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xitronics 22nd May 2007 10:54 AM

Doubts regarding a 1Watt FM TX Kit
2 Attachment(s)
In the VHF oscillator circuit I have attached to this post, what should the approximate power output across a dummy load be..?

Btw this the 1st stage of a 1W FM transmitter I purchased recently from a local seller. It is hardly outputting 50-100mW of power.. I'll post the complete circuit diagram of it soon.

In the mean time can anyone assist me on predicting the output of this oscillator stage..?


richie00boy 22nd May 2007 11:52 AM

What load are you using? The load in real use will be extremely high impedance as it's just the aerial wire capacitive reactance. You are probably just loading the output stage too much and it's collapsing.

xitronics 22nd May 2007 01:45 PM

Final 1WATT Ckt Uploaded..
2 Attachment(s)
Alright.. I have attached the complete FM circuit. In that I have indicated the expected o/p power level across each stage as specified in my user manual.


For a 75cm aerial, radiation resistance is around 40ohms (@100Mhz) which has been transformed to a desired value by impedance matching section across the output of final stage. (see the new schematic I have posted). Anyways it must be that low for 1WATT RF I guess..

I do not know the input impedance of the driver and output stages. How can I estimate that..? I think we may do so with reasonable accuracy by referring to s-parameter data in their datasheets. But I am not at all sure abt it. Kindly share anything you might have on this subject..

PS: DC current from battery is measured around 20mA at 12V meaning a 240mW power input. Surely something is wrong with the circuit. pls help..!


richie00boy 22nd May 2007 02:38 PM

The stated 1W output will be only if you are driving the circuit right up to clipping.

Just build it, test it with music or whatever your input signal is, and listen to the radio in the next house to see how much inpout you can give it before it sounds distorted. Then take the radio how far away you want it to send. If it still picks up with good quality then don't obsess over power numbers.

destroyer X 22nd May 2007 06:49 PM

Multiply the current you have into the transistor by the voltage

About the first diagram posted.

It is a class A radio frequency oscilator/transmitter.

The real power will be 10 percent of your current multiplied by the output voltage.

The real power transmitted will depend of standing wave ratios, and this depends from antena matching...size, material, position, way used to couple to the antenna and errors of phase.

Normally the power offered, suggested, advertised, informed, is the consumption power, as the real transmitted power will depends from the matching between the transmitter and the load (antenna)

Because of antenna whip impedance...this circuit cannot produce more than 300 miliwatts of is more probable that you are having 35 to 70 miliwatts.

This can be listened 12 meters distant...crossing 4 walls...inside your home.

Outside you may be able to reach 300 meters maximum with visual contact.

My informs are more feelings than calculations..based into many years of experience as Radio Amateur..but errors can reach more than 50 percent if you think on numbers.

BTW...that battery can supply 50 miliamps maximum keeping 8.5 volts.... the maximum power the circuit can drain will be 425 miliwatts..but this is consumption...maximum consumption.... the real output, if perfectly transfered to the Antenna will be no more than 10 percent of that...maybe 45 miliwatts.....and this makes sense...because the transistor power.



xitronics 23rd May 2007 01:21 PM

@destroyerX :

You mean the device's power efficiency would only be 10%..! Hard to imagine that. Could you please explain over this in a bit more detail..

Secondly, lets not connect a Monopole Whip at the output point. If it were to be a matched dummy load what could be the maximum power delivered at 12V regulated supply..? I dont want a strict 1W ouput but I can make it radiate around 300-500mW it'll serve my purpose..

Btw the circuit from Vegakit. And its a single sided PCB with no ground plane.. Is this the crux of the issue..?

PS: In the 2nd pic i forgot to show the 10pF feedback cap across oscillator EC terminals.. did it by mistake..


destroyer X 23rd May 2007 02:08 PM

Class A, single ended, normally only 10 percent of the consumption power is
transformed in usefull radio frequency signal that would be irradiated.....this.....if the antenna is matched.

That other one, using 2n3886 or something alike and 2N4427 is able to produce from 500 to 800 miliwatts without the second schematic is biased in class C...and the efficiency of class C is very big.....but.....class C produce a lot of harmonics...twice your fundamental frequency and so on....this means losses of energy in other frequencies other than the needed ones and interferences too.

Class A..... with a CCS is something more produce 1 watt of Radio frequency you will force a transistor to operate to 10 or a little bit more watts.... the transistor will be expensive..... something alike SD1018 (25 or more watts and only 13.8 volts)....or a 1477 (100 watts of radio frequency up to 250. Megahertz)

Those ones, able to dissipate 5 watts maximum (2N3866) are enougth only to 500 miliwatts of class A power....and those ones can work with voltages upper than 13.8 volts...they can work using 23 to 25 maximum.

Efficiency is small to radio frequencies...and small to class A.... this is the ratio related the consumption and the real usefull signal....consumption is your current multiplied by your voltage...normally, in class A, you will have NO MORE, than 10 percent of that fast calculation, converted in usefull signal into the antenna.

Measure you current into the output and multiply by the voltage DC you have when the circuit is working connected to the antenna...multiply that value and observe 75 percent to class C and 10 percent to class A



xitronics 23rd May 2007 04:02 PM

@destroyer: Thanks a lot for the info.

One thing I have seen in many RF design guides is about RF transistors having limited power gains of around 10-20dB depending on internal capacitances.

What it means is if I intend to get 500mW out of an RF FET(or BJT) at 10dB i must feed in 50mW of input power, real power. My million dollar question is what inside the trans'tor consumes this "real" power.

Look, it cant be the gate-source junc resistance coz its too high in reality. If it is the internal capacitance(limiting the power gain) it never would dissipate that much of real power.

Pls let me know. I havent been able to find a satisfactory answer to this Q anywhere.


destroyer X 23rd May 2007 08:19 PM

In the class A sittuation, the efficiency is a normal consequence of advanced bias
To avoid crossover distortions (reducing it) we force transistor into a heavy conduction....let's say that it will dissipate 10 watts of heat when one watt will be really produced and transfered to the next stage....the transference to antenna use to produce losses too.

When you advance your bias point, means also that you have advanced current...bigger current entering the device..... every device has minimum and maximum points of operation...if you start them to work already "acelerated" into mid range, you will have less room to increasings and decreasings of current..but, you will have "that room" to increase and decrease in the same proportion...because you are in the mid point between the maximum and minimum.

Advanced bias point also represents looses of sensitivity.... more current will be needed to obtain a good multiplication into the output.... advancing bias you will enter a low gain region of the device.

Well...this is the way i can "see" those things.... i am not absolutelly sure and others can produce better replies...let's listen others..there are engineers in our forum...i am not....all i know is a consequence of experiences made.... long time doing those things...RF power amplifiers was my hobby for some years (around 10).

But class C, class AB will provide you much more efficiency.

Radio frequency power transistors are normally constituted by several small transistors (1 watt) in 10 watts unit may have 10 small transistors ...and to equalize them you need emitter resistance to each device...i suppose this may reduce the it is normal to find small gain into Radio Frequency transistors.....normally gain is lower than 20....and when transistors turn old, the gain use to reduce.

Yes, capacitances will go "eating" the device power when you increase the frequency..the same device, operating at 10 Megahertz will provide you bigger gain.... result of smaller losses.



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