# Antenna Coupling via Capacitor 3

This question appears at first glance to be almost identical with this one Antenna coupling via capacitor but I think that in the following case the involved coupling capacitor might play a different role than in the linked question and I would like to find out which role it has.

Consider the models of antenna + matching modules (I found them in: https://www.gsm-modem.de/M2M/m2m-faq/how-to-design-an-antenna-matching-circuit/)

Question: What is the job of the capacitor in first model? Unfortunately, the author waived any explanation on the function of this coupling capacitor, he just noted that a final matching circuit could be just one capacitor or inductor in row. Why?

Indeed I think that here the reason d'etre of this coupling capacitor might be another one than of that one from Antenna coupling via capacitor which was based on the circuit I found here: http://makearadio.com/misc-stuff/regennotes.php (but I don't know, it's just a conjecture of mine)

The reason for implementation of the coupling cap there was (see Andy aka's answer) that the author was dealing with a concrete regenerative receiver, where the (variable!) coupling capacitor's job was to set the right amount of the Q-factor which is roughly proportional to the quotient $$\ Ctank/Ccou \$$, where $$\ Ctank \$$ is the capacitor in the tank, and $$\ Ccou \$$ the coupling cap.

Now in https://www.gsm-modem.de/M2M/m2m-faq/how-to-design-an-antenna-matching-circuit/ the author not says to which kind of receiver the antenna is coupled. So it seems that the role of coupling capacitor which he introduces there is of more "universal" manner and not depends on concrete receiver type which sits at the other end.

So could somebody explain what the job of the coupling capacitor in first model is?

So could somebody explain what the job of the coupling capacitor in first model is?

From the rather weak web site you linked it says this: -

Question: What is the job of the capacitor in first model? Unfortunately, the author waived any explanation on the function of this coupling capacitor, he just noted that a final matching circuit could be just one capacitor or inductor in row. Why?

If the antenna is "short" it can have a significant amount of capacitive reactance as per this impedance plot for a monopole antenna when the length is below 0.25 λ: -

And, as per answers given on previous occasions on this subject, you might choose to add capacitance in series to raise the reactance and make tuning tighter (more selective). This of course suits the old-fashioned receivers like crystal sets and regenerative receivers.

But, you may also choose to cancel out the inherent capacitance of a "short" antenna by base loading with an inductor. This causes series resonance and therefore, you can obtain a higher degree of power transfer (at the expense of a fairly low radiation resistance that requires some form of matching network to bring it to a sensible value).

Indeed I think that here the reason d'etre of this coupling capacitor might be another one

No it isn't. Of course, anyone can say that the purpose of the capacitor is DC blocking but that is missing the whole point about what the awkward website is trying to tell you.

• When you say that 'choosing to add capacitance in series to raise the reactance and make tuning tighter (more selective)' could you exactly point out how this effect of adding capacitance in series makes the tunning tighter in precise mathematical terms? The way you describe it in your answer, seems a bit vague to me. Could you formalize it? How you quantify here the 'tightness' or 'selectivity' of the tuning precisely? By Q factor? If yes, the Q factor of what? Do you implicitely connect a 'test tank' to the antenna about and analyze it's Q? Dec 13, 2021 at 21:27
• @katalaveino unaccepting my answer is not the right way to go. I don't respond to that sort of "stick" mentality (just because you have a follow-up query that you need help with). If you want a derivation of proof of how the Q factor is affected then you should ask for that but, remember what I said on the other question about pure math answers (that are as much related to mechanics as they are related to EE and quite possibly other areas). This answer has been answered. Dec 13, 2021 at 22:48
• Strictly speaking, your answer indicates the role capacitor there. But of course, for a non-pro like me, a few more detailed explanations of the exact relationships between the Q and the coupling capacitor would be more desirable. But if you think that that's requires a separate discussion ok, I will post it next time. Dec 15, 2021 at 1:46

This question is relevant to transmitter / antenna coupling.

• Circuit # 1: The purpose of the capacitor is DC blocking.

• Circuit # 2: The capacitor-input Pi tank is a low-pass filter.

• Circuit # 3: The inductor-input Pi tank is a high pass filter.