of the distance from the ends.

This occurs because every part of the antenna has coupling to every other part of the antenna and because the antenna has ends with boundary conditions, and is one of the reasons why the equations for feed point impedance are so complicated and an approximation only.

If a perfect sine wave was applied at the center feed points of an ideal resonant 1/2 λ dipole, then because the phase difference between voltage and current of the standing wave is only exact at the end of the elements, and changes along the length of the elements, then the standing wave can't be a perfect sine wave and must have some distortion as it is "twisted" in accordance with the change in phase along the elements.

Does this departure of the standing wave from a sine wave cause distortion ? and if so then the distortion must increase as the Q increases.

Obviously there is no distortion otherwise everyone would know about this so does the antenna at the receiving end have the reverse effect and undo the distortion ?

Or is there something else going on ?

Improve this question
$\endgroup$
5
  • 4
    $\begingroup$ Giving minus points with no explanation doesn't help anyone, if you can't understand the question or don't think it's useful, how about saying why so we can all learn something ? as per the guidelines for this site ? Otherwise the minus points don't mean anything. $\endgroup$
    – Andrew
    Commented Aug 30, 2021 at 23:20
  • $\begingroup$ Agreed. Actually this is a really good question and the down votes are probably from folks who didn't stop to think through all the different modulation schemes and the effects of frequency dependent amplitude and phase shifts would have on them. $\endgroup$
    – uhoh
    Commented Aug 31, 2021 at 14:36
  • $\begingroup$ I don't quite understand your question. Antennas are linear devices, and only non-linear devices can cause distortion. $\endgroup$
    – Mike Waters
    Commented Sep 1, 2021 at 16:09
  • $\begingroup$ @MikeWaters If you feed a sine wave into a dipole the standing waves aren't sine waves they are distorted because radiation skews the phase difference between voltage and current of the standing wave, even thought a dipole is a linear device this departure away from a perfect sine wave of the standing wave doesn't represent the input exactly so i my mind this must cause some distortion. $\endgroup$
    – Andrew
    Commented Sep 1, 2021 at 22:52
  • $\begingroup$ @MikeWaters And yes i think the one minus vote is due to a lack of general understanding of the subject of the question for whoever gave that minus vote. $\endgroup$
    – Andrew
    Commented Sep 1, 2021 at 22:53

1 Answer 1

Reset to default
6
$\begingroup$

As far as I can see, no. The shape of the standing wave might have an effect on the antenna's spatial pattern, but the fact that it is a standing wave pretty well implies that it's steady-state, periodic, and all points vary in time at the same frequency, so it's not distorting anything.

If you're wondering about the case where the driving signal is at a different frequency from the antenna's resonance (SWR ≠ 1:1), then the answer becomes yes, there's some distortion, but at the frequencies and bandwidths we're accustomed to using, the effect is insignificant, even for quite high SWR (and for HF, at least, we use those small relative bandwidths because ionospheric scintillation would ruin the coherence of a wider signal, before other effects did).

If you're wondering about the case where the driving signal isn't an unmodulated sine wave, the answer is again "yes, but it generally doesn't matter", for similar reasons as the previous linked answer. You can look at the antenna as forming a bandpass filter, which reflects or attenuates signals far from its resonant frequency. Like any filter, this inevitably causes some amount of distortion, but as with any well-tuned filter used appropriately, this distortion isn't objectionable. The signal fits well within the "filter bandwidth" and nothing bad happens.

If you really push the Q, though (for example, take a small (but low loss) magloop for 40m, which can have a Q of over 2,000 and a 2:1 SWR bandwidth under 3kHz) then you might see some noticeable effects. If you were to put a 6kHz-wide AM signal into such an antenna, and your transmitter didn't mind the SWR, then you might actually be able to detect the resulting distortion; this high-Q antenna forms a filter that really wants to "ring". But Q of up to several hundred corresponds to filter bandwidths that don't give us any trouble in practice.

Improve this answer
$\endgroup$
1
  • $\begingroup$ Depending on the specific type of modulation and demodulation used (especially some digital ones), a frequency-dependent phase shift may also result in signal distortion and degradation. $\endgroup$
    – uhoh
    Commented Aug 31, 2021 at 14:38

You must log in to answer this question.

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