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Is there any reason to think that changing the Q of an antenna would affect the directional pattern, assuming that is the only variable that changes?

Restricting to values of Q say less than 1000.

Assume this is an HF loop antenna which could reach Q values of <1000 somehow, and assume it is made of magic metal so that I can vary the conductivity and therefore the Q as desired without changing anything mechanically. The capacitor is of course perfect.

So as I increase resistance of the metal, does the pattern change?

I miss having access to XFDTD, I could have just simmed it.

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  • $\begingroup$ How will you change the Q and not the current distribution? $\endgroup$
    – tomnexus
    Oct 18, 2017 at 21:38
  • $\begingroup$ Varying resistance. Assume that I can. $\endgroup$
    – user103218
    Oct 18, 2017 at 21:40
  • $\begingroup$ Well if the resistance is somewhere that has no effect on the pattern, like across the feedpoint, then it will have no effect on the pattern. If it's in series with the middle of the radiator, it will affect both the Q and the pattern. That said, the pattern of a short dipole and an ideal dipole are so similar that the pattern won't change much. $\endgroup$
    – tomnexus
    Oct 18, 2017 at 21:45
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    $\begingroup$ Just for argument, say it was something like using lead pipe (or worse) for the radiating elements. No lumped resistor. I understand it's a horrible idea, but I'm trying to find documentation for or against the idea that a change in Q without any other change like resonant freq or mechanical arrangement would distort the pattern. $\endgroup$
    – user103218
    Oct 18, 2017 at 21:59
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    $\begingroup$ @user103218 could you update your question with more information, what type of design you are looking at? what materials? what frequencies? how do you plan to change/restrict Q? for what purpose? what is it what you are trying to accomplish? and any other information which could help us understand your particular situation and question. $\endgroup$
    – Edwin van Mierlo
    Oct 19, 2017 at 8:44

2 Answers 2

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In general for a given antenna evaluated on a given frequency, if whatever is changing the Q occurs uniformly or universally and does not result in a change to the effective aperture, phasing, or geometry of the antenna then the pattern will not be altered in a meaningful way.

An example of a uniform change would be changing the wire gauge of a wire antenna. An example of a universal change would be changes to a matching network that do not affect element phasing. Another example of a universal change is adding resistance to the antenna feedpoint. Any of these examples could alter the Q without changing the pattern.

Changes that affect the Q may change the gain of the antenna without any changes to its pattern. In antenna engineering, the pattern of the antenna is called Directivity. It is the Efficiency of the antenna times its Directivity that yields its Gain: $$ Gain=Efficiency \times Directivity $$

Since the Q of the antenna relates to its efficiency , it is clear that the gain can be altered through changes in Q.

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Honestly, I can't give a definite answer to this question with the given information. But the general answer is that the Q alone probably will have little or no noticeable effect in the pattern. Only the bandwidth and efficiency are likely to be altered in any significant way, since you are talking about "magically" changing the material used. However, other related things (almost ALL parameters are interrelated SOMEHOW) MAY be affected and thus cause a change. The amount of change is still likely to be minimal, if even noticed. You could simulate the effects using a good antenna simulator program. Simply change the material used in the simulation and rerun it. In my experience this rarely has any profound effect. And many others that have posted their experiences online report little to no difference, no matter what material they use for the conductor.

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