Harold14370,
agree with Dishmaster. If the transmitter and receiver are fixed, the frequency cannot change. Frequency is the number of beats or pulses per unit time, and every beat or pulse transmitted must also be received.
Every time the speed of a wave changes relative to an observer, the frequency of that wave will change relative to the observer as well.
Maybe I complicated the way I stated what I was thinking. Let me try to simplify it using an analogy.
Let's say you have a audio speaker that's stationary on the Earth. You turn on the speaker, and it produces a sound of a specific tone (frequency). Now, you're one hundred meters away and you hear the speaker. Since you are stationary relative to the Earth's atmosphere (the medium of sound), the frequency of the sound will be equal to the same frequency as the speaker produced.
Now, let's say that you start moving towards the speaker at a specific speed. Because of this, the speed of the sound wave will speed up relative to you, and the frequency of the sound wave will increase. The frequency of the sound increased because the speed of the wave increased. But let's forget about the frequency shift of the sound for a second. It's kind of irrelevant to the point I'm trying to make.
As state above, let's say that there's a speaker making a sound of a certain frequency and you're moving towards it at a certain speed. Now, lets say you pulled out a ruler that's one meter long and you hold it out in front of you so that it's pointing towards the speaker. For the sake of argument, let's say that you have "audio vision" so that you can actually see the sound, and let's say that you have a very quick mind. When you're traveling towards the speaker, you look at your ruler and count how many wavelengths of the sound fit on the length of your one meter ruler. You find that there are N number of wavelengths of sound across your one meter ruler.
Now, you decide to speed up towards the speaker and count the number of wavelengths of sound on your one meter ruler again. You find that the count did not change, there are still N wavelengths of sound there. Next, you decide to stop, and start moving in the opposite direction of the sound. You look down at your ruler to find out that there are still N wavelengths of sound on your ruler. You conclude that regardless of the speed that you are moving relative to the sound, the number of wavelengths of sound on your ruler remained constant even though the frequency and speed of the sound, relative to you, has changed.
You're surprised at the results at first, but when you think about it, it makes sense. You see, if the frequency of the speaker is constant, then the wavelength of the sound is constant. And if the length of your ruler is constant (1 meter), and the length of of wavelength of the sound is constant, then the total number of wavelengths of that sound on your ruler must be constant as well.
Now, lets take the same example I gave above and replace the sound with light, the Earth's atmosphere with light's medium, and the ruler with two mirrors (a semi-silver mirror on one end, and a full mirror on the other). As in the example above, you'll find that no matter how fast you're moving towards, or away from, the light, the number of wavelengths of light between the two mirrors will remain constant. Actually, the number of wavelengths of light between any two points remains constant regardless of how fast those points are moving relative to the light (as long as the distance between the points remain constant).