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.

There are two ways to consider this antenna:
First way:
15" of hook-up wire will constitute a short monopole, a quarter wavelength at 100Mhz would be about 29".
To operate correctly the monopole will require a groundplane connected to the 0v line of the transmitter. The groundplane should be a set of wires a quarter wavelength long extending radially from the transmitter.
This is getting messy.

The second way:
Consider the transmitter to be directly connected to the centre of a dipole, your antenna wire makes one leg of the dipole, the supply wiring makes the second leg of the dipole.
Another messy poorly defined situation.

The "look" of the electromagnetic field in your room will be messy,
The antenna is badly defined and the field will be affected by every conducting item in the room, as well as a few outside it.

To state that the RF output from the transmitter is about 50mW is a BIG assumption which is likely to be VERY wrong.
The power output of the transmitter will be very dependant on the load presented by the undefined antenna.

What the signal strength will be in you receiver 5 meters away is anyones guess.
There is a formula for calculating field strength, but to use it with any accuracy, the transmit and receive antennas must be well defined, the path between them must be unobstructed. Also when the antennas are close together (in terms of wavelength) the formula does not work very well.

From a practical point of view, radio with odd bits of wire for antennas will work, but not necessarily very well, and doing calculations on the likely signal strength is just about impossible.

JimB

__________________
Experience is directly proportional to the value of the equipment ruined.

 

of course it's a "toy" what do you expect? i'm using it so i can learn from it..

Imageshack - screenshot1wx.jpg

i did this thing long time ago but i left it aside because i couldn't make it
work...the frequency was always driffting even from a single touch at the battery..or at the antenna (monopole as you told me)

so as you understand the frequency is not standard... 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.

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.

 

nobody?

 

That is a seriously poor circuit for many reasons.
The antenna is directly connected to the oscillator tank circuit, there is no way that is going to be stable, especially when someone touches any part of the circuit.
"Toy", I see these circuits presented here and cringe, they work, just, but there is nothing good about them from an electronic engineering point of view.

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?

There are too many unknowns to define the impedance of your antenna.
As for the transmitter, it does not have a defined output impedance or a designed load impedance.
Any load (ie the antenna) on that circuit will shift the frequency of and untimately stop the oscillator.

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.

JimB

__________________
Experience is directly proportional to the value of the equipment ruined.

 

i know it and i don't care... as i said i did that circuit long time ago when
i was at my very first steps in electronics. i had it in a box just to remember my first builds and i got it now so i can play a bit and learn something more...
there are tons of such circuits out there..also audioguru did an improved little FM transmiter.

my objective is to understand RF.


i'm using a Lutron FC-1200 frequency meter and a Sony AIR-8 receiver


i hope you somehow find it exciting helping me.......

 

OK, I found some enthusiasm

Using an antenna analyser, read about it here:
Vector Impedance Antenna Analyzer AIM4170

I did some scans of a couple of antennas, a dipole made from 19inch long hook-up wire, and a groundplane antenna (a monopole) made up using 19inch lengths of steel rod.

The dipole.

With the dipole and the antenna analyser just laying on my desk, I got the result as shown in the attachment below.
The legs of the dipole were 19inches long, this should give a resonant frequency around 145Mhz, in reality it was showing multiple resonances around 137Mhz due to the effect of the desk.

Picking the analyser up in may hand and holding it clear of the desk and other stuff, I got a totally different picture.
The general apperance was much more what would be expected from a crude dipole, the resonant frequency was higher than expected, and much higher than when on the desk.

Attached Thumbnails

     

__________________
Experience is directly proportional to the value of the equipment ruined.

 

amazing....

this instrument could be very handy to someone who builds his own antennas...usually many guys test their home made antennas directly on their transmitter with a SWR bridge.

i'll come back later on with more questions...

 

And now the groundplane (monopole) antenna.

I will let the pics speak for themselves.

JimB

Attached Thumbnails

       

__________________
Experience is directly proportional to the value of the equipment ruined.

 

i'm back!

i did some study and some experiments...and i think i got improved a bit...
so if planning to improve the fm transmiter by myself by the way it helps alot if i imagine the antenna as a speaker and the FM transmiter as an audio amplifier...

so here's what i'm thinking to do... starting from the power supply... i'm going to use a VK200 at the power source so i can filter any noise going to the fm transmiter when i'm not using batteries..

but also i got a better idea... i'm thinking of using an lm7805 with the needed capacitors to stabilize the power supply and then use a Murata BLM31AJ601SN1L which i got available from past projects so i can keep the regulator safe from oscillations.. and keep a clean power supply for my FM transmitter.


now about the antenna that is directly connected to the oscillator tank...what can i do about that??? can i use another transistor as a buffer?!

antenna is a load to the FM transmiter as the speaker is a load for an audio amplifier right?

what do you want to know so you can tell me the impedance of my monopole antenna other than the length of the antenna and the frequency of my trasmiteter?

antenna is a load to the FM transmiter as the speaker is a load for an audio amplifier right? so an audio amplifier that outputs 100W at 4ohm it outputs less at 8 ohm is it the same?

i also need results from a monopole antenna without groundplane as the one i'm using and on frequencies close to the ones used on FM radio broadcasting... as do i ask too much?

 

I suggest that you look at the FM transmitter circuit by Audioguru on this site, it used an amplifier stage to isolate the oscillator from the antenna.

Yes.

Yes.


This is a completely pointless exercise, because...

The antenna has to have two "wires", a dipole has two wires, the second wire of the monopole is the groundplane.
An antenna without a ground connection for the current to flow through a circuit (yes, there is current flowing in a transmitting antenna), is like connecting up a speaker with just one wire.

JimB

__________________
Experience is directly proportional to the value of the equipment ruined.

 

as i said before i'm aware about the FM transmitter Audioguru did...
i need someone to explain me where is that stage and how i can copy it...


interesting.. i could ask why it uses only one wire...but i thought it's similar withthe radios...

what about my ideas for improvement do you agree with them?
i'm also thinking to change the air coil with an axial inductor of the same inductance is that possible?

 

i have started a PCB based on the simple FM transmitter where i intend to make my modifications...

i will try to substitude as many components as i can with SMDs or atleast with smaller thru hole so i need to know if i can use an 100nH inductor in the place of the air coil..

The followin pcb is done on windows paint program i don't want to use any specified tools like EAGLE because i want also to be able to do my designs without any fancy tools.

Attached Thumbnails

 

 

In my FM transmitter I used a low-dropout 5V regulator because the voltage of a 9V battery quickly drops lower than the minimum input voltage for a 7805 regulator.
I used a 3rd transistor as an RF amplifier to isolate the tuned circuit of the oscillator from stray capacitance near the antenna.

Audio is completely different to RF. The output impedance of an audio amplifier is extremely low, 0.04 ohms or less.
The output impedance of an RF amplifier matches the impedance of the antenna to avoid high frequency reflections that cancel the output signal.

__________________
Uncle $crooge

 

i don't have 5V low drop out... either i'm going to use 3.3V LDO or a normal 7805.. what do you think?

on the other thread you told me i can replace the second LC tank with a resistor if the transistor before the antenna is biased correctly for it...

can you explain it a bit more? why you used that LC? isn't it much difficult to regulate two LC tanks at the same values??

aproximately what value of resistor do i need in the collector so i can replace the LC tank?

 

Every semiconductor manufacturer makes 5V low-dropout regulators.

The DC current gain of a 2N3904 transistor is from 100 to 300.
With 20mA collector current and a collector resistor of 220 ohms then a base resistor of 83k will be good when the gain is 200 but not good if the gain is higher or lower or if the temperature changes. The operating point will also change as the battery voltage runs down. So the transistor must be biased properly with more resistors so that any 2N3904 transistor will work at any reasonalble temperature and at any reasonable supply voltage.

With the tuned circuit then the output is still a pretty good sine-wave when the transistor is saturated or is cutoff. Then the interference is low.

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Uncle $crooge