## New guy here. I got interested in learning more about antennas and so

## New guy here. I got interested in learning more about antennas and so

(OP)

I bought a book to try and learn from. I hope you will be able to answer some questions for me that the book asks at the end of the chapters. Some of them I have trouble with and I can not seem to find them.

The first one is how is the resonant freq of and antenna determined. I came up with either by length or by the frequency you use it with? Which one is right? And if it is by length how do you calculate it? I dont seem to see any calculations for it.

Thanks for the first one

The first one is how is the resonant freq of and antenna determined. I came up with either by length or by the frequency you use it with? Which one is right? And if it is by length how do you calculate it? I dont seem to see any calculations for it.

Thanks for the first one

## RE: New guy here. I got interested in learning more about antennas and so

The resonant frequency of anything is a characteristic of that system. It has nothing to do with the frequency at which you actually operate it. That seems a strange answer because clearly you choose the frequency at which you will operate the device depending on its natural responses.

Suppose I have a motor driving a cam which pushes a spring which in turn pushes a mass. At a certain motor speed the mass will move much more than at other frequencies. This is the resonant frequency. The resonant frequency relates to the size of the mass and the size of the spring. The speed at which you drive the motor doesn't change the characteristics of the system. We have a "natural response" and a "forced response". Look these up in a text book or google them.

## RE: New guy here. I got interested in learning more about antennas and so

So then based on the size is there a calculationn I could you or would it be directly porportional to the size I would assume?

## RE: New guy here. I got interested in learning more about antennas and so

The key things about an antenna are directivity and output reflection coefficient (matching). If the antenna is highly reactive it will require more matching to get power n or out. If the antenna is more directive you will get a higher output signal for a given incident field strength (higher "gain").

## RE: New guy here. I got interested in learning more about antennas and so

## RE: New guy here. I got interested in learning more about antennas and so

## RE: New guy here. I got interested in learning more about antennas and so

Here are the basics. The formula for determining the resonant frequency is f(res) = 1 / [2 * pi (sqrt LC)]. A practical means of using this information could be as follows:

First, you construct an antenna of specific dimensions. You measure the capacitance. You measure the inductance. You plug both values into the above formula and that will tell you your resonant frequency or f(res). Let's assume that your DESIRED resonant frequency is higher than your calculated resonant frequency. You can see from the formula that you can either decrease the inductance L or decrease the capacitance C, or both, and the result of any of these actions will be an increase in the resonant frequency. Any action you take that makes the denominator of the formula smaller makes the resonant frequency higher in frequency, thus more closely approaching your DESIRED resonant frequency.

Second, you can see from the formula XfL = 2 * pi * f * L that decreasing the L as in our example above decreases the inductive reactance (XfL) while raising the resonant frequency. Increasing the L by adding a series inductor would decrease the resonant frequency, as you correctly state in your last post.

Finally, you can see from the formula XfC = 1 / 2 * pi * f * C that decreasing the C as in our example above increases the capactive reactance (XfC) while raising the resonant frequency. Decreasing the C by adding a series capacitor would increase the resonant frequency as you correctly state in your last post.

The whole point of my original post was this. You asked if length is the determining factor in resonant frequency. It is ONE factor, and a major factor as well. However, anything in the construction, materials and/or feed method that affects either the XfL or XfC or both will also be a factor. Therefore to directly answer your original question, "...how is the resonant frequency of and (sic) antenna determined.", it is determined by the value of the total XfL (which is dependent on the total L) and the value of the total XfC (which is dependent on the total C).

debodine