Antenna terminated in an impedance

I have a basic question about terminating an antenna with an impedance (L,C or anything). In this case, is it possible to analyze the circuit by assuming impedance division between the connected load and the impedance of the antenna? If that's true how is the reflection (possibly due to mismatch or scattering) modeled? If we take it further, and connect the antenna directly between the gate-source of a transistor (without any transmission line or waveguide), do we need to do the s-parameter analysis or can simply use the circuit model of the transistor and Thevenin model of the antenna and do KVL-KCL?

since an antenna is, by definition, an electrically large object, you can not do a lumped element analysis of it.

WHY would you want to terminate an antenna anyway? that would reduce the transmitter efficiency significantly...as you heat up your termination.

I am basically considering the antenna in the receiver mode. How should I model the circuit in this case (if the lumped analysis is not possible). So the thevenin model for the antenna will not work?

Its not often that an antenna directly feeds the gate/source of a transistor, unless its a mast head pre amplifier. Usually there is a transmission line involved.

But basically you would want all of the available signal power to drive the gate capacitance to maximum signal voltage, and that requires correct impedance matching if coupling losses are to be minimised.

Almost anything will work, but if you are chasing highest possible signal to noise ratio, correct impedance matching as well as resonance, can make quite a difference.



The idea is to resonate the input capacitance of the device with an inductor to form a parallel tuned tank. Then couple your transmission line into the inductor for maximum energy transfer.

This works because the gate is a high impedance load, so you need voltage magnification and an impedance step up from the transmission line.

Circuit Q may be important if you require some bandwidth. If its for one spot frequency make the Q as high as possible.

we are doing Thz detection using MOSFETs. I am just saying if we want to analyze the voltage on the gate (in the aformentioned configuration), is it possible to use the thevinin model of the antenna and then calculate the voltage induced on the gate ? (using the circuit model of the transistor)



Thanks for your comprehensive reply. I totally understand your explanation of the resonance. But I still did not get my answer if it makes sense to do the analysis simply by using the Thevenin model of the antenna and circuit model of the transistor...

Its not just resistive matching, the gate will be a complex capacitive load and you need a conjugate match, which is effectively a resonance.

I have no idea how to do it at Thz, but at much lower frequencies usually an adjustable noise source is used. The lower the amplitude of the external introduced noise source required to raise the receiver audible noise output by say 6db is a good guide.

Its really signal to noise ratio you are chasing, and anything that reduces noise is just as good as something that increases signal amplitude. Gain is easy to get further down the chain if you need more.

You know I think my case is different from the case of a power amplifier....I just one to get the maximum voltage on the gate source of the transistor and use the nonlinear nature of the transistor to rectify that voltage.....So basically I want to have the maximum gate source voltage.... That's why I am asking is it possible to simply analyze the situation using the Thevinin model of the antenna and the circuit model of the transistor....

I believe the antenna produces either:
* greater V, less A, or
* less V, greater A.
What you get depends on what impedance you hook up to it.

A unpowered 'crystal' radio set can drive crystal headphones (which is to say, high impedance type). As for ordinary low impedance headphones, you don't hear much.

The antenna is influenced by energy from photons. As for antenna impedance, it's hard to say whether it's high or low. It might have to do with the amount of energy coming from the photons. If it's a lot of photons then they convey a lot of energy.

If it's a few photons then it's a small amount of energy. So is it low V and low A? Is it high V and miniscule A? (I have trouble picturing it as high A, but maybe it really is, and the voltage is miniscule.)

We need a resonating circuit to lock onto a particular frequency coming from the antenna.
What resonating circuit is best? Should it be LC tank? LC series? One or more combinations of both?

I suppose my above thoughts are not direct answers. They are a way of looking at the question, in hopes of leading to an answer.

Referring to your original question.

The port of an antenna can be represented by a single complex impedance (for each point of the frequency range). Respectively the power transfer to a load can be calculated as voltage divider (with complex impedances).

Thanks for ur answer. I have another question, suppose we connect a load to the antenna (receiver mode), does the power burnt in the impedance of the antenna (in this circuit model, voltage source in series with a complex impedance) kinda represent the ohmic loss and the reflected power from the antenna?

The real antenna impedance component includes also the radiation resistance which doesn't represent a physical resistor.

If you have an ideal antenna (no ohmic losses) with 50 ohm impedance, then the "50 ohm" represents the transformed impedance of free space. No power is "burned" by this real impedance.

Maybe I used a wrong terminology. Let me put it this way. In the case of an ideal antenna in receiver mode (no ohmic losses), does the power across the 50 ohm component of the antenna impedance represent the reflected power from antenna ?

Power reflected by what? If you load the antenna with 50 ohm, there's no reflection.

again, maybe I am using the wrong terminology. Sorry for that. Does the power across the 50 ohm component of the antenna impedance represent the "re-radiated or scattered power" from the antenna?

I don't think so. If terminated with the characteristic impedance, the antenna will absorb maximal field power and reflect/scatter nothing. When mismatched, part of the available power will be retransmitted.

if that's the case, does the power across the 50 ohm component of antenna impedance have any physical interpretation?

As said, it's no physical resistor rather than transformed free space impedance, a factor linking electrical and magnetical wave magnitude. https://en.wikipedia.org/wiki/Impedance_of_free_space

The impedance has a clear physical interpretation, but the power is somehow virtual, I think.

Thanks for your clarification. I have another question. Two cases of a receiver antenna (open circuit and short circuit): What is physically happening? Is the antenna radiating back the received power? If yes, is it clear quantitatively how much of the received power?