When waves encounter an impedance discontinuity at least two things happen. Part of the energy in the wave is reflected back toward the source and part of the energy continues on it's way.
Standing waves are indeed a direct result of a mismatch between an antenna and the amplifier.
I've given you the best advice I have; I can't do any more.
If you want to learn a bit further, here is a very good explanation of why an antenna actually radiates:
ARRLWeb: Why an Antenna Radiates
If we connect our 75ohm dipole to a 50ohm feeder, there will be a VSWR (Voltage Standing Wave Ratio) of 1.5 to 1 in the feeder.
What this means is, if we measure the voltage at various points along the feeder, we will find alternate maxima and minima at quarter wavelength intervals along the feeder, the ratio of the maximum to minimum voltage will be 1.5 to 1.
Enough for now, bedtime I think.
JimB
i have a small one transistor FM transmitter and today i decided to experiment a bit...
i was using the transmitter without any microphone connected... and i was trying to find the carrier frequency on the radio which was difficult because the transmitter is unstable and the frequency was rolling...but i got surprized when i noticed that if i hit a component with the back of the screwdriver then i can hear the noise from the radio like if i was recording the sound.
my opinion is that the vibration was transformed into electrical signals which then transmitted to the radio...and probably the variable capacitor was responsible for that effect... the sound has nothing
to do with bad conduction..
what do you think?!
Yes, as I said previously, a halfwave centre fed dipole has an impedance of 75ohms, etc. Some practical antennas will have matching circuits built in them to give other impedances (usually 50 ohms).so...antenna impedance is a property of the antenna type?!.
A dipole has two wires (or rods), each wire is a quarter wavelength long at the design frequency.can you show me a feedpoint, a feeder and a dipole? although i know what these are...you're getting me confused because i don't know what you mean when you say to connect a 75ohm dipole to a 50ohm feeder.
You do not state the frequency of the transmitter. You do say that it is an FM transmitter so I assume you mean one of the little toys to make a noise on a VHF FM broadcast receiver, so I guess the frequency will be around 100Mhz.i'm using a 15'' hook up cable for antenna and i would like to know how it radiates in my room (how the electromagnetic field looks like around the cable
and inside my room while the antenna is looking horizontally over my desk)
what type of antenna is the cable i'm using and what's the impedance of it and of the FM transmitter
It is based on a 2n3904 transistor and i'm using a 9V battery source and it draws about 20mA so it decipates about 180mW and the output RF power must be ~50mW so the output power in dB must be +17dBm but what's the signal level in μV that reaches my radio at a distance of 5 meters?!
You do not state the frequency of the transmitter. You do say that it is an FM transmitter so I assume you mean one of the little toys to make a noise on a VHF FM broadcast receiver, so I guess the frequency will be around 100Mhz.
That is a seriously poor circuit for many reasons.of course it's a "toy" what do you expect? i'm using it so i can learn from it..
Imageshack - screenshot1wx.jpg
How do you know that it is running on 54Mhz? Do you have a receiver for that frequency or some other kind of frequency meter?right now it operates at about 54 MHz and i'm catching the second harmonic on my radio at 108 MHz... i need to change the coil so i can make it work a bit better.
There are too many unknowns to define the impedance of your antenna.so...now can you tell me what's the impedance of this monopole? and what's the output impedance of the transmitter?
you told me about a monopole with ground plane..can you give me a photo so i can know how it looks like?
i need to understand how an electromagnetic field radiates to the enviroment from an antenna like the one i'm using....
i also need to have a better understanding about the importance of the antenna size... (you said the size of the monopole is small it should be 29''..ok i know that but the schematic states 15'' and in fact it still radiates at a very long distance.
That is a seriously poor circuit for many reasons.
How do you know that it is running on 54Mhz? Do you have a receiver for that frequency or some other kind of frequency meter?
I have been out all day, just got in.
If I get the enthusiasm a bit later I will have a play with some antennas and get some results and picture for you.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?