# Q factor of antenna in receiver circuits

I have a quite broad question about the significance of the Q factor (and especially that one of the antenna) in receiver circuits.

Say we have an abstract receiver consisting of an antenna and the receiver box (including all these necessary parts of a common receiver circuit, such as the tuned circuit, filter, matching box, feedback etc.)

Note that 'receiver box' is of course a non standard terminology; here I use it to emphasize that I'm speaking about the part of the complete receiver system without the antenna:

The most elementary model we could have here consists only of an antenna, coupling capacitor and tuned circuit, where on the right hand side we passed to antenna replacing circuit:

I found several times on the internet (for example, here) that one is often interested in improvement of the Q factor of the antenna. Why?

In general the Q factor of a resonant system determines the bandwidth issues, ie. low Q receiver tend to receive more different frequencies, but has bad quality (damping) and selectivity behavior, and a receiver with high Q has good selectivity but has only a narrow band of frequencies it can receive. Q seems to be about the bandwidth of the receiver.

Why and when is (or might be) it necessary to take also the Q of the antenna into account if one is going to adjust bandwidth issues of a receiver? Does it completely suffice to adapt the "right" Q factor (and therefore the desired bandwidth) only for the receiver box if one wants a receiver with certain properties? Can the Q of antenna in rough words be simply "ignored"?

In simple words: When one wants to build a receiver with narrow or wide bandwidth, about Q of which component of the receiver system should one really care? I guess that the Q of the receiver box is most important, but the question is if the Q of the antenna sometimes (and if yes, when) matters in these tuning issues.

In case of the elementary model pictured above seemingly in order to adapt the desired receiving properties and so the bandwidth around resonant frequency (ie. the inverse of Q), it suffice to adapt the $$\L_1 K_1\$$-tank alone without taking care for the Q of the antenna, or not?

If I'm wrong, and the Q of the antenna is more important then I thought, what is it's usage?

If one is interested in a receiver with certain bandwidth, which Q factor should one try to adapt (Q of which component?) That of the receiver box alone or should one also take into account how the Q of the receiver box is related to the Q of the antenna?

What kind of relation between these two Q's should be tried to be achieved in order to improve the receiver system?

I hope my question is not too broad. My motivation is that I read several times that the bandwidth issues of a receiver are managed over adjustment in the receiver box only ("matching network,") but on the other hand there are a lot of sites dealing with adaptation of Q factor of the antenna itself, which seems in light of fist "philosophy" irrelevant. What is my thinking error?

• Systems I work with design the antenna (element) for the maximum frequency range (bandwidth) needed for the application, which usually means low Q. Then the amplifier or receiver behind the antenna is tuned to use a narrow portion of the antenna's bandwidth, what is sometimes called the instantaneous bandwidth. This can be considered the high-Q part, or more selective part of the system Nov 9, 2021 at 2:34
• @SteveSh: so in summary one can regard it as a general principle when dealing with bandwidth issues in receiver systems: The Q of antenna should tend to be not too high (in order to allow certain "flexibility" for wide sortiment of receivable frequencies), while the Q of what I called the "receiver box" above (which comprises all this amplifier, filter etc stuff) should be designed which preferably high Q, right? Thats essentially the "philosophy" on the relation between the Q factor of the antenna and the receiver box one strives for when designing receiver circuits, right? Nov 9, 2021 at 16:44
• Yes, although I usually don't use the term Q-factor much. Bandwidth (BW) and Instantaneous Bandwidth (IBW) are used in my industry. Nov 13, 2021 at 0:33
• @SteveSh: But the meaning of this terminology is coherent in the sense when we are talking about receiver/transmitter systems then the 'the Q-factor of the antenna' corresponds to 'THE bandwidth (BW)' (of complete network), while 'the Q-factor of the tuning unit' of the receiver corresponds to 'the Instantaneous Bandwidth (IBW)'? Is this statement correct? Or is this also highly non standard terminology? Nov 13, 2021 at 3:08
• Btw: The reason why I'm also a bit uncertain about usage of Q-factor for receivers/transmitters is that I several times read a phrase like 'the Q-factor of the receiver/transmitter blabla...'. But the point is that in general Q is associated to a swinging system (like an abstract LC-circuit like the LC-tank in out case, but also the antenna due to it's replacement model as imaged above). And the question is in that case to what of the total receiver/transmitter system the mentioned Q is associated now? Nov 13, 2021 at 3:10

one is often interested in improvement of the Q factor of the antenna. Why?

This was most often done on simple radio receivers such as a crystal set. Complexity of circuitry was just not possible in those days so, the antenna (and it's effective capacitive impedance due to it being "short") was utilized as part of the tuned-circuit to enable a listener to hear one transmission and avoid hearing an adjacent transmission.

The basic antenna (shorter than a monopole) itself was just a wire but, because it projected a capacitive reactance, it could form part of the electrical tuning of the whole system. So, we use words or phrases like "tuning the antenna" or "improving the Q-factor of the antenna" but, in reality we are just kidnapping the projected impedance of the antenna and using it in part of a tuned circuit.

Why and when is (or might be) it necessary to take also the Q of the antenna into account if one is going to adjust bandwidth issues of a receiver?

On more modern receivers (super-heterodyne) we want a low-Q antenna i.e. an antenna that produces (within reason) the same signal levels at one end of the band compared to the other end and, doesn't produce large resonant peaks in the middle. In other words we want a flat response from the antenna because the selectivity circuitries are not part of the antenna any more; they are part of the I.F. strip. Think about a bat detector - pretty standard microphones are used that would probably pick-up audio quite well but, the electronic circuits down-stream from the microphone are aligned to pick-up mainly bat ultrasound frequencies. They can of course be retuned to different bat frequencies but, this is generally done using a super-heterodyne method just like a more modern radio receiver.

I guess that the Q of the receiver box is most important, but the question is if the Q of the antenna sometimes (and if yes, when) matters in these tuning issues.

It's most important when we want to be able to tune a station at one end of the dial and then have equal success tuning a station at the other end of the dial. A tuned antenna circuit can do this but the components become bulky and cumbersome compared to using heterodyne techniques.

If one is interested in a receiver with certain bandwidth, which Q factor should one try to adapt (Q of which component?) That of the receiver box alone or should one also take into account how the Q of the receiver box is related to the Q of the antenna?

You have to be careful here. A "certain bandwidth" might refer to either the full range that a receiver can tune stations (bottom of the dial to top of the dial) or, it might mean the bandwidth of the selectivity applied to the incoming signals at a particular frequency in order to amplify it and reject adjacent channels from causing interference. If you have a radio system that is only interested in one particular radio frequency then you can apply Q at the antenna or, you can apply Q in the circuits that follow the antenna (or both).

• thank you, i will take a closer look at it in the coming days and ask about some aspects that i have not yet fully understood Nov 12, 2021 at 2:11
• @katalaveino this site is all about questions and answers and does not work well with either questions that evolve or comments that snake through a whole learning process. If you have certain (a few) comments to make about my answer so that things can be clarified, that's fine but, do not digress and do not expect a lengthy teach-in session. Learning is about raising a new question on this site, getting an answer and moving on. Nov 12, 2021 at 8:35
• a nitpick on your usage of language: when you say 'it (antenna) projected a capacitive reactance...' you just mean that it partially behaves in some sense like a capacitor and therefore in practice it is reasonable to require that a model of an antenna should have a capacitor. That's what you mean by 'projected a capacitive reactance'? I have to admit that the word 'project' in that context confused me a bit. Nov 13, 2021 at 3:18
• It has an effective capacitive reactance when shorter than a full monopole or dipole. Nov 13, 2021 at 8:54
• On your last paragraph: "You have to be careful here... " Yes, there I indeed meant by "certain bandwidth" the full range of frequencies that a given receiver can receive (after we have manually dialed the one desired frequency we want to listen now). Now in light of this clarification: say we want to build a receiver that can receive every frequence from a given band when we choose it via a manual dial. Can in this setting it summarizingly be said that the general "mantra" on involved Q's Nov 18, 2021 at 1:00

The Q factor is a Figure of Merit of resonant circuits.

If you consider just the antenna of your circuit, then you calculate the Q factor of the antenna and possibly try to maximize it.

If you consider some other resonant part of your system, then you calculate the Q factor of the that part and possibly try to maximize it.

The experienced RF engineer, when designing the receiver/transmitter part of a radio circuit rarely starts from the Q factor.

He/she evaluates/measures the impedance Z of the antenna at the frequency of interest and then designs the matching R, L, C network to the receiver/amplifier in order to maximize the maximum power transfer.

That il called RF impedance matching.

RF impedance matching calculations are used inside RF chips and outside RF chips whenever the RF signal leave a block of the system.

• Yes, you emphasized another important aspect in design of receiver/transmitter circuits - impedance matching (en.wikipedia.org/wiki/Impedance_matching) - between the antenna and "receiver box" impedance. There - even if in practice sometimes may become hard - the answer to question "What we want?" can be easily written down: the equality of $Z_S =Z^*_L$. And this is exactly what our adaptation is based on as ultimate goal. That aspect of matching I (presumably, at least theoretically) understand. Nov 9, 2021 at 17:09
• But then the a bit provocative question is if (and when) the Q of the antenna in receiver design at all matters? So say we want to construct a good receiver system and we already succeed in matching the inpedance of the antenna and the "receiver box" via the method you sketched above. Now, are we finally already done and the Q of antenna shouldn't matter any more? What is then reason why some people are interested in properties of the antenna's Q? Is that just an "end in itself"? So my concern is about antenna Q's "raison d'être". Nov 9, 2021 at 17:10
• You may try, and some RF simulators like Optenni Lab do, to design a second or third order matching network. In that case, maximizing Q becomes a design target. Good question, thanks. Nov 9, 2021 at 19:28
• maximizing which Q? Of the antenna or of the matching box? My question is: if we know the Q of the antenna and the Q of our receiver box and assume we can control them both and we want increase the quality of our receiver system, so that means the receiver should be able to select every possible frequence we want within a fixed given frequency band. Then in ideal case: how the Q of antenna should be related to the Q of the receiver box? Nov 9, 2021 at 23:22
• (eg should they both have a similar Q, or should the Q of receiver box be much bigger the the Q of antenna, or or or ...?). which relationship between these two Qs should ideally be aimed for? That's the core of my problem. Nov 9, 2021 at 23:22